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Teacher Guide

LS3 Material & Equipment List page 157

Learning Set Three Material & Equipment List(quantity listed is the amount needed per group unless otherwise noted)

LESSON 13: DOES MASS CHANGE IN A CHEMICAL REACTION?Materials:

• 6 Alka-Seltzer tablets• Water• 2 small cups• 1 empty, clear soda-pop bottle (20 oz) with the cap (to be used twice)• Matches

Additional materials for optional activities:• 8 oz. plastic or paper cups• 6 teaspoons (~30 mL) glue (e.g., Elmer’s® or comparable brand)• 5 mL water• 15 mL sodium borate solution (liquid laundry starch e.g. Linit)• Plastic bags (for after the investigation)

Equipment:• Balance or electronic scale• 1 small test tube or vial

Additional equipment for optional activities:• Popsicle sticks (for stirring)• Graduated cylinder


LESSON 14: WHY DOES MASS STAY THE SAME IN A CHEMICAL REACTION?Materials:

• Gumdrops of 5 different colors (at least 3 of each color)• Cups or baggies to hold the gumdrops• Toothpicks

LESSON 15: IS MY SOAP A NEW SUBSTANCE?Materials:

• 50 ml rubbing alcohol (store bought: 70% isopropyl alcohol, 30% water)• Multiple broken-off pieces of students’ soap• Warm tap water, warm cooking oil (approx. 15 mL each)• Water

Additional materials for optional activity:• All concept maps (Lessons 1, 6, and 12)• Colored markers• 2 large sheets of paper (11” x 18”)• Sticky notes (two different colors)• Tape

Equipment:• 1 balance (electronic recommended)• 100-mL graduated cylinder• Calculator• 2 test tubes• Test-tube clamp• Stopper• Temperature probe, or thermometer• Hotplate• Ring stand• 500 mL beaker• Small metal spatula, or popsicle stick


LESSON 16: HOW DOES MY SOAP COMPARE? OR HOW CAN I IMPROVE MYSOAP?Materials for Option A:

• A broken-off piece of students’ soap• A broken-off piece of commercial brand soap• Plastic wrap or aluminum foil• Rubber band• 3 pieces of cloth• Items to “dirty” the cloths – e.g., charcoal, dirt, markers

Materials for Option B:• Masking tape• 4 paper cups or other containers for the salt, water, fat, and rubbing alcohol• 2 coffee filters or paper cups to mass the salt and fat• 50 g table salt (sodium chloride)• 175 mL water• 11 g fat• A selection of oils (e.g., coconut oil, palm oil, olive oil)• A selection of extracts (e.g., vanilla, almond, peppermint)• 20 mL rubbing alcohol• 20 mL 6 M sodium hydroxide solution

[Flinn Scientific Inc. (catalog number is S0242; cost for 500 mL is $8.70)]• Paper towel or paper plate (This is for the soap to dry and harden)

Equipment for Option A:• 100-mL graduated cylinder

Equipment for Option B:• Large beaker (ex. 250 mL) or large plastic cup

[Students leave their soap in this cup until the next day. Consequently, thiscontainer cannot be reused in subsequent classes.][A large plastic disposable cup can be used instead of a large beaker. The cupneeds to be a relatively thick, high quality plastic cup because the soap is basicwhen it is poured into the cup. The basic solution can cause a thin cup todeteriorate. Do not use paper or styrofoam cups. It is preferable for the cup to beclear.]

• Small beaker (ex. 100 mL)[This beaker does not have to be exactly 100 mL. There just needs to be onesmall beaker and one large container.]

• 2 stirring rods or 2 plastic spoons• An eye dropper, or plastic pipette• Hotplate• Balance• 1 graduated cylinder

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Does mass change in a chemical reaction? Lesson 13

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LESSON 13 –DOES MASS CHANGE IN A CHEMICAL REACTION?

OVERVIEW

The previous two Learning Sets have focused on substances, properties, and chemicalreactions. This Lesson introduces Learning Set 3, which addresses the concepts ofconservation of mass, open systems, and closed systems. Learning Set 3 presents a finalsub-question: Do new substances always come from old substances? The purpose oflesson 13 is to introduce the final sub-question and the concepts of conservation of mass,open systems, and closed systems. The lesson includes one optional activity (13.A) thatcan precede the core activities (13.1, 13.2). In Optional Activity 13A students conduct achemical reaction to produce gloop and investigate whether mass stays the same orchanges during a reaction that does not include a gas. In Activity 13.1 studentsinvestigate whether measuring the mass of reactants and products could provideevidence of whether new substances always come from old substances. Students conductan investigation of total mass before and after a chemical reaction (Alka-Seltzer) in anopen system. They find that the total mass after the reaction appears to be less than thetotal mass before the reaction. You facilitate a discussion about why the mass might havechanged and introduce the terms open system and closed system. Then in Activity 13.2,you challenge students to design an investigation to account for the mass of the gasproduced in the reaction. Students design procedures that incorporate a closed systemand test their design. They find that the total mass before the reaction is equal to thetotal mass after the reaction. The lesson concludes with a discussion where you introducethe concept of conservation of mass and discuss the discrepancy in students’ findings fortheir first and second investigations.

SAFETY GUIDELINES

• For Optional Activity 13.A, Gloop is not toxic. Students can touch the gloop withtheir bare hands.

• For Activities 13.1 and 13.2, goggles should be worn.• Students should be reminded not to taste anything, including the gloop and the Alka-

Seltzer.Learning Performance Assessment Criteria:Students define that the principle of“conservation of mass” is that the totalmass before a chemical reaction isequal to the total mass after a reactionin a closed system because no material(atoms) can enter or leave the system.Material (atoms) is neither created nordestroyed in chemical reactions.(LP19)

Students’ definitions includes that the mass of thereactants before the Alka-Seltzer experiment equals themass of the products after the chemical reaction when itis conducted in a closed system because the gas cannotleave the system.

Students identify the type of system,open or closed, for a process. (LP 20)

Students identify the first Alka-Seltzer investigation asoccurring in an open system and the secondinvestigation as occurring in a closed system.

Students design an investigation todetermine whether total mass remainsthe same before compared to after aprocess. They make a prediction,identify variables, control variables,and communicate scientificprocedures. (LP 21)

Students’ experimental design includes: a predictionabout whether mass will stay the same, identify theimportance of closing the system, designing andcommunicating a procedure that involves closing thesystem and finding the total mass before and after thechemical reaction for Alka-Seltzer.


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Learning Performance Assessment Criteria:the same before compared to after aprocess. They make a prediction,identify variables, control variables,and communicate scientificprocedures. (LP 21)

importance of closing the system, designing andcommunicating a procedure that involves closing thesystem and finding the total mass before and after thechemical reaction for Alka-Seltzer.

Students conduct a scientificinvestigation to gather data aboutmass before and after a process. (LP22)

Students’ investigation includes completing the Alkaseltzer reaction in both an open and closed system. Ineach case, students gather data in a clear record (such asa data table) for their observations and mass data.

Students construct a scientificexplanation that includes a claimabout the mass before compared toafter a chemical reaction, evidencethat includes the mass before and afterthe reaction and the type of system,and reasoning that mass before andafter the reaction must be equalbecause of the principle ofconservation of mass. (LP 23)

Students’ explanations for Lesson 13.1 include a claimthat the mass decreased, evidence in the form of massbefore and after the reaction and reasoning that the masschanged because the gas escaped the bottle.

Students’ explanations for Lesson 13.2 include a claimthat mass was conserved, evidence in the form of massbefore and after the reaction and reasoning that massstays the same in a closed system because the gas couldnot leave the system.

PREPARATION

Time

2 class periods (Optional add 1 class period)MaterialsOptional Activity 13AFor Each Student• Activity Sheet 13A: Does mass change when we make gloop?For Each Group• 8 oz. plastic or paper cups• 6 teaspoons (~30 mL) Elmer’s® glue• 5 mL water• 15 mL sodium borate solution (liquid laundry starch e.g. Linit)• Popsicle sticks (for stirring)• Graduated cylinder• Mass balance• Plastic bags (for after the investigation)Activity 13.1For Each Student• Activity Sheet 13.1: Does mass change when Alka-Seltzer reacts?For Each Group• 3 tablets Alka-Seltzer• 50 ml water• 1 small cup


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• 1 empty, clear soda-pop bottle (20 oz) without the cap• 1 balance or electronic scaleActivity 13.2For Each Student• Activity Sheet 13.2: Does mass really change when Alka-Seltzer reacts?For Each Group• 3 tablets Alka-Seltzer• 50 ml water• 1 small test tube or vial• 1 empty, clear soda-pop bottle (20 oz) with the cap• 1 small cup• 1 balance or electronic scale• 1 match

• This is not for students to use in their designs for 13.2. You will instruct studentsto use the match to test the gas produced in the reaction after they have finishedtheir experiments.

Set-upOptional Activity 13A• Prepare the sodium borate solution by mixing 20 g (1/6 cup) of laundry borax

(sodium tetraborate decahdrate, Na2B4O7 • 10 H2O) into 1 L (about 1 qt.) of waterwhile stirring. All of the borax may not dissolve, but this is OK.

Activity 13.2• For the beginning of Activity 13.2, gather all of the equipment the students can use

for their experiment at the front of the room. You should show them the materialsthey can use to design their procedure.

INSTRUCTIONAL SEQUENCE

Introducing the Lesson:Tell students that they are now about to start the third sub-question to the drivingquestion “How can I make new stuff from old stuff?” This is the last section of the unit,which will allow them to completely answer the driving question. The third sub-questionis “Do new substances always come from old substances?” In order to answer thisquestion, you would like students to think about some of the concepts they have alreadylearned about in the unit. Ask students what happens in a chemical reaction.

Two or more substances interact and their atoms combine in new ways to formnew substances with different properties than the old substances. The newsubstances are made of the same atoms as the old substances, but the atoms arearranged in new ways.

Ask students whether they think that the new substances always come from oldsubstances or if they think that new substances could come from somewhere else.


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Students may provide a variety of answers. The point of this question is to makestudents thinking visible and to help students realize that new substances cannotjust appear from nowhere. New substances are always made by recombiningexisting atoms from old substances. Students may offer ideas like - In order tomake new substances, you need atoms that can recombine to make those newsubstances. Those atoms need to come from somewhere. That somewhere is theold substance. They cannot just appear.

Ask students how could they investigate whether new substances come from oldsubstances and not from somewhere else. Ask students if there is anything that theycould measure to provide evidence for whether new substances come from oldsubstances. Is there anything that they think is the same about the reactants andproducts?

Students might say either yes or no and provide different reasons for theirresponses. The point of this question is to access their thinking.

If students do not bring up the concept of mass, ask students if they think the mass of thereactants is the same or different than the mass of the products after a chemical reaction.You may want to ask them about a specific reaction that they have completed in class.For example, you could ask them if they think the copper and acetic acid had the samemass as the copper acetate and hydrogen gas produced during the reaction in Lesson 8.

Students may provide a variety of responses. The purpose is to elicit students’thinking – The mass stayed the same because there were the same number andtype of atoms before and after the reaction. OR The mass decreased becausethere was a gas produced and the gas escaped or gases have less mass. OR Themass increased because there was another substance on the penny so the pennyhad a greater mass.

Ask students if they think measuring mass would provide evidence to answer the sub-question “Do new substances always come from old substances?”

If the mass of the new substances and the old substances were the same, thiswould provide evidence that new substances always come from old substances. Ifthe mass of the new substances is greater, this suggests that the new substancesare coming from somewhere else or that matter is created. If the mass of the newsubstances is less, this suggests that some of the new substances are escaping orthat matter is destroyed.


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Teacher Background Knowledge• Mass is conserved for all chemical reactions. On an atomic level, the numbers of

each type of atom are conserved for all chemical reactions. In a closed system,conservation of mass can be measured.

• A chemical reaction in a closed system means that nothing enters or leaves thesystem. A chemical reaction in an open system means that something can enter orleave the system, but atoms are still neither created nor destroyed.

• On an atomic level, a chemical reaction in a closed system means that atoms do notenter or leave the system. A chemical reaction in an open system means that atomscan enter or leave a system.

• Students will be introduced to these ideas during this lesson and the next lesson.

Optional Activity - Does mass change in a chemical reaction? Lesson 13

Optional Activity page 166

Optional Activity 13A: Does Mass Change When I Make Gloop?1. Introduce Activity 13ATell students that each group will conduct an experiment to test whether mass stays thesame or changes when they perform a chemical reaction - this time, they will make“gloop”. Write the new (word) reaction on the board:

Reactants ProductsGlue + Water (H2O) + sodium borate in water gloop

Provide each student with Activity Sheet 13.1: Does mass change when I make gloop?Review with students the procedure on the student sheet.Ask students if they know why they can include the mass of the cups. Why don’t theyhave to subtract the mass of the containers out?

They will compare masses before and after the chemical reaction, so the constantmass of the containers doesn’t matter.

Ask students if they know why they need to include the mass of the empty cups after thereaction.

Since they included the mass of the cup with the reactants, they have to includethe mass of the cup with the products.

Teacher Content KnowledgeThe reaction for making gloop can be explained by the model below. In this example, aboron atom connects two glue molecules. This connection happens with all of the gluemolecules, to make long chains. These long chains are called polymers. The molecules,and the atoms that make up the molecules, all have mass. From the reaction, you can seethat there are the same number of atoms before the reaction as after the reaction. Youcan do this by counting the number of atoms to the left of the arrow, and the number tothe right of the arrow.

glue + water + sodium borate solution gloop

+ H2O + B(OH)4- + 5H2O

Optional Activity - Does mass change in a chemical reaction? Lesson 13

Optional Activity page 167

2. Students Investigate Whether Mass Changes When They Make GloopHave students work in groups to complete Activity 13A. Provide each group with thematerials.3. Class Discussion of Student Results and Conclusion QuestionsHave students share their observations of what happened during the gloop reaction. Then,discuss students’ responses for the two conclusion questions. Examples of studentobservations, as well as examples of student responses to the conclusion questions areprovided in the teacher version of Activity Sheet 13A.

Common Student Conceptions• Students may think that mass changes during a chemical reaction based on their

everyday experiences. For example, students may think that mass changes whenmaterials burn or rust because the reactants seem to get “used up” or the products are“added.” They may think that the materials “disappear” or “appear.”

• Students’ perception of “mass” (if they mix up of density, weight, and mass) mayinfluence what they think happens to mass during a chemical reaction.

4. Wrap-up: Discussion of Whether Mass Always Stays the SameTell students that in this reaction mass stayed the same. Ask students if they think massstays the same in all chemical reactions. Can they think of any reactions where it wouldnot stay the same?

Students will provide a variety of comments. This question is intended to set thestage for their participation in Activity 13.1.

Tell students that in the next activity they will explore a different chemical reaction to seeif mass still stays the same.


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Activity 13.1: Does Mass Change when Alka-Seltzer Reacts?1. Introduce Activity 13.1Tell students that each group will conduct an experiment to test whether mass stays thesame or changes when 3 Alka-Seltzer tablets react in water. In other words, is the totalmass before the reaction the same as or different from the total mass after the reaction?Provide each student with Activity Sheet 13.1: Does mass change when Alka-Seltzerreacts? Review with students the procedure on the student sheet.Ask students if they know why they are breaking the Alka-seltzer into pieces.

The opening of the soda-pop bottle is small. It will be easier to add pieces ofAlka-seltzer rather than the whole tablet to the bottle.A student may also say that breaking the tablet into pieces increases the surfacearea of the Alka-seltzer, which will make the chemical reaction go faster.

Ask students if they know why they can include the mass of the cup and the bottle in theirtotal. Why don’t they have to subtract the mass of the containers out?

They will compare masses before and after the chemical reaction, so the constantmass of the containers doesn’t matter.

Ask students if they know why they need to include the mass of the empty cup.Since they included the mass of the cup with the reactants, they have to includethe mass of the cup with the products.

2. Students Investigate Whether Mass Changes When Alka-Seltzer Reacts in WaterHave students work in groups to complete Activity 13.1. Provide each group with thematerials.3. Class Discussion of Student Results and Conclusion QuestionsHave students share their observations of what happened when Alka-Seltzer reacted inwater. Then, discuss students’ responses for the two conclusion questions. Examples ofstudent observations, as well as examples of student responses to the conclusionquestions are provided in the teacher version of Activity Sheet 13.1.4. Wrap-Up: Discussion of the Gas Escaping the BottleUse the students’ results from this investigation to lead into the next activity wherestudents will repeat the same chemical reaction, but this time in a closed system.


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Teacher Background Knowledge• The bubbles indicate that a gas was produced due to a chemical reaction. The mass

seemed to change in the reaction because students did not measure the mass of thegas that was produced. The procedure allowed the gas to escape from the bottlewithout a cap, so the mass of the gas was not included in the students’ total mass afterthe reaction. Many students may think that a gas does not have mass, so this couldcause confusion.

• This is a chemical reaction in an open system, so the gas product leaves the system.Conservation of mass cannot be measured in an open system. However, atoms are notdestroyed. In an open system, you do not mass all of the substances. Students need touse a closed system (e.g. a capped bottle) to test whether or not conservation of massholds for this reaction, which is the focus of Lesson 13.2. In a closed system, none ofthe substances can escape, so the mass of all the atoms can be measured.

Ask students: Why do you think the mass changed during the Alka-Seltzer reaction? Doyou think there is anything that you could do differently and the mass would stay thesame?

The Alka-Seltzer made bubbles or a gas. The bubbles escaped the bottle so thatthe mass of the gas produced in the reaction was not “counted,” or included, inthe total mass after the reaction.If students do not bring up the bubbles or gas, suggest that they look at what theyrecorded for observations during the reaction. Encourage students to considerwhether any of these observations could have caused the change in mass.

Ask students if they think it is important to “count,” or include, the masses of all products(including the gas) to test whether mass stays the same or changes in a chemical reaction.

Common Student Conception• Many students may think that a gas does not have mass, so this could cause

confusion.

I need to “count” the masses of all products in their measurement of the totalmass after the reaction.

Ask students how they could include the mass of the gas in their measurement of the totalmass after the reaction.

Students may say that they can trap the gas to keep it from escaping.The goal of this discussion is to convince students that they need to re-test whether massstays the same or changes in the chemical reaction. They need to “count” or measure themasses of all products, including the gas, in their measurement of the total mass after thereaction.Introduce the concepts of “open system” and “closed system” to students. Tell studentsthat a system is what is included in a chemical reaction. In chemistry, a scientist hasto state what is and is not included in a system.Ask students what was and was not included in the system of the chemical reaction fortheir investigation?


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The system of the chemical reaction included the Alka-Seltzer and water in thebottle. The system did not include the gas that was produced. (The system alsodid not include everything else outside of the Alka-Seltzer and water in the bottle.)

Tell students that the system of the chemical reaction for their investigation was an opensystem. An open system means that something can enter or leave the system. Intheir investigation, the gas left the open system and was not included in the total massafter the reaction. In the next investigation, they are going to redesign the procedure forthe investigation in order to create a closed system. A closed system means thatnothing can enter or leave the system. They will redesign the procedure so the gas isincluded in the total mass after the reaction.

Teaching Alternative• To help introduce an experimental design that incorporates a closed system, you may

want to do the following demonstration or show the iMovie on KNOW for Lesson 13(http://know.soe.umich.edu/) that demonstrates the following reaction.

• Procedure 1 (Open system): Put 1-2 matches into a large Erlenmeyer flask. Mass theflask with the matches and record the “total mass before the reaction” on the board.Heat the flask with matches using a hotplate on high. After the matches ignite andburn, mass the flask with the burned matches and record the “total mass after thereaction” on the board.

• Total mass should be lower after than before the reaction because the carbon in thematches reacts with oxygen gas in the air producing carbon dioxide that escapes theflask. There is less carbon in the flask so the mass decreases (an open system).

• Procedure 2 (Closed system): Put 1-2 matches into a large Erlenmeyer flask and sealthe flask with a rubber stopper. Follow the remaining steps of Procedure 1.CAUTION – STOPPER MAY FLY OFF. Since carbon dioxide is produced andbecause the gas in the flask is heated causing the pressure to increase, the stopper mayfly off the flask. Make certain the flask is pointing away from people.

• If you have time to practice and have all of the equipment, you might want to try thisas a demonstration.

Common Student Conception• Students may believe that the total mass of the closed container increases because

smoke is produced during the chemical reaction.

5. Homework; Reader 13.1Assign Student Reader 13.1. Students should write responses to questions found withinthe reader.


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Activity 13.2: Does mass really change when Alka-Seltzer reacts?1. Student Groups Brainstorm Ways to Include the Mass of the GasTell students that each group has to re-design the experiment to test whether mass“really” stays the same or changes when Alka-Seltzer reacts in water. Their designsshould be a closed system to include the mass of all products of the reaction, includingthe gas.Have students break into their groups. Give students Activity Sheet 13.2: Does massreally change when Alka-Seltzer reacts?First, have each group write a purpose for its investigation. Tell students that theirpurpose should state what they are testing with their design. Ask students to share theirpurposes.

The purpose of my investigation is to test whether mass stays the same or changeswhen Alka-Seltzer reacts in water. My design will “count” or measure the massof the gas produced in the reaction to see if it has an effect.

Show students the equipment they have to re-design the experiment: an empty soda popbottle, the cap for the bottle, cup, and a small test-tube or vial.

Teaching Alternative• You may want to provide students with other materials (other containers, baggies,

balloons) to use in their redesign of the procedure. One thing to keep in mind is thatthe type of container used in the experiment can influence its results. For example,using a balloon to seal the container will result in displacement of air when gas isproduced in the reaction and the balloon expands. This will cause the mass after thereaction to be slightly lower than the mass before the reaction. We chose a sodabottle because no air is displaced and the mass before is equal to the mass after. Amore detailed explanation about why the change in mass occurs is available on theKNOW website (http://know.soe.umich.edu/).

Have each group brainstorm a new procedure for a closed system that accounts for themass of all products of the reaction. They should draw and/or write their procedure ontheir student sheet. See the teacher version of the student sheet for possible designs.After the groups have finished their designs, have each group report to the class what itsprocedure will be.Emphasize important steps that students mention for their designs that differ compared tothe original procedure. You may want to record these steps on the board or an overhead.If they do not bring up the following ideas, specifically ask them:• What containers will they use for water and the Alka-Seltzer tablet?• What quantity of each material will they use?

If students report that they will use different quantities of the materials for theirdesign compared to the original procedure, tell them to use the same quantities: 50mL water and 3 tablets Alka-Seltzer. They need to control these variables so theycan compare their results from this experiment to the last experiment.


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How will they find the total mass before the reaction?Emphasize that students include the mass of the cap in their measurements. Thecap has to remain on the bottle and be included in the measurement of total massafter the reaction, so it also needs to be included in the measurement of total massbefore the reaction.

• What procedures will allow them to include the mass of the gas produced in thereaction in their total mass after the reaction?

• How will they find the total mass after the reaction?Tell students that they should record the same pieces of evidence as in their firstinvestigation of Alka-Seltzer: total mass before the reaction, total mass after the reaction,and observations of the chemical reaction.Explain that students should include the masses of the bottle, the cap, and the vial in theirtotals, without subtracting. The containers are not involved in the reaction. They willcompare masses before and after the chemical reaction, so the constant mass of thecontainers doesn’t matter.Have students return to their groups and revise their initial procedures, if needed. Tellthem to create a data table or other means of recording their data. You may want torequire that each group check its procedure and data table with you before starting theinvestigation.2. Students Repeat Alka-Seltzer Investigation in a Closed ContainerProvide students with the necessary materials for their investigation. Have them work ingroups to complete their investigation.3. Class Discussion of Student Results and Conclusion QuestionsAsk students to share their observations of what happened when Alka-Seltzer reacted inwater. Then, discuss students’ responses for the two conclusion questions. Examples ofstudent observations, as well as examples of student responses for the conclusionquestions are provided in the teacher version of Activity Sheet 13.2.Ask students what they think would happen to the mass if the cap were taken off theirsoda bottle. Why do they think that would happen?

The mass would decrease by 1.5 grams. This happens because it would then be anopen system and the gas can escape the bottle.

Have students take the cap off their bottles, set the cap on the balance (next to the bottle),and then observe what happens to the mass. If students no longer have sealed sodabottles, you may want to do this as a demonstration.Ask students if the last three chemical reactions they completed in class were in opensystems or closed systems. If they had determined the mass before and after thereactions, what do they think they would have found? Would the mass have changed?

The baggie was in a closed system because we did it in a sealed plastic bag.Nothing could enter or leave the system. The mass would have stayed the same.


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The copper penny experiment was in an open system because the cup did notcompletely cover the Petri dish. So the vinegar could still evaporate out of thePetri dish and the gas could escape because the seal was not airtight. The masswould have changed. Students may initially think this experiment was a closedsystem. You may want to show them or draw the set-up.Making soap was in an open system. The mass would have changed. Everythingwas combined in an open beaker so materials could enter or leave the system.

Ask students if they think when you burned magnesium in class as a demonstration if thereaction was in an open or closed system. If they had determined the mass before andafter the reactions, what do they think they would have found? Would the mass havechanged?

Burning magnesium was in an open system because materials could enter orleave the system. The magnesium reacted with the oxygen in the air to form thenew substance, magnesium oxide. The magnesium oxide had a greater mass thanmagnesium because it was made of both magnesium and oxygen. Students maynot think that the magnesium oxide has a greater mass because it was a powderand the magnesium ribbon was a solid metal. You may want to demonstrate thisto students by repeating the experiment and massing the magnesium ribbon beforeburning and then the magnesium oxide after burning.

Teaching Alternative• You may want to repeat either the baggie experiment from Lesson 7 or the copper

penny experiment in Lesson 9 and this time measure the mass before and after thereaction. You could also set-up two versions of each experiment, one in an opensystem and the second in a closed system.

Concluding the LessonTell students that in a closed system the mass always stays the same in all chemicalreactions. Tell students that scientists call this concept the “conservation of mass”. Theconservation of mass states that the total mass before the reaction is equal to thetotal mass after the reaction in a closed system (or the total mass of reactants wasequal to the total mass of products). Emphasize that conservation of mass in a closedsystem is always true. Tell students that the reason mass is always conserved is becausematerial (atoms) can never be created or destroyed. In the next lesson, they will usemolecular models to explore what is happening with the atoms.Ask students why conservation of mass cannot be measured in open systems. Tell themto think about the first Alka-Seltzer activity.

In our first investigation of the chemical reaction for Alka-Seltzer, the gas left theopen system. Mass appeared to change because the mass of the gas was notincluded in a closed system. This is why evidence for the conservation of masscannot be obtained in an open system

Explain that conservation of mass also applies to other processes besides chemicalreactions. Conservation of mass occurs in boiling, freezing, creating mixtures and otherprocesses as well.


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Ask students how they would now answer the third sub-question “Do new substancesalways come from old substances?” Did the last two experiments with Alka-Seltzerprovide support for or against this idea?

Since mass stayed the same in the closed system, this suggests that newsubstances only come from old substances. Material cannot be created ordestroyed.

Tell students that in the next lesson they will continue to explore this idea by usingmodels to examine what happens to the atoms during a chemical reaction.

Check Point• At this time your students should have a basic understanding of conservation of mass,

open systems, and closed systems. If you feel that your students are confused by theseideas and you skipped 13A, you may want to add this activity before continuing to thenext Lesson. These ideas will be further explored in terms of atoms and molecules inthe next Lesson.

Homework: Reader 13.2Assign Student Reader 13.2. Students should write a response to the questions.Discussion of students’ answers will be used to introduce lesson 14.

Why does mass stay the same in a chemical reaction? Lesson 14

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LESSON #14 –WHY DOES MASS STAY THE SAME IN A CHEMICAL REACTION?

OVERVIEW

The previous lesson focused on the sub-question: Do new substances always come fromold substances? Students gathered data in the form of mass to show that since mass staysthe same during a chemical reaction in a closed system that this provides evidence thatthe new substances are coming from the old substance. The purpose of this lesson is toexplore what is happening during the conservation of mass at the particulate level.Students learn that conservation of mass occurs because you have the same number andtype of atoms before and after a chemical reaction. Students construct gumdrop modelsto represent the reactants and products in a chemical reaction similar to, but simplerthan, the Alka-Seltzer reaction. Students examine what happens to atoms in the reaction.They find that the numbers of each type of atom before the reaction are equal to thenumbers of each type of atom after the reaction. You lead a discussion to help studentsrelate the consistency in the numbers of each type of atom in the reaction to the idea ofconservation of mass. To conclude the lesson, you help students connect these newconcepts to the learning set sub question and to the driving question.

SAFETY GUIDELINES

• Students should be reminded not to taste anything, including the gumdrops.Learning Performance Assessment CriteriaStudents construct molecular models torepresent the arrangements of atomsand molecules composing substancesbefore and after a chemical reaction.(LP 16)

Students construct gumdrop models of baking sodaand hydrochloric acid (reactants in a chemicalreaction similar to the Alka-Seltzer reaction);separate the atoms (gumdrops) of the reactants; andrecombine the atoms to form new models of carbondioxide, water, and table salt (products of thereaction).

Students define that the principle of“conservation of mass” is that the totalmass before a chemical reaction is equalto the total mass after a reaction in aclosed system because no material(atoms) can enter or leave the system.Material (atoms) is neither created nordestroyed in chemical reactions. (LP19)

Students’ definitions include that the number andtype atoms before a chemical reaction is the same asthe number and type of atoms after a chemicalreaction in a closed system because no atoms canenter or leave the system. Atoms cannot be createdor destroyed.

Students use molecular models ofsubstances before and after a chemicalreaction to reason that the number ofeach type of atom is the same beforecompared to after the reaction, which iswhy mass is conserved. (LP 24)

Students use their molecular models of baking soda,hydrochloric acid, carbon dioxide, water, and tablesalt to reason that the number of each type of atom isthe same before compared to after a chemicalreaction, which is why mass is conserved.

PREPARATION

Time 1 class period


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MaterialsActivity 14.1For Each Student• Activity Sheet 14.1: Why does mass stay the same in a chemical reaction?For Each Group for Student Investigation 14.1• Gumdrops of 5 different colors (at least 3 of each color)• Cups or baggies to hold the gumdrops• ToothpicksFor Discussion following Lesson 14.1:• Overhead: Chemical reaction of baking soda + hydrochloric acidSet-up• You may want to create group sets of gumdrops. For example, you could put three

gumdrops of 5 different colors in a baggie for each group ahead of time.Alternatively, you could have a station set-up where students gather their owngumdrops.


Introducing the LessonAsk students if they remember what question they were trying to answer in the lastLesson. Why had they decided to examine the mass before and after a chemical reaction?

We were trying to answer the questions: Do new substances always come fromold substances? If mass stayed the same that would suggest that the newsubstances were only coming from the old substances and that they came from allof the old substances. If the mass increased, that would suggest that mass wascoming from somewhere else as well the old substances or that matter wascreated. If mass had decreased, that would suggest that the new substances didnot use up all of the old substances or that matter was destroyed.

Ask students what were their results from the Alka-Seltzer investigations.The first time we tested the Alka-Seltzer the mass decreased because we did this inan open system. An open system is when something can enter or leave the system.In the case of Alka-Seltzer, the gas left the system (soda bottle). When we testedthe Alka-Seltzer in a closed system, the mass stayed the same. A closed system iswhen nothing can enter or leave the system. We prevented the gas from leaving.

Ask students how would they now answer the question: Do new substances always comefrom old substances?

Since the mass stayed the same in a closed system, this suggests that newsubstances come from old substances.

Tell students that they are now going to construct molecular models to explore why massalways stays the same and why new substances always have to come from oldsubstances.


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Activity 14.1: Why does mass stay the same in a chemical reaction?1. Introduce Activity 14.1Tell students that they are going to construct models of substances that are very similar tothe reactants of their Alka-Seltzer reaction. The chemical reaction for Alka-Seltzer occursbecause one Alka-Seltzer tablet contains different substances that can interact. Then askstudents if Alka-Seltzer is a substance or a mixture?

Alka-Seltzer is a mixture. Alka-Seltzer is a combination of different substances.Remind students that many materials are mixtures. Ask them for examples of mixturesthey remember from their investigations or the reader.

Students may provide a number of responses such as: lemonade, salt water, tapwater, hot chocolate, store-bought soap

Tell students that Alka-Seltzer is a mixture of many substances. Explain that two of thesubstances in Alka-Seltzer can be reactants in a chemical reaction. Ask students what thescientific terms “reactant” and “product” mean?

A reactant is an old substance in a chemical reaction, or a starting substance inthe reaction. A product is a new substance made in a chemical reaction.

Tell students that when an Alka-Seltzer tablet is added to water, two reactants in thetablet interact to form products in a chemical reaction. Write the two reactants on theboard or on an overhead: Baking soda + Acid

Teacher Background Knowledge• Alka-Seltzer contains baking soda (sodium bicarbonate), citric acid, and other

substances such as aspirin. When Alka-Seltzer is added to water, the water provides amedium for the chemical reaction between the baking soda and citric acid in theAlka-Seltzer to occur. The products of the reaction are: carbon dioxide, water, and acomplex salt. The word equation for the reaction is:baking soda (aq) + citric acid (aq) → carbon dioxide (g) + water (l) + salt (aq)

• The purpose of Lesson 14.1 is for students to use gumdrop models to investigate whymass is conserved in the Alka-Seltzer reaction. However, citric acid and the saltproduct are too complex for students to examine on a particulate level. StudentActivity 14.1 simplifies the reaction by substituting hydrochloric acid for citric acidin the reaction. The reaction that students investigate is:baking soda + hydrochloric acid carbon dioxide + water + saltNaHCO3 (aq) + HCl (aq) CO2 (g) + H2O (l) + NaCl (aq)

Tell students that the acid in Alka-Seltzer is very complex. The acid is too complex forthem to investigate using a model. Tell them that they will examine a simpler acid thanthe one in Alka-Seltzer. Tell students that they will figure out why mass stays the samein the chemical reaction between baking soda and acid (the reactants) using molecularmodels.


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Teaching Alternative• Students can either create the molecular models using gumdrops and toothpick or

using the program “Chemation” on handheld (palm pilots). This program allowsstudents to make ball and stick models. Then students can manipulate the models andsave them in multiple frames. The use of multiple frames allows students the abilityto “animate” the chemical reaction. The program helps students understand thedynamic nature of chemical reactions. A detailed description of how to useChemation is provided in the introductory materials and also on KNOW.

• You may also want to show students this reaction between baking soda andhydrochloric acid before having students construct models of the reaction.

Teaching Strategy• Show students the reaction between baking soda and hydrochloric acid before having

students construct models of the reaction. This will provide students with the visualphenomena to relate the molecular models.

Review gumdrop models: Show students the different color gumdrops and the toothpicksthey will use in their investigation.Ask students what a gumdrop represents? Ask students what a toothpick shows?

A gumdrop represents an atom. A toothpick shows how two atoms are chemicallyattracted/combined together.

Ask students why there are different color gumdrops?The different color gumdrops represent different types of atoms.

Demonstrate how to make a gumdrop model for baking soda. Tell students that you aregoing to show them how to make a gumdrop model for baking soda, one of the reactants.On the board next to “baking soda,” write the chemical formula for the substance:NaHCO3. Tell students to look at the chemical formula you wrote on the board. Askthem how many different colors of gumdrops you will need to make a model of bakingsoda? Ask them why?

Four different colors of gumdrops. There are four types of atoms in baking sodabecause there are four different chemical symbols in its chemical formula.

Ask them to name the different types of atoms in baking soda. Tell them they shouldknow some of the names of the atoms because they saw their symbols in other lessons.

Students should know that: “H” means hydrogen, “O” means oxygen, and “C”means carbon.Students may not know that: “Na” means sodium or “Cl” means chlorine. If noneof the students know this, tell them. You may need to explain to students that anuppercase letter with a lowercase letter next to it means one type of atom.

Ask students how many atoms of sodium there are in baking soda? Hydrogen? Carbon?Oxygen? Tell them to use the chemical formula.


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Baking soda has: 1 sodium atom, 1 hydrogen atom, 1 carbon atom, and 3 oxygenatoms.

Show students how the atoms are arranged for a model of baking soda. On the board oran overhead, write the type of atom that each color gumdrop represents. Demonstratehow to put together the gumdrops (atoms).

Teacher Background Knowledge• The model for baking soda looks like:

• In this model, we do not differentiate the types of bonds (i.e. the ionic bond betweensodium and the carbonate ion and the covalent bonds of all the other connections).Instead, we use each stick as a representation of a chemical attraction/connectionbetween the atoms. Students will learn the intricacies of bonding in later grades.

• The link between carbon and the oxygen at the top of the model (where oxygen is notlinked to any other atoms) is actually a double bond. To simplify the model forstudents, we recommend treating this double bond as a single link like all of the otherlinks and using only one toothpick.

Keep your model assembled for students to refer to in their investigation.3. Students Construct Models for Conservation of MassHave students break into groups. Give each student: Activity Sheet 14.1: Why does massstay the same in a chemical reaction?On the board or on an overhead, write the word equation for the reaction shown on thestudent sheet. Tell students that during this investigation they will try and determinewhat the “?” is in the chemical reaction.

Teacher Background Knowledge• The word equation on the student sheet shows a “?” in place of “salt.” In their

investigation, students determine that salt (NaCl) is the third product of the reactionby using conservation of mass (or conservation of atoms). Do not write “salt” in theword equation you put on the board.

Remind students that carbon dioxide is the gas produced in the Alka-Seltzer reaction (i.e.the gas in the capped bottle in their experiment).


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Teaching Alternative• Some of the questions on the student sheet are challenging, so it may be helpful for

student groups to share their answers to each question before moving to the nextquestion. You can have a class discussion about each question, during which differentgroups share their answers, after groups complete the question.

Teaching notes about this activity are provided in the teacher version of the student sheet.4. Class Discussion of Students Models and the QuestionsAsk students to share their responses from Activity sheet 14.1. Examples of students’molecular models and responses to the questions are provided in the teacher version ofthe student sheets.Ask students how they defined conservation of mass in the last lesson where they testedthe mass of the Alka-Seltzer in both an open and closed system.

Conservation of mass means that no matter how substances interact with eachother, the total mass of the closed system stays the same because nothing canenter or leave the system. The total mass before the chemical reaction is equal tothe total mass after the reaction.

Ask students how they might add on to their definition considering what they justobserved using molecular models.

Mass is conserved because the number and type of atoms always stays the sameduring a chemical reaction. There are the same number and type of atoms beforethe reaction than after the reaction. The atoms are just combined in new ways toform new substances.

Tell students that scientists sometimes call this conservation of matter. Conservation ofmatter states that no matter how substances interact with each other, the numbers ofeach type of atom in a closed system stay the same. This is because atoms can neverbe created or destroyed. The number of atoms before and after a chemical reaction is thesame and the types of atoms before and after a chemical reaction is the same. If thenumber of atoms stays the same no matter how they are rearranged, then their total massstays the same.

Common Student Conceptions• Students may have a difficult time understanding that atoms cannot be created or

destroyed. They may think that since chemical reactions make new substances thatthey also make new atoms. You need to help students understand that the newsubstances are not the result of new atoms, but rather the same atoms are combined innew ways.

Concluding the LessonAsk students if a chemical reaction can create any new atoms? Ask them if a reactioncan destroy any atoms?A chemical reaction does not create or destroy any atoms. The number of each type ofatom stays the same in a reaction.


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Show students the overhead of the chemical reaction of baking soda + hydrochloric acid.Tell students that the reaction can be represented in three different ways by a: molecularmodel, word equation, or chemical equation. Discuss the kind of information provided byeach model: The word equation shows the names of the reactants and products. Thechemical equation shows the numbers and types of atoms in each reactant and product.The molecular model shows how the atoms are arranged in each reactant and product.Have students tell you what happens to the atoms in baking soda and hydrochloric acidwhen the substances react. Ask them how the atoms make the new substances: table salt,water, and carbon dioxide?

The atoms in baking soda and hydrochloric acid separate from each other. Thenthose same atoms rearranged and into new combinations to make table salt,water, and carbon dioxide.

Based on what students have done during the last two lessons, ask them how they wouldnow answer the sub-question: Do new substances always come from old substances?

New substances always come from old substances because atoms cannot becreated or destroyed. This is why mass is always conserved. You always have thesame number and types of atoms. In a chemical reaction, the atoms justrearrange to make new molecules. You always have the same atoms. So newsubstances are always the atoms from the old substances just in a newarrangement.

Check Point• At this time students should understand conservation of mass in terms of the number

and type of atoms always staying the same in a closed system.

HomeworkHave the students read and answer the questions in the Student Reader, Lesson 14.1

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Learning Set 3: Do new substances always come from old substances? Lesson 14

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Hydrogen atom: whiteCarbon atom: grayOxygen atom: redChlorine atom: greenSodium: blue

+ + +

HydrochloricAcid

+ Baking Soda Salt + Carbondioxide

+ Water

HCl + NaHCO3 NaCl + CO2 + H2O

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Is my soap a new substance? Lesson 15

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LESSON 15: IS MY SOAP A NEW SUBSTANCE?

OVERVIEW

The purpose of this lesson is to investigate the soap that students made in Lesson 12 todetermine whether students completed a chemical reaction and produced a newsubstance. This lesson engages students in using science principles and tools introducedin Learning Sets 1 & 2. Students begin their investigation by determining the color,hardness, solubility, melting point, and density of their newly formed soap. They thencompare the properties of their soap to the properties of one of the original substances(fat) to determine whether a chemical reaction occurred. Students write a scientificexplanation to support their claim as to whether they believe a chemical reactionoccurred as a result of combining substances. Then you show the class an overhead of aparticulate model of the chemical reaction for soap making, and discuss with studentshow the sodium hydroxide and lard interact and their atoms combine in new ways toform soap. In Optional Activity 15A, students revise their concept maps for the final time.They brainstorm a list of concepts, which need to be added to their maps and create newmaps based on their knowledge of properties, substances, chemical reactions andconservation of mass gained over the last three learning sets. They present their finalmaps to the class to showcase their learning. The lesson culminates with a discussionabout how students think their soap compares to commercial brand soap.

SAFETY GUIDELINES

• Students must wear gloves when handling their soap.• Refer to Lesson 2 for safety guidelines for testing solubility.• Refer to Lesson 3 for safety guidelines for testing melting point.• Students should wear goggles when testing solubility, density, and melting point.• Students’ soap should not be given to students to take home.

Learning Performance Assessment CriteriaStudents conduct a scientific investigationto gather data about properties ofsubstances, such as color, hardness, density,melting point, and solubility. (LP 6)

Students’ data table for the newly formed soapincludes the color, hardness, density, solubility,and melting point.

Students construct a scientific explanationthat includes a claim about whether aprocess is a chemical reaction, evidence inthe form of properties of the substancesand/or signs of a reaction, and reasoningthat a chemical reaction is a process inwhich substances interact to form newsubstances so that there are differentsubstances with different properties beforecompared to after the reaction. (LP 15)

Students’ scientific explanation about substancesin a chemical reaction includes: a claim that a newsubstance was formed after the set of originalsubstances was combined; evidence of specificproperties that differ between the new substanceand one of the original substances; and reasoningthat different substances have different properties.


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PREPARATION

Time

2 class periods (Optional Activity adds one more class period)Materials

Activity 15.1: Is my soap a new substance?

For Demonstration:• 3 overhead transparencies of Molecular Model of Fat and Soap

For Each Student:• Activity Sheet and Procedures Sheet (15.1): Is my soap a new substance?

For Each Group:Measuring Density• 1 balance (electronic recommended)• 1 100-mL graduated cylinder• 50 ml rubbing alcohol (store bought: 70% isopropyl alcohol, 30% water)• 1 calculator• A broken-off piece of student’s soapMeasuring Solubility• Warm tap water, warm cooking oil (approx. 15 mL each)• 1 test tube• 1 stopper• A small broken-off piece of student’s soapMeasuring Melting Point• A broken-off piece of students’ soap• 1 temperature probe, or thermometer• Hotplate• 1 test tube• 1 test-tube clamp• Ring stand• 500 mL beaker• Tap water• Small metal spatula, or popsicle stick

Optional Activity 15A: Final revision of concept maps

For Each Student:• All concept maps (Lessons 1, 6, and 12)• Colored markers• 2 large sheets of paper (11” x 18”)• Sticky notes (two different colors)• Tape


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Set-up

Activity 15.1: Is my soap a new substance?

Set up the materials for testing the properties similar to Lessons 2, 3 and 4. Gather all ofthe materials in a central location for easy student access.

• For solubility, the homemade soap dissolves better in warm water and oil. Warmwater from a faucet will work well. To warm the oil, place a beaker filled withenough oil for all of the groups on a hot plate set on low. Another method forwarming the oil is to place it in a microwave. The oil should be warm to thetouch, yet comfortable enough to leave your finger in.

• For melting point, set-up one apparatus as a model for students to use in setting uptheir experiment. See Lesson 3 for complete description and illustration.

• For density, the piece of soap will need to fit into the mouth of a 100-mLgraduated cylinder. You may want to cut appropriate-sized pieces of students’soap before class and then dispense them. We recommend electronic balancesbecause they are easy to use, reducing chances of error. However, triple-beambalances are an option.


Introducing the Lesson

Begin by revisiting the soap-making experiment from lesson 12. Ask students toconsider whether their soap is a new substance that was formed after combining fat andsodium hydroxide solution.

Students’ responses will likely fall into three categories: some will argue that theirsoap is a new substance, others will suggest that their soap is not a new substance,and still others will say that they need more information to be certain either way.

Ask students how they might be able to determine whether their soap really is a substancethat is different than the original substances.

To find out if our soap really is a new substance, we would need to test itsproperties and compare them to the properties of the original substances, like fatand sodium hydroxide solution. If our soap has different properties, then it is anew substance.

Describe the investigation for students: they will measure the properties of their soap andcompare these properties to the properties of one of the original substances (fat) todetermine whether or not the soap is a new substance made as a result of a chemicalreaction. The properties that students will investigate today include color, hardness,density, solubility, and melting point.Tell students that they only need to compare their homemade soap to the fat and not thesodium hydroxide because sodium hydroxide is in a different phase (liquid versus solid)at room temperature. Since sodium hydroxide is a liquid at room temperature and their


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soap is a solid at room temperature, they do not need to measure the properties. They canactually already tell that they have different melting points (because they are in differentphases).Activity 15.1: Is my soap a new substance?

1. Student Investigation–Examining and Comparing the Properties of Soap and Fat

Provide students with Activity Sheet 15.1: Is my soap a new substance?Have students work in groups to complete this investigation.

Teaching Strategy:• One way to structure this lesson is to allow student groups to proceed in their own

order and at their own pace as they measure properties.• Another way to structure this lesson is to have all student groups conduct the same

measurements at the same time, one measurement of each property after another.• A third way to structure this lesson is to have testing stations. This would allow

students to move from station to station to measure the properties of their soap.• A fourth way to structure this lesson is to have each group in charge of one property.

For example, you could have one group determine the melting point of all thedifferent soaps produced in the class (they could place a number of test tubes in onebeaker).

Show students the equipment they will need to conduct their investigations. Then, brieflyreview Activity Sheet 15.1. Remind students that in earlier lessons (Lessons 2, 3, & 4),they measured the properties of fat and commercial soap. In today’s lesson, they will bemeasuring only the properties of their newly made soap. Alert students that they willneed to refer back to their Activity Sheets from lessons 2, 3, & 4 (Sheets 2.1, 3.1, & 4.1)for procedures for measuring solubility, melting point, and density. They will also need torefer back to their Activity Sheets to complete the data table on properties for fat.Emphasize with students that they will need to wear gloves at all times when they handletheir soap. This is because there may still be some trace amounts of sodium hydroxide intheir soap.2. Class Discussion of Activity Sheet 15.1

Begin by asking groups to report on their different properties for their student-made soapand for the fat. Record student data on an overhead or the board.Discuss any differences in student data and possible causes of experimental error. Then,discuss students’ response to the conclusion question. Examples of a completed datacollection table and student responses for the questions are provided in the teacherversion of Activity Sheet 15.1.3. Class Discussion of the Molecular Model of Fat and Soap

Introduce the discussion by telling students that you are going to show them molecularmodels, or representations of soap and fat. These molecular models are more complicated


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than the other models students have seen. Both fat and soap have many atoms. Tellstudents that the models that you are going to show them are one way that scientistsrepresent soap and fat. Then, show students the first overhead transparency of fat andsoap.Ask students: What does each colored ball represent?

Different atoms – Carbon (gray), Oxygen (red), Hydrogen (white), and Sodium(blue)

Ask students: What do the lines connecting the atoms represent?These show chemical connections or chemical attractions between the differentatoms.

Tell students that molecular models have different shapes, such as a ring (sugar), a longchain (soap), and double helix (DNA). Fat molecular models are made of three longcarbon chains.Place the second transparency on the overhead.Ask students how they think the soap and fat molecular models are similar and different.

The chains for the fat and soap are similar, but not exactly the same; fat has threechains while soap only has one chain; and soap has a sodium atom and fat doesnot.

Place the third transparency on the overhead.Ask students to describe how they think the chemical reaction occurs.

The sodium hydroxide and fat interact and their atoms combine in new ways toform soap. The sodium from the sodium hydroxide sticks to one of the longchains. The other part of the sodium hydroxide becomes part of the glycerol. Youneed 3 sodium hydroxides because fat contains 3 long chains. One sodiumhydroxide is needed for each chain.

Teacher Background Knowledge• The glycerol from the chemical reaction was part of the solution that students poured

out the day after they made their soap. The glycerol is not part of their soap.

Ask students whether this description fits their definition of a chemical reaction: Achemical reaction is a process in which two or more substances interact and their atomscombine in new ways to form new substances with different properties from the oldsubstances. The new substances are made of the same atoms as the old substances, butthe atoms are arranged in new ways.

Yes because the fat and sodium hydroxide interact and their atoms recombine andstick together to form soap and glycerol.

Ask students if they think this chemical reaction follows the law of conservation of mass


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Yes. If you counted the number and types of atoms before the chemical reaction,they would be the same as the number and types of atoms after the chemicalreaction.

Tell students that their soap (if it has completely reacted) is soap all the way throughout.If the reaction did not go to completion, there may be trace amounts of sodium hydroxidestill in their soap. That is why students must wear gloves when handling the soap. Askstudents what they think a molecular model of store bought soap would look like.

It would be a mixture with other molecules besides the soap molecules.Tell students that while pure soap is a substance, the soap from a store is often not asubstance. Store bought soap is usually a mixture because it has dyes (color), fragrance,and moisturizers added to it. The soap that students made in class is a substance becauseit is only made of soap throughout.4. Homework

Assign Student Reader 15.1: Making Soap in Colonial Times versus Today. This readingpresents a brief comparison between soap making in Colonial times and soap makingtoday. Students should write a response to the question found at the end of this section.

Optional Activity 15A has students construct final concept maps in class. The finalconcept map activity is intended to help students organize their knowledge of unitconcepts and showcase what they learned in the unit. If you choose not to complete thisactivity in class, have students construct a concept map as homework. Similar to therevisions in Lesson 12, they should use a different color pen or marker to circle anyconcepts that they still agree with and cross out (with a simple X) any concepts that theyno longer agree with. Then they should add new sticky notes with any new ideas thatthey would like to add to their concept map. If your students used PiCoMap to createtheir concept maps, they should revise their concepts using PiCoMap.

Optional Activity 15A: Final revision of concept maps Lesson 15

Optional Activity 15A page 191

Optional Activity 15A: Final revision of concept maps

1. Discussion of Concept Maps

Remind students of the concept maps they created in Learning Set 1 to summarize whatthey had learned about substances and properties. Ask students if they were to drawanother concept map now, would it look the same or different? Why?

Students will probably say that their concept maps would look different becausethey learned many new things since Learning Set 1.

Tell students that they are going to look at their old concept maps to see what they stillagree with and to see what they think they should change. Then they will create newconcept maps to summarize their understandings of unit concepts.

Teaching Alternative• You may want to have students create concept maps in groups instead of individually.

2. Students Examine Concept Maps from Learning Set 1

Hand students the concept maps that they constructed in Learning Set 1. Give students(or groups) different color sticky notes and/or different color markers than they used ontheir original maps.Ask students to examine their concept maps to see what ideas they still agree with andwhat ideas they would like to change. Tell students to take one color marker and circleany concepts or linking words that they still agree with. Then they should take a differentcolor marker and cross out (with one line only or a simple X, so they can still read it) anyconcepts or linking words that they want to change. For example, perhaps they can thinkof better linking words to connect “stuff” and “substance,” or they did not connect twoconcepts that they now think should be connected.

Tell students that making a change does not mean that they were “wrong,” it justmeans their thinking has changed. Being able to look at how your thinking haschanged is an important skill that can help you develop a deeper, betterunderstanding of a scientific concept. Also, expert scientists use this skill touncover new concepts, ideas, and ways of looking at the world.

3. Discussion of Concepts to Add to Concept Maps

Ask students to share some of the changes they would want to make to their maps.Students will most likely want to add more concepts to their maps. As a quick review, askstudents to list the concepts from their initial maps, and list these on the board. The listwill most likely include the following concepts but may include others: substance,mixture, stuff, material, atoms, molecules, property, solubility, melting point, density,hardness, color, chemical reactions, phase changes, and mixing.Ask students what concepts they have learned since they last made their concept maps.Ask them to name new concepts that they want to include in their new concept maps.



You may want to encourage students to look back at their activity sheets and readers toremind them of the concepts they have learned.Add the new concepts to the list on the board. Prompt students for examples and relatedconcepts. This list may get quite long and may include some of the following words:

conservation of massconservation of atoms

systemclosed system, open system

Students may suggest concepts like “model” or “scientific explanation.” Although theseare important ideas, have students focus exclusively on chemistry concepts.Point out that the list of concepts to include in the concept map is much longer than it wasafter Learning Set 1 or Learning Set 2. So that the new maps are not confusing, tellstudents that they will need to organize their concepts. Tell them that the most generalconcepts should be placed at the top of the map. More specific concepts and examples ofthem are placed below. Similar concepts should be at the same level.4. Students Construct Final Concept Maps

Have students construct their final concept maps, reflecting everything they have learnedin the unit. As students work on their concept maps, walk around the room and help themby asking leading questions. Questions could include: Why did you put certain conceptsabove or below others? Why did you group certain concepts together? What are otherexamples of concepts that you can include on your map? Why did you pick thisparticular link? Has your understanding of the concepts changed since the first map?How has your understanding changed?If students are using sticky notes, have them tape the sticky notes in place when they arefinished adding concepts, linking words, and rearranging their maps. This will preventthe notes from coming unstuck. Or you may want students to rewrite their concept mapson a second piece of paper without using sticky notes.



CheckpointStudents’ final maps will provide a great deal of information about what they havelearned during the unit. You may want to focus your attention on any one of thefollowing:• Look at the concepts and links the students circled on their original maps. How did

they change these in their new maps?• Look at the arrangement of concepts in the new maps. Is there a hierarchy? Are

concepts grouped in ways that make sense?• Look at the linking words on the new maps. Are they accurate descriptions of the

relationships between concepts? Are there obvious connections missing from themap?

5. Students Present Final Concept Maps

Have students present their final concept maps to the class. Ask them to describe thebiggest change they have made to their maps since the beginning of the unit.


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Concluding the Lesson

End this lesson with an open-ended discussion about students’ soap in comparison tocommercial brand soap. If given a choice, would students rather use the kind of soap thatthey made, or a commercially made soap? Encourage students to share their reasons.

Some students may say that commercial soap is more ideal because of thefragrances and moisturizers that are added. Others may comment that they wouldrather use their soap because it does not have the fragrances and moisturizers so itis pure soap. They may think that pure soap will do a better job cleaning.

Ask students whether people still make their own soap today. Why would people maketheir own soaps when they can purchase soap from stores?

People still make their own soaps today. For many, soap making is a hobby.People enjoy trying different ingredients, such as different oils and moisturizers.Other people make and sell their own soaps.

These questions are intended to set the stage for Lesson 16, which involves students incomparing their soap with commercial brand soap and/or making new soap with differentoils or other reactants.

Learning Set 3: Do new substances always come from old substances Lesson 15

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Fat and Soap Molecules

Fat molecule Soap molecule

C15H31COO—CH2

|C15H31COO—CH

|C15H31COO—CH2

C15H31COONa

Oxygen (red)

Carbon(gray)

Hydrogen(white)

Carbon

Oxygen

Sodium(blue)

Hydrogen

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Compare the structures of fat and soap

Soap molecule

Fat molecule

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How did we make soap from fat?

+

+

Fat + SodiumHydroxide

Soap + glycerol

C15H31COO—CH2

|C15H31COO—CH |C15H31COO—CH2

+ 3NaOH 3 C15H31COONa +

HO—CH2

HO—CH

HO—CH2

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Compare or Improve? Lesson 16

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LESSON 16: HOW DOES MY SOAP COMPARE ORHOW CAN I IMPROVE MY SOAP?

16A: How does my soap compare with commercial brand soap?OR

16B: How can I improve my soap?

OVERVIEW

In this culminating lesson, we offer two activities that engage students in closelyexamining their soap. We suggest that you select one of these activities as a final lessonfor the unit. In Activity 16A, students compare their own soap with commercially madesoap to determine ways in which their soap is similar and different from the commercialbrand soap. This activity involves students in designing and conducting their ownexperiments to test the quality of their soap versus commercial brand bar soap. InActivity 16B, students revisit Lesson 12 and try to improve upon their prior soap-makingefforts. Students alter their original soap-making procedure to make a soap that isqualitatively different from their first soap. Lesson 16 concludes with a class discussionabout what students learned from their unit-long inquiry into how new substances can bemade from old substances.

SAFETY GUIDELINES

Activity 16A• Students must wear gloves when handling their soap.• Students must wear goggles when testing soaps.• Students’ soap should not be given to students to take home.Activity 16B• Sodium hydroxide solution (NaOH) is caustic. Sodium hydroxide can cause severe

burns. The sodium hydroxide solution should not come in contact with skin or eyes.If the sodium hydroxide solution touches the skin, the area should be immediatelyrinsed with water. If the sodium hydroxide solution contacts the eyes, they should beflushed with water. In both instances, a school nurse or medical personnel should benotified.

• Students must wear goggles at all times.• Students must wear gloves at all times.• Students should be reminded that hot plates are like hot burners on a stove, and that

touching the surface of the plates could result in a serious burn. All plugs and cordsshould be kept away from the hotplates.

• Disposal of extra sodium hydroxide solution: Dilute the solution further by pouringthe solution into a beaker of ice water. After the sodium hydroxide is diluted, thesolution can be poured down the drain.


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Learning Performance Assessment CriteriaStudents design an investigation todetermine whether a process is a chemicalreaction. They make a prediction, identifyvariables, control variables, and communicatescientific procedures. (LP 13)

Students’ experimental design includes: aprocedure that involves testing their soap andcommercial soap, and a clear record (such as a datatable) for their observations and data (Activity16A); OR a procedure for making an improvedsoap (Activity 16B).

Students conduct a scientific investigationto gather data about properties of substancesbefore and after a process (chemicalreaction, phase change, mixing). (LP 14)

Students’ investigation produces: data aboutwhether their soap performs better than anothersoap on comparison tests (Activity 16A); OR dataabout whether they were able to make a better soap(Activity 16B).

PREPARATION

Time

1 class period (Activity 16A)2 class periods (Activity 16B)Materials

Activity 16A: How does my soap compare with commercial brand soap?

For Each Student:• Activity Sheet 16A: How does my soap compare with commercial brand soap?

For Each Group:Measuring Lather• A broken-off piece of students’ soap• A broken-off piece of commercial brand soap• 100-mL graduated cylinder• Plastic wrap or aluminum foil• Rubber bandTesting how well the soaps clean• A broken-off piece of students’ soap• A broken-off piece of commercial brand soap• 3 pieces of cloth• Items to “dirty” the cloths – e.g., charcoal, dirt, markers

Activity 16B: How can I improve my soap?

For Each Student:• Activity Sheet 16B: How can I improve my soap?

For Each Group:• Large beaker (ex. 250 mL) or large plastic cup


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Note: Students leave their soap in this cup until the next day. Consequently, thiscontainer cannot be reused in subsequent classes. For example, if you have 5periods of science and 6 groups in each period, you will need 30 containers.Note: A large plastic disposable cup can be used instead of a large beaker. Thecup needs to be a relatively thick, high quality plastic cup because the soap isbasic when it is poured into the cup. The basic solution can cause a thin cup todeteriorate. Do not use paper or Styrofoam cups because these may leak. Also, itis preferable for the cup to be clear. If the cup is clear, students can see the soapform at the top of the salt solution.

• Small beaker (ex. 100 mL)Note: This beaker does not have to be exactly 100 mL. There just needs to be onesmall beaker and one large container.

• 2 stirring rods or 2 plastic spoons• An eye dropper, or plastic pipette• Hotplate• Balance• 1 graduated cylinder• Masking tape (or some way students can label their large beaker with their names)• 4 paper cups or other containers for the salt, water, fat, and rubbing alcohol• 2 coffee filters or paper cups to mass the salt and fat• 50 g table salt (sodium chloride)• 175 mL water• 11 g fat• A selection of oils (e.g., coconut oil, palm oil, olive oil)• A selection of extracts (e.g., vanilla, almond, peppermint)

Note: These are all oils and extracts that can be purchased from local grocerystores.

• 20 mL rubbing alcohol• 20 mL 6 M sodium hydroxide solution - Note: You can order sodium hydroxide

solution through Flinn Scientific Inc. (catalog number is S0242; cost for 500 mLis $8.70)

• Paper towel or paper plate (This is for the soap to dry and harden)Set-upActivity 16A: How does my soap compare with commercial brand soap?

• For lesson 16A, gather all of the materials your students will use for their latherand soap cleaning investigations in a central location for easy student access. Youwill want to show students these materials when they are designing theirexperiments.

Activity 16B: How can I improve my soap?• You may want to have cups containing table salt, water, fat, and rubbing alcohol

for each group. You could place these in a basket or on a tray and pass them outto each group. Be sure to tell students that you have not provided them with exactquantities.


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• Another alternative is to set up a table with all of the substances and empty papercups. The students can get their own materials from the table.

• Do not provide your students with sodium hydroxide in cups or leave the sodiumhydroxide sitting out on a table. We suggest that the sodium hydroxide solutionbe kept away from students and in your possession until students need the sodiumhydroxide. For step 4 of the experiment, we recommend that you pour the sodiumhydroxide for students.

• Be sure to wear gloves and goggles when handling the sodium hydroxidesolution.


Introducing the Lesson

Begin with a discussion of Student Reader 15.1: Making Soap in Colonial Times versusToday. Discuss with students their responses to the question found at the end of thissection. Possible student responses are provided in the teacher version of Student Reader15.1.Describe the investigation for students. They will either participate in Activity 16A, aninvestigation in which students compare their soap with commercial brand soap; orparticipate in Activity 16B, a soap-making experiment in which students try to improveupon their soap making efforts and produce a “better” soap.Activity 16A: How does my soap compare with commercial brand soap?

1. Student Investigation–Comparing Students’ Soap and Commercial Brand Soap

Provide students with Activity Sheet 16A: How does my soap compare with commercialbrand soap?Tell students that their task is to design and conduct an investigation to compare theirsoap with commercial brand soap. They will need to consider the kind of tests that theymight conduct to compare the soaps. Ask students what qualities they might look for insoap that performs well. Have students consider their own experiences in using soap. Askstudents: What criteria would you want to use to compare the soaps?

Students may talk about the ability of a soap to clean an object or remove dirtfrom an object. Students may talk about bubbles or lathering ability as beinganother comparison for the soaps. Students may also talk about fragrance.

Ask students to predict whether their soap or the commercial soap would perform betteron comparison tests. Ask students what type of test they might perform (lather, cleaning,etc.), and whether their soap or the commercial soap would perform better.

Students may provide a variety of responses. Some possible responses include:In a lather test, the store soap will work better because it is going to producemore bubbles. This is because companies design their soap to have lots ofbubbles.


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In a cleaning test, our soap will work better because it is pure soap. Since it doesnot have other fragrances or lotions in it, our soap will clean better.

Ask students how they could test the soaps to make their comparisons. Remind studentsthat they cannot put the soap on their skin.

Teaching Strategy• We anticipate that students will want to compare the soaps on lathering ability and

cleaning power. In the preparation section of this lesson, we suggest materials forconducting tests that compare on those criteria. However, your students may suggestother comparisons. You may want to allow other comparisons and tests that studentscome up with as long as these tests are safe.

• One strategy for helping students to generate ideas for their comparison tests is tohave them conduct a web search.

Show students the materials they will be able to use to design and conduct theircomparison tests. Have students work in groups to design and carry out their test. Eachgroup should draw and write their procedure on Activity Sheet 16A.Once groups have finished recording their procedures, have each group report to the classwhat their procedure will be. Provide feedback and encourage other groups to makesuggestions. Then, have student groups revise their initial procedure. You may want tocheck each group’s procedure before allowing them to start the investigation. Sampleprocedures for measuring lather and for testing how well two soaps clean are provided inthe Teacher Version of Activity Sheet 16A.Prior to allowing students to begin their soap testing, be sure that all students are wearinggloves and goggles. Students will need to wear gloves because their soap might not havecompletely reacted and, as a result, may still contain trace amounts sodium hydroxide.

Teacher Background Knowledge• Students’ soap may contain trace amounts of sodium hydroxide, which may irritate

skin. For this reason, do not allow students to test their soap by applying it to theirskin.

2. Class Discussion of Results and Activity Sheet 16A

Discuss students’ responses to the conclusion questions. Possible student responses and ateaching strategy are provided in the teacher version of Activity Sheet 16A.16B: How can I improve my soap?

1. Student Investigation–Making Soap

Begin by asking students: If you were going to make soap again, what could you do tomake a “better” soap? Think about soap that you use to wash your hands. What wouldyou want to be in that soap? Why would that make it “better”?

To improve their soaps, students may suggest that they add a fragrance. Theymay also suggest that they add a different ingredient to make the soap a better dirt


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cleaner. Students may reason that their soaps were made of only soap and thatstore soap has other ingredients added to make them more fragrant and better atcleaning.

Teaching Strategy• One way to support students in thinking about how to improve their soaps is to have

them explore different soap-making websites. A web search may lead students toconsider other reactants that may used in their soap-making. If students do find newprocedures on the web, you should check them to make sure they are feasible in theclassroom and not dangerous. We suggest that you limit the reactants students canuse for this experiment.

Tell students that in this activity, they will have an opportunity to try making a “better”soap. They will alter the same soap-making procedures that they followed in lesson 12.Have students look back at the procedure from Lesson 12. Ask students for differentways that they might change the procedure. Remind students that they only want tochange one variable in the procedure so they can compare whether altering that variablechanged the quality of their soap.

Students may suggest some of the following possibilities:1. Substitute an oil (coconut, soybean, olive oil, etc) for the lard2. Use a combination of an oil and lard3. Add an extract (vanilla, peppermint, etc) to the existing procedure4. Increase or decrease the heating time5. Increase or decrease the amount of rubbing alcoholYou may want to show them the different extracts and oils that you haveavailable.

Tell students that there are two things that you do not want them to change because ofsafety issues. You do not want them to increase the amount of sodium hydroxide. Sincesodium hydroxide sodium is a potentially harmful chemical, you do not want students toincrease the amount of sodium hydroxide in their procedure. You also do not want themto increase the temperature of the hotplate because it could cause the solution to bubbleor splash out of the beaker.Provide each student with Activity Sheet 16B: How can I improve my soap? Have eachgroup choose one variable to change. Then have them revise the student procedure fromLesson 12 to write their new procedure. You may want to check each group’s procedurebefore they begin to make sure the procedure is feasible and safe.Provide each group with the appropriate materials or tell them where to obtain thematerials. Then, review safety guidelines with students.Remind students to be cautious with sodium hydroxide solution. Emphasize that sodiumhydroxide solution is a caustic chemical. They must wear gloves and goggles while


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making soap. Emphasize that if sodium hydroxide spills, or splashes on their body orclothes, they need to notify you right away.Review the soap-making procedure with students. Emphasize that students shouldmeasure the substances carefully for the chemical reaction to occur properly. Additionalteaching notes are provided in the teacher version of Activity Sheet 16B.Have students work together to complete Activity 16B.

Teaching Strategy• The chemical reaction to make soap takes an entire class period. Your students will

need to work quickly and efficiently to complete the reaction in one period. If youare concerned that your students will take awhile to select oils and collect and massthe materials, you may want to split the reaction over 2 days. You could have yourstudents decide on what oils and extracts they will use and measure the water,rubbing alcohol, salt, fats and oils on Day 1. Then on Day 2 you could have studentscomplete the soap making activity.

2. Wrap-up: Transition to the Next Day

Tell students that their soap needs to continue reacting over night. Tomorrow they willbe able to remove a bar of soft soap from the salt water.3. Complete Soap Making

Students must wear gloves and goggles when they are removing their soap from thesaltwater. Have students complete Step 8 on Activity Sheet 16B. Step 8 providesdirections for removing the soap from the saltwater.

Teaching Alternative• The soap making reaction will take about a week to reach completion. At that time,

you may want to do a comparison between students’ new soap and their old soap, orbetween their new soap and commercially made soap. Activity 16A providesguidelines for comparison testing of soaps. Lesson 15 may be referred to formeasuring and comparing properties.

Concluding the Lesson

End this lesson with an open-ended discussion about what students learned frominvestigating the driving question: How can I make new substances (stuff) from oldsubstances (stuff)? Discuss some of the specific concepts (e.g. substance, property,chemical reaction, atoms, molecules, conservation of mass) and things learned from theinvestigations, as well as ideas about how this unit connects with their daily lives. Togenerate discussion on this later topic, you may want to ask students why learning aboutmaking new stuff from old stuff might be of interest or use to them.

Responses will vary and the discussion could go in any number of directions. Youmay want to focus discussion on what students learned and enjoyed about the


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unit, or direct student thinking toward how what they learned may be usefulbeyond this classroom and this school year.Discussion could also address why scientists might be interested in the drivingquestion, or how society benefits from chemical reactions in general.You may want to discuss some of the practical benefits of chemical reactions, likehow chemical reactions are used in the making of new stuff like soaps, plastics,and bread; in photography to make pictures, in burning and rusting; and evenwhen a banana ripens.

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