NAME____________________________________ PER____________ DATE DUE____________

ACTIVE LEARNING IN CHEMISTRY EDUCATION

“ALICE”

CHAPTER 3

PHYSICALSTATES

OFMATTER

3-1 © 1997, A.J. Girondi

NOTICE OF RIGHTS

All rights reserved. No part of this document may be reproduced or transmitted in any form by any means,electronic, mechanical, photocopying, or otherwise, without the prior written permission of the author.

Copies of this document may be made free of charge for use in public or nonprofit private educationalinstitutions provided that permission is obtained from the author . Please indicate the name and addressof the institution where use is anticipated.

© 1997 A.J. Girondi, Ph.D.505 Latshmere DriveHarrisburg, PA 17109

alicechem@geocities.com

Website: www.geocities.com/Athens/Oracle/2041

3-2 © 1997, A.J. Girondi

SECTION 3.1 Physical Properties of Matter

You have already seen that chemistry is a science that deals with the composition of substancesand the changes in composition that they may undergo. By simply observing a chemical substance, youcannot always determine the composition of that substance. You can usually distinguish one chemicalsubstance from another by experimentation. Sometimes the experiments you perform will be qualitativeactivities, and at other times they will be mainly quantitative.

Chemical substances are usually distinguished by their appearance, taste, odor, feel, and othersimilar properties. Such characteristics are called physical properties. We can recognize specificsubstances by their physical properties. Just as you can recognize your friends by their physicalappearance, you can also recognize chemical compounds by their physical appearance and properties.To obtain a better understanding of physical properties, we will attempt to distinguish between twodifferent metals on the basis of their physical properties.

ACTIVITY 3.2 Identifying Metals By Physical Properties

Get a sample of metal A and a sample of metal B from the materials shelf. Metals A and B look quitesimilar. It would be difficult for the untrained eye to determine the identities of these metals based on theirappearance alone. But, there are ways of identifying these two metals based on their physical propertieslisted in Table 3.1.

If you were to refer to a reference book, such as the Handbook of Chemistry and Physics, youwould probably have a difficult time trying to determine the identity of metal A and metal B. There aresimply too many metals with the same or similar properties because the physical properties that you havedescribed are very general.

Would you categorize the first four observations called for in Table 3.1 as quantitative or qualitative?

{1}_____________________ Why?{2}_______________________________________

There are several physical properties of substances that are more quantitative in nature. Manysubstances are more easily identified by quantitative means. For example, every pure substance (elementor compound) has a specific temperature and pressure at which it boils. If we were to melt metals A and B,we could identify them by comparing their measured melting points with the melting points of metalsfound in a reference book.

Another physical property that is a good quantitative measure is the density of a substance.Density is the mass of a given volume of a substance. You would probably agree that a block of cement isheavier than a block of styrofoam of the same volume. As a result, we would say that the cement is moredense than styrofoam. We will try to identify metals A and B by calculating the density of each.

density =

mass (grams)

volume (mL)

Density can be measured in a very simple way. Themathematical formula for density is given at right. Themeasurements you will need to determine density are themasses of your metals and the volumes occupied by themetals. To determine the densities of metals A and B use thefollowing procedure. .

Procedure:

1. At this time, make the first four observations of metals A and B listed in Table 3.1. Record yourobservations in the table. Following the remaining steps to determine the densities of metals A and B.

3-3 © 1997, A.J. Girondi

2. Obtain a 50 mL graduated cylinder. Add 40.0 mL of water to the graduate. Measure the mass of thegraduate containing 40.0 mL of water. Record this mass on a piece of paper.

3. Add as many pieces of metal A as needed to the water in the graduate to make the total volumebetween 47.0 and 50.0 mL. All of the pieces of metal must be totally submerged. Calculate the volume ofthe metal in the graduate by subtracting the original volume of water (40.0 mL) from the final volumereading. Record this result as the volume of metal A in Table 3.1.

4. Weigh the graduate and content (water + metal). From this final mass subtract the original mass of thegraduate + water. Record this difference as the mass of metal A in Table 3.1.

5. Dry the pieces of metal A with a towel and return them to the materials shelf.

6. Repeat steps 1 through 4 using metal B.

Table 3.1Observed Properties of Metals A and B

Physical Property Metal A Metal B

1. Physical State ________________ ________________(solid, liquid, gas)

2. Color ________________ ________________

3. Luster ________________ ________________(shiny, dull, etc.)

4. Texture ________________ ________________(rough, smooth)

Table 3.2Densities of Some

Common Metals

Metal Density (g/mL)

lead 11.34zinc 7.14mercury 13.59tin 5.75nickel 8.90platinum 21.45aluminum 2.70

Table 3.2 lists various common metals and their densitiesincluding metals A and B. Calculate the density of each metal in thespace provided below. Be sure to include units with your answers.Enter the results into Table 3.1. Compare your calculated densitiesof metals A and B to the densities of the metals in Table 3.2. Find themetals in Table 3.2 whose densities most closely match those formetals A and B. Enter the identities of metals A and B in Table 3.3.

Using the Handbook of Chemistry and Physics or whateverother reference resource is available to you, look up metals A and Band list any other properties that are given. A copy of the handbookmay be available in your lab area and/or in the school library. Enterthe additional properties of these metals into Table 3.3.

3-4 © 1997, A.J. Girondi

Metal A: Show density calculations in space below.

Metal B: Show density calculations in space below.

Table 3.3Identities and Densities of Metals A and B

Metal A. Mass: _____________ g; Volume: _____________ mL

Density: ____________ g/mL; Identity: _____________________________

Metal B. Mass: _____________ g; Volume: _____________ mL

Density: ____________ g/mL; Identity: _____________________________

ACTIVITY 3.3 Comparing the Densities of Liquids

Liquids also have densities, but they are determined in a slightly different way. Use the procedurebelow to determine the density of liquids.

1. Measure the mass of a clean, dry 10 mL graduated cylinder to the nearest 0.01 g. Record the mass inTable 3.4.

2. Obtain a bottle of alcohol, oil, or vinegar from the materials shelf. Carefully measure exactly 10.0 mL ofone of the liquids into the graduate. It is extremely important for you to read the meniscus correctly in thisactivity. Use a dropper to adjust the volume to exactly 10.0 mL, if needed.

3. Measure the mass of the liquid to the nearest 0.01 g, and record this mass in Table 3.4. Discard theliquid into the sink after use.

4. Repeat steps 1 through 3 for the other liquids. When finished with the oil, wash the graduate using asmall test tube brush and some detergent. Rinse well with water.

5. Calculate the mass of each liquid using the data collected. Calculate the density of each liquid bydividing its mass by its volume (10.0 mL). Be sure that each measurement or calculation in your table hasa unit with it.

6. Wash and return all glassware.

3-5 © 1997, A.J. Girondi

Table 3.4C a l c u l a t e d D e n s i t i e s o f S e l e c t e d L i q u i d s

Alcohol Oil Vinegar

Mass of graduate + liquid ________ ________ ________

Mass of graduate ________ ________ ________

Mass of 10 mL of liquid ________ ________ ________

Density of liquid (g/mL) ________ ________ ________

Volume of liquid used _10 mL__ _10 mL__ _10 mL__

Use the data in Table 3.4 to answer the following questions.

1. Which liquid appears to be most dense? _____________________________________________

2. Which liquid appears to be least dense? _____________________________________________

3. If oil is added to water, 2 layers form. One layer is water, while the other is oil. Which substance wouldyou expect to find on the bottom and which on top? (The density of water is about 1.00 g/mL) Explain:

_____________________________________________________________________________

_____________________________________________________________________________

Put about 2 mL of oil into a test tube. Carefully add about 2 mL of water. Observe the behavior of the

water as you add it. Which liquid is on top?_________________ Was your prediction correct?_______

(Use detergent and water again to clean the oily glass.) Discard the liquids into the sink.

4. Specific gravity is the ratio of the density of a liquid compared tothe density of water (1.00 g/mL). Since it is a ratio, specific gravityhas no units. For example, suppose the density of a liquid is 1.80g/mL. The specific gravity of that liquid is calculated as shown atright.

S.G. = 1.80 g/mL1.00 g/mL

= 1.8

Notice that the only difference between the density of a liquid and its specific gravity is thatdensity has units and specific gravity does not. What is the specific gravity of the oil that youused?__________

SECTION 3.4 Phase Change Equations

Phase changes are examples of changes in physical properties. Phase changes can bedescribed in the form of equations. Most solids when heat sufficiently undergo a change in state from asolid to a liquid phase. Most of us are familiar with the process called melting. This phase change can bewritten as an equation:

water(s) ----------> water(l)∆

The ∆ symbol means “heat added” while the subscripts (s) and (l) refer to solid and liquid, respectively.

3-6 © 1997, A.J. Girondi

The reverse process can be written as: water(l) ----> water(s) and is called freezing. Using these twoexamples, write a similar equation to show the phase change involved in boiling. Use the subscript (g) torepresent a gas or vapor. Use any appropriate symbols as well.

{3}_______________________________________________

Condensation is the opposite of boiling. Write an equation to show the phase change involved incondensation.

{4}_______________________________________________

ACTIVITY 3.5 Energy Changes Which Accompany Phase Changes

In this activity you will be observing a phase change firsthand. You are going to obtain data anddraw what is known as a cooling curve. Remember to always wear safety glasses and an apron.

1. Fill a 250 mL beaker about 1/3 full of tap water and bring the water to a boil. Obtain a stopwatch, ifpossible, or have available a wrist watch or classroom clock with a second hand.

2. Obtain a test tube which already contains a small amount (about 2 cm) of paradichlorobenzene (PDB)from the materials shelf. Keep the tube of PDB away from any flame! Your lab should have goodventilation during this activity.

3. Turn off the heat under the beaker of water and place the tube of PDB into the hot water. Allow it toremain there until the solid is completely melted. At that point, place a thermometer directly into themelted PDB. Do not remove the tube of PDB from the hot water until the temperature of the melted PDBis between 65oC and 80oC. (The higher the better within that range if you have time.)

Do not try to use a flame around or under the melted PDB. It is very flammable!

4. When the appropriate temperature has been reached, remove the tube of PDB from the hot water andplace it into a test tube rack on your lab table. Start timing immediately, and record the temperature of thecooling PDB every 30 seconds.

5. Record the temperature as precisely as possible to the nearest 0.5oC, and use the thermometer tocarefully stir the PDB 10 seconds before each reading is taken. You will need a partner to help keep time,read the thermometer, and record the temperatures. Enter all data into Table 3.5.

6. After you have obtained 3 or 4 consecutive readings which are within 1 degree of each other, you canstop if your class is close to ending. However, if you have time you should continue taking readings tocomplete as much of the data table as possible. If the PDB gets too solid to stir, discontinue that. Stopthe readings if the temperature reaches 40oC.

7. When you have collected all needed data, reheat the water in your beaker and put the tube of PDB init. Remelt the PDB so that you can remove the thermometer. Remove the tube from the hot water andallow it to cool. DO NOT remove the PDB from the test tube. Return all equipment to the materials shelf.

3-7 © 1997, A.J. Girondi

Table 3.5Cooling Temperatures

Time (sec) Temp(oC)

____0____ _________

_________ _________

_________ _________

_________ _________

_________ _________

_________ _________

_________ _________

_________ _________

_________ _________

_________ _________

_________ _________

_________ _________

_________ _________

_________ _________

_________ _________

Time (sec) Temp(oC)

_________ _________

_________ _________

_________ _________

_________ _________

_________ _________

_________ _________

_________ _________

_________ _________

_________ _________

_________ _________

_________ _________

_________ _________

_________ _________

_________ _________

_________ _________

Prepare a graph of your data on the grid below. Record temperatures on the Y axis and time on

the X axis. What was the dependent variable in this activity?{5}____________________ What was the

independent variable?{6}______________________. Compare your graph to the graph of a normal

cooling curve for a pure substance (element or compound) shown in Figure 3.1. Notice how the graph

has been divided into five sections. The data you collected reflects only a part of the overall cooling curve

for PDB. An interesting thing to notice on the cooling curve is that there are two plateaus where

temperature remains constant. Each plateau signifies a phase change (phase changes occur at constant

temperatures). In section B-C of the graph in Figure 3.1, condensation is occurring. Section D-E

represents freezing.

3-8 © 1997, A.J. Girondi

Temp (oC)

A

B C

D E

F

Time (sec) ----->

vapor vapor + liquid liquid liquid + solid solid

Figure 3.1 Cooling Curve of a Pure Substance

If we were to pretend that the curve in Figure 3.1 is a cooling curve for PDB, what letter on the curve mostclosely approximates the area in which you collected data in this activity?{7}__________

Before you go any further, draw a circle around that area of the graph in Figure 3.1 above which bestrepresents the range in which you collected your cooling curve data. Seek help if you need it.

SECTION 3.6 Kinetic and Potential Energy Changes During Phase Changes

You may ask, “Why is the temperature constant while a phase change is occurring?” In order tounderstand the answer, you must be aware that temperature is a measure of kinetic energy - the energydue to the motion of the particles. When particles lose kinetic energy, their temperature decreases (sincetemperature is a measure of kinetic energy). However, when particles move farther apart or closertogether, they gain or lose potential energy which is energy of position. Particles move closer togetherwhen they lose potential energy, and farther apart when they gain potential energy. Potential energychanges do not involve temperature changes.

When molecules have a lot of kinetic energy, their motion keeps them from being drawn closertogether by attractive forces that exist between them. However, if the particles lose kinetic energy, therewill come a point at which the attractive forces will pull them closer together. This involves a loss ofpotential energy which occurs at a constant temperature. The result will be a phase change likecondensation or freezing.

When substances are heated, the molecules move faster and gain kinetic energy as temperatureincreases. Eventually, they may gain enough kinetic energy to overcome some of the attractive forcesbetween them and move farther apart. When they move apart they are gaining potential energy, and theydo this at a constant temperature. This results in a phase change such as melting or vaporization. Thereare plateaus on a warming curve just as there are on a cooling curve. On such a curve, particles are movingfaster when temperature is rising. They are moving farther apart when the temperature is constant duringthe heating process. Pure liquids boil at constant temperatures (at constant pressure) because the

3-9 © 1997, A.J. Girondi

particles are moving farther apart and potential energy is being stored as the liquid becomes a vapor.

Refer back to Figure 3.1 as you answer the following questions. In which sections of the cooling

curve is the temperature changing?{8}____________________________ According to the curve, does

the temperature change or remain constant during a phase change?{9}__________________________

In which section(s) of the curve is the substance changing temperature and getting

cooler?{10}__________________ In which section(s) of the curve are the particles moving

slower?{11}_____________________ In which section(s) of the curve are the particles moving at a

constant rate?{12}______________________ In which section(s) are particles moving closer

together?{13}_______________________

The freezing point of a pure substance (element or compound) is the temperature at which it

changes from the liquid to the solid phase. On the graph, the freezing point (FP) of that substance is the

temperature at which the plateau occurs. Look at the graph you plotted in Activity 3.5. What is your

estimate of the freezing point of the PDB? {14}_______________

Steam at 100oC contains more heat and can, therefore, cause a more serious burn than boiling

water at the same temperature. Explain this in terms of the kinetic and potential energy content of the

steam and of the boiling water. {15}_____________________________________________________

_____________________________________________________________________________

_____________________________________________________________________________

Figure 3.2 is a graph showing the warming curve of water. It is the opposite of a cooling curve.Divide the graph into sections. Label with a pencil which phase or phases (solid, liquid, vapor) are presentin each section of the graph, and label those areas of the curve where phase changes (melting andvaporization) are occurring.

According to Figure 3.2, what is the melting point of ice?{16}__________ In the same way, what is

your estimate of water’s boiling point based on Figure 3.2?{17}_____________ How do the melting and

freezing points compare? {18}_________________________________________________________

One rather interesting point about phase changes is that even though phase changes occur, youalways have the same chemical substance you began with. The solid, liquid, and gaseous phases of wateror PDB are easily converted back and forth by simply adding or removing heat energy. (See Figure 3.3)

3-10 © 1997, A.J. Girondi

-25

0

25

50

75

100

125

Time (sec) ------->

oC

Figure 3.2 Warming Curve of Water

LIQUID WATER

LIQUID WATER

WATER VAPOR

ICE

add energy

remove energyremove energy

add energy

Figure 3.3 Energy and Phase Changes

Every pure substance melts and boils at aspecific temperature. Table 3.5 contains a list ofsome selected pure substances. As can be seen,each substance has its own characteristic meltingand boiling point. As a result, some substancesexist as gases at room temperature while others aresolids or liquids. Use the melting and boiling pointinformation provided to determine which of thesubstances listed are solids, which are liquids, andwhich are gases at room temperature (25oC) andnormal pressure.

Table 3.5Melting and Boiling Points

Substance M.P. (oC) B.P. (oC)

Neon -248.7 -245.8Water 0 100.0Acetone -95.0 56.6Sodium 97.5 899.0Ethyl Alcohol -117.3 78.5Copper 1083.0 2582.0

3-11 © 1997, A.J. Girondi

The solids are: {19}________________________________________________________________

The liquids are: {20}________________________________________________________________

The gases are : {21}________________________________________________________________

ACTIVITY 3.7 Physical and Chemical Changes

Now that we have learned the fine points about phase changes, we are going to look at two otherkinds of changes - physical changes and chemical changes. A physical change refers to a change in theappearance of a substance. Like a phase change, when a physical change occurs, you also end up withthe same substance you began with - it just looks different. If you tear a sheet of paper into small pieces,you have brought about a physical change. You still have paper, which is chemically unchanged. Is

tearing a piece of paper also a phase change?{22}_________Why or why not?{ 23}__________________

______________________________________________________________________________

When ice is changed to liquid water and then to steam, is water undergoing a physical change?________

Why or why not? {24}_______________________________________________________________

A somewhat different type of change is called a chemical change. After a chemical changeoccurs, you no longer have the same chemical substance. A new and different chemical substance isformed. If you were to set fire to paper, would you still have paper? Obviously not! Chemical changesresult in the formation of substances with new chemical compositions. There are some key signs that youcan look for to determine whether or not a chemical change has occurred. For example, if a gas is given offor if a precipitate (solid) is formed during a change, you can be fairly certain that a chemical change hasoccurred. Often (but not always) a color change can indicate a chemical change. A temperature changemay or may not indicate that a chemical change has occurred. Color changes and temperature changessometimes occur during physical changes, so they are not clear evidence of chemical changes.

In this activity you will witness some physical and chemical changes. You will need 4 test tubes, atest tube rack, and the chemicals labeled “ALICE 3.7” on the materials shelf. Follow the directions below,and record all of your observations in Table 3.6. In Table 3.6, you must place check marks in the columnthat best describes what happens during each change. You may check more than one column for eachchange as needed. Pay close attention to whether or not a solid substance (precipitate) forms. It willeither lie on the bottom of the tube, or will remain suspended and make the liquid contents cloudy. Clearsolutions do not contain precipitates. (Do not confuse the terms clear and colorless. Clear liquids may ormay not be colorless.)

Chemical Change 1: Rinse a small test tube with distilled water. Add about 20 drops of AgNO3 (solutionA) to the tube. Add about 10 drops of K2CrO4 solution (solution B) to the same tube. Record anyobservations in Table 3.6 by placing check marks in the proper boxes.

Chemical Change 2: Add about 10 drops of Pb(NO3)2 (solution C) to a small clean, rinsed test tube. Addabout 20 drops of NaI (solution D) to the same tube. Shake the contents gently. Record observations.

Chemical Reaction 3: Add about 2 mL of HCl (solution E) to a clean 150 mm (medium) test tube. Place asmall piece of zinc metal into the tube. Record observations.

3-12 © 1997, A.J. Girondi

Chemical Reaction 4: Add about 2 mL of H2SO4 (solution F) to a 150 mm test tube. Note the temperatureof the solution by feeling the outside of the tube with your hand. Next, add about 4 mL of NaOH solution(solution G). Caution: keep these solutions off skin and out of eyes! Feel the tube again. Record.

Table 3.6Evidence of Chemical Changes

Color Change Gas Given Off Precipitate Forms Temp. Change (yes or no) (yes or no) (yes or no) (yes or no)

Change 1:

Change 2:

Change 3:

Change 4:

In chemical change 4, an energy change was the only easily noticeable evidence of a chemicalchange. However, energy changes by themselves are not enough evidence to indicate for sure that achemical change has occurred, because many physical changes are also accompanied by energychanges. To observe a physical change in which there is an energy change, place about 2 mL of water ina clean test tube. Note the general temperature of the water by pressing the tube on the inside of yourwrist. Using forceps, place 2 or 3 pellets of solid sodium hydroxide (NaOH) into the tube of water. Shakethe tube gently for 30 seconds or so, and press the bottom of the tube on the inside of your wrist. Do notlet the solution or the NaOH pellets contact you! Note the relative temperature of the contents of thetube. NaOH does not react with water, it merely dissolves. Dissolving is a physical change. What change

in temperature did you observe?{25}____________________________________________________Discard the contents of the tube into the sink and run the water. Rinse the tube with water.

Your experience from this activity should tell you that if you note a color change, a gas beingproduced, a temperature change, or the formation of a precipitate, you should at least suspect that achemical change has occurred. This is particularly true if more than one of these events has occurred,because color and temperature changes themselves are not conclusive evidence.

Identify each of the following changes as either physical (P)or chemical (C).

a. getting a flat tire {26}___________

b. a campfire burning {27}___________

c. seeing your “breath” when you exhale on a cold day {28}___________

d. digesting food {29}___________

e. rain changing to ice {30}___________

f. drying the laundry {31}___________

g. a bomb explosion {32}___________

3-13 © 1997, A.J. Girondi

SECTION 3.8 Density Problems

Earlier in this chapter, you worked with the concept of density as a physical property of matter. Dothe problems below. The units used to express density are usually g/mL or g/cm3. The g/mL unit is usedmore frequently for liquids, while g/cm3 is used more often for solids. This is really just a matter ofpreference, since 1 mL and 1 cm3 represent exactly the same volume. That is, 1 mL = 1 cm3. Usedimensional analysis wherever possible. Show all work neatly, even if only one or two steps are needed.

Problem 1. What is the density of gasoline if 350 mL has a mass of 245 grams?

Problem 2. A block of metal has the dimensions 5.0 cm X 7.0 cm X 20.0 cm, and its mass is 5.0 kg.What is the density of the metal in g/mL?

Problem 3. What is the density of hydrogen gas (in g/mL) if 100. liters have a mass of 8.93 X 10-3 kg?

Problem 4. What is the density of milk in g/mL if 1.0 quart has a mass of 1.0 kg? (1.06 qt. = 1.00 L)

ACTIVITY 3.9 Determining the Thickness of Aluminum Foil

In this activity you will use what you have learned about density to calculate the thickness ofaluminum foil. Now you can imagine how tough this would be to do using only a ruler. Aluminum foil isn’tvery thick! We will accomplish this task in an indirect way. The formulas you will use are already familiar toyou. The volume of a regular object can be found using the formula V = L x W x H, where L = length, W =width, and H = height. If the object is a piece of aluminum foil, we can alter the formula to V = L x W x T,where T = thickness. Since this activity involves finding the thickness, we can solve the formula for Twhich gives T = V/(L x W). From this formula you can see that in order to determine the thickness, youneed to know the length and width of the piece of foil, and you also need to know the volume of it.

3-14 © 1997, A.J. Girondi

How do you calculate the volume of a piece of aluminum foil? That’s where density becomesuseful. The density (d) of aluminum is 2.70 g/1 cm3. If you measure the mass of the foil and if you knowthe density of the metal, then you can use the formula at right below to find the volume:

density = g

cm3V = g X

cm3

g= cm3 V = g X

1

dSince: and since: then:

Use this formula to get the volume of the foil

Do you see how these two formulas can be used to derive this one?

T = V

L x W

Once you have the volume, you can substitute that value into theformula shown at right. Measure the length and width (in cm)of therectangular piece of foil, and substitute those values along with thevolume into the formula. When you solve the formula for T, the units willcancel and leave you with centimeters. You will have calculated thethickness using the mass and density!

Procedure: Cut or obtain 2 square pieces (about 10.0 cm x 10.0 cm) of aluminum foil - one regular andone heavy duty. Weigh each piece on the laboratory balance, and measure the length and width of eachpiece in centimeters. Enter the data into Table 3.7 below. Return the foil squares to the appropriatecontainers. Complete the calculations, showing all work neatly in the spaces provided.

Table 3.7Aluminum Foil Data

Regular or Heavy Duty Length (cm) Width (cm) Mass (g)

_________________ _________ _________ _________

_________________ _________ _________ _________

SHOW your calculations in the spaces below. Express results to two decimal places.

Step 1. Calculation of the volume of the foil.

a. regular foil

b. heavy duty foil

Step 2. Calculation of the thickness of the foil.

a. regular foil

b. heavy duty foil

3-15 © 1997, A.J. Girondi

Step 3. According to your calculations, determine how many times thicker the heavy duty foil is comparedto the regular foil. (Divide the thickness of the heavy duty foil by the thickness of the regular foil.)

Result: the heavy duty foil is _____________ times thicker.

SECTION 3.10 Accuracy, Precision, Percentage Error and Percentage Deviation

Let’s use your data from Activity 3.9 to determine your accuracy and your precision . Accuracyrefers to the closeness of your result to the actual or accepted value. Precision refers to the closeness ofyour results to each other (if you did an experiment more than once) or to that of other lab groups who didthe same experiment. It is possible to be precise without being accurate. In other words, you can get aresult that is close to what others got, while still being inaccurate. This can happen if your class is usingpoor measuring instruments or a bad technique. However, in order to be accurate you must also beprecise. In other words, if everyone in your class is getting a result which is close to the accepted value(accurate), then those results will also be close to each other (precise). Accuracy is often expressed aspercentage error, while precision is often expressed as percentage deviation .

Step 1. Calculation of Error. Obtain the accepted value for the thickness of the two kinds of aluminum foilfrom your instructor. Calculate your error and percent error as shown below. Quantities enclosed invertical bars such as: |O - A| refer to absolute value.

Error = |O - A| O = your observed value A = the accepted value

% error =

|O - A|

A X 100

Be careful when you use the formula above to calculate % error. It involves subtraction, division, andmultiplication. When using a calculator to solve such equations, you must do all your addition andsubtraction and then get a subtotal before doing the multiplication and/or division. If you follow the rulesfor significant digits, you will need to round twice. List your error results below.

% error for the regular foil: __________% % error for the heavy foil: __________%

In this experiment, you should be able to achieve less than 5% error. How did you do? ______________

______________________________________________________________________________

Step 2. Calculation of Deviation. Get thickness results from three other lab groups in your class. Enterthe data into Table 3.8. Calculate the average (mean) thickness for each foil. To calculate your deviation(precision) use the formulas below.

Deviation = |O - M| O = your observed value M = average (mean) value

% deviation =

|O - M|

M X 100

Note that the calculation of deviation also involves an absolute value. Use the same precautions whenusing a calculator as you did when calculating % error. List your deviation results below Table 3.8.

3-16 © 1997, A.J. Girondi

Table 3.8Lab Group Data

Thickness of Thickness ofGroup Regular Foil (cm) Heavy Foil (cm)

Your Group ____________ ___________

Lab Group 1 ____________ ___________

Lab Group 2 ____________ ___________

Lab Group 3 ____________ ___________

Average (Mean) ____________ ___________

% deviation for the regular foil: __________% % deviation for the heavy foil: __________%

Did you manage to achieve a small deviation (5% or less)? ___________________________________

If your group had a small error but a high deviation, that means that your small error was probablyjust due to luck, or maybe off-setting errors. If your group had a large error (over 5%) but a small deviation,that means that your high error was due to the measuring instruments or the procedure – not to humanerror. If your group had a small error and a small deviation, that means that the instruments and your labtechniques and skills were good (you were both accurate and precise)! If you got a large error and also alarge deviation, plan to pursue a career in something other than science, because the error was all your

fault! Comment on the performance of your group: ________________________________________

______________________________________________________________________________

______________________________________________________________________________

SECTION 3.11 Review Problems

Problem 5. A piece of nickel sheet metal has a width of 11.5 cm, a length of 4.66 cm, and a mass of58.18 grams. What is the thickness of this sheet of nickel metal? (Use any needed data from Table 3.2.)

3-17 © 1997, A.J. Girondi

Problem 6. A student experimentally tried to determine the thickness of the metal mentioned inproblem 5. Her observed (O) result was 0.126 cm. The results from three other students were 0.130 cm,0.114 cm, and 0.134 cm.

a. Calculate her % error. (Your answer to problem 5 can be used as the accepted (A) value.)

b. Calculate her % deviation. (You will need to calculate the mean, M, of her value and the three others.)

c. Based on her % error and % deviation, comment on these results assuming an acceptable value of 5%

for both error and deviation. _________________________________________________________

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Problem 7. The “Miracle Thaw” is a heavily advertised sheet of “space age” metal on which you canplaced frozen foods for rapid defrosting (according to the manufacturer). The Miracle Thaw is 45 cm long,25 cm wide, and 0.50 cm thick. It has a mass of about 1519 grams. What is the “space age” metal whichcomposes this “miracle?” Use data from Table 3.2. (Let the consumer beware.)

Problem 8. A cork cube weighs 500.0 grams and has a density of 0.025 g/cm3. Calculate the length ofone side of the cube in cm. Hint: the volume formula is V = s3 where s is the length of a side. You willneed to take a cube root in this problem. Use the appropriate key(s) on your calculator.

3-18 © 1997, A.J. Girondi

ACTIVITY 3.12 The Barge Problem

--> --> --> Teacher Demonstration <-- <-- < --

With the background you have gained thus far in this chapter, explain what would happen in thesituation described below. You must use the concept of density in your explanation. Once you haveformulated your explanation, you will carry out an experiment to illustrate what actually happens. Take thedensity of water as 1.00 g/mL , and keep in mind that , in order to float, a boat must displace a volume ofwater equal to its own mass.

A barge is hauling metal beams through the Panama canal. While the barge is in a closed lock inthe canal, all of the metal falls overboard into the water in the lock. Do you think the water level in the lockwill rise, fall, or remain unchanged when the metal falls in? ___________________________________

Your teacher will demonstrate an experiment to test your hypothesis. A 100 mL beaker will be

used as the barge, a 400 mL beaker about half-full of water for the lock, and pieces of lead metal for the

beams. Enough metal will be used to make a difference, but not to sink the barge! A piece of tape or a

grease pencil will be used to mark the water level before and after the metal spills. Describe the result of

the experiment:_________________________________________________________________

Do the results support your original hypothesis?____________ Whether your answer is yes or no,

explain this phenomenon: _________________________________________________________

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ACTIVITY 3.13 Sublimation

--> --> --> Teacher Demonstration <-- <-- < --

Because toxic fumes are involved, your teacher will conduct this demonstration under an exhaustfume hood in your lab area. You are going to witness a rather unusual type of phase change. It involvessolid iodine crystals which change directly to a vapor without going through the liquid state. This is knownas sublimation. You may be familiar with dry ice (which is solid CO2) which also undergoes sublimation.Care must be taken because iodine vapors are very dangerous and can cause serious stains and burns.

Look at Figure 3.4. Note that an ice cube is placed on top of a watch glass which is located on topof a beaker or evaporating dish containing some iodine crystals. As heat is applied, the iodine crystalsslowly turn into a deep violet-colored vapor which is much heavier than air. When the vapor comes intocontact with the cool underside of the watch glass it turns directly from the vapor phase back to the solidphase in a phase change known as deposition. Crystals may also form on the cool sides of the container.Do not allow iodine to come into contact with you or your clothing! On the line below, write a phasechange equation for the sublimation of solid iodine. Remember to use the symbol for added heat.

{33}_______________________________________

3-19 © 1997, A.J. Girondi

ice cube

watch glass

deposited crystals

iodine vapor

iodine crystals

Figure 3.4 The Sublimation of Iodine Crystals

SECTION 3.14 Learning Outcomes

Read through the learning outcomes below. Place a check mark in front of each outcome that youhave mastered. When complete, arrange to take the test on Chapter 3, and move on to Chapter 4.

_____1. Identify general physical properties of matter (color, phase, texture, luster, etc.)

_____2. Solve problems involving the density of a solid or a liquid.

_____3. Given proper information, determine the specific gravity of a liquid.

_____4. Label, interpret, and explain the parts of heating and cooling curves for pure substances.

_____5. Distinguish between physical changes, chemical changes, and phase changes.

_____6. Distinguish between accuracy and precision, and calculate percentage error and percentage deviation given the necessary information.

3-20 © 1997, A.J. Girondi

SECTION 3.15 Answers to Questions and Problems

Questions:

{1} qual; {2} no measurement involved; {3} water(l) ---∆--> water(g); {4} water(g) ------> water(l) ;{5} temperature; {6} time; {7} area around D and up to E; {8} A-B, C-D, E-F; {9} remains constant{10} A-B, C-D, E-F; {11} A-B, C-D, E-F; {12} B-C, D-E; {13} B-C, D-E; {14} about 53oC;{15} Steam at 100oC has the same amount of kinetic energy but has more potential energy than water at100oC since it has gone through an additional phase change.{16} 0oC; {17} 100oC; {18} the same; {19} sodium, copper; {20} water, ethyl alcohol, acetone; {21} neon;{22} no; {23} no change in phase - still a solid; {24} yes, because it is still water in each phase{25} increase; {26} physical; {27} chemical; {28} physical; {29} chemical; {30} physical; {31} physical;

{32} chemical; {33} I2(s) ----∆----> I2(g)

Problems:

1. 0.70 g/mL

2. 7.1 g/mL

3. 8.93 X 10-5 g/mL

4. 1.1 g/mL

5. 0.122 cm

6. a. 3.28 %b. 0 %c. Since both error and deviation were under 5% in this type of activity, she has done well. The fact that there was 0% deviation indicates that the 3.28% error was probably due to the equipment or procedure.

7. aluminum

8. 27 cm.

3-21 © 1997, A.J. Girondi

SECTION 3.16 Student Notes

3-22 © 1997, A.J. Girondi