www.ck12.org

CHAPTER 2 Matter and ChangeChapter Outline

2.1 PROPERTIES OF MATTER

2.2 CLASSIFICATION OF MATTER

2.3 CHANGES IN MATTER

2.4 REFERENCES

Freshly baked bread and cherry pie are two delicious parts of any meal. What happens to the ingredients that go intothe bread and the pie as they are heated in the oven? If you have ever done any baking, you certainly know that thebread dough that is put into the oven changes dramatically as it bakes. Many chemical reactions occur, and one ofthose reactions releases a gas that causes the bread to rise. In this chapter, you will learn about some of the basicways in which matter is capable of changing and how we can make sense of those changes. You will also learn howto classify the various types of matter that are part of the world around us.Flickr: jcroninone. www. f lickr.com/photos/31867606@N04/4038864078. CC BY 2.0.

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2.1 Properties of Matter

Lesson Objectives

• Describe a substance according to its physical properties.• Distinguish between extensive and intensive properties.• Describe the three states of matter.• Identify physical changes to matter.

Lesson Vocabulary

• extensive property• gas• intensive property• liquid• mass• physical change• physical property• pure substance• solid• vapor• volume

Check Your Understanding

Recalling Prior Knowledge

• What are some physical properties that can be used to describe matter?• What are the three states of matter? Describe their similarities and differences.

The science of chemistry is largely concerned with the understanding and classification of matter. This understandingnaturally begins with careful observations. What does a particular sample of matter look like? Is it a solid, liquid, orgas? Is it hard or soft? Is it shiny or dull? These types of questions are where we will begin our study of matter andits properties.

If you would like to refresh your memory about states of matter, go to “The Phantom’s Portrait Parlor: Phases ofMatter” at www.miamisci.org/af/sln/phases/watersolid.html and view solids, liquids, and gases at the molecularlevel.

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Description of Matter

Recall that matter is considered to be anything that has mass and occupies space. Matter can be best understood interms of its properties. Some properties of matter can be determined without altering the identity of the matter, andit is there that we will begin.

Physical Properties

Pictured in the Figure 2.1 are two very different objects. Each has a different chemical composition, meaning thateach is made up of a different type of matter.

FIGURE 2.1(A) This lamp from the country of SriLanka is made of silver. (B) These largesalt mounds from Bolivia were producedduring a salt mining operation.

Silver and ordinary table salt are two examples of chemical substances. A pure substance is matter that has auniform and definite composition. All samples of pure substances have identical properties.

A physical property is a characteristic of a substance that can be observed or measured without changing theidentity of the substance. For example, silver is a shiny metal that conducts electricity very well. It can also bemolded into thin sheets, a property called malleability. These are all physical properties of silver. In contrast, saltis dull and brittle, but it can also conduct electricity after being dissolved in water, which it does quite readily. Inaddition to color, hardness, malleability, solubility, and electrical conductivity, some other physical properties ofmatter include density, melting point, and boiling point.

There are many examples of physical properties that can be further investigated on the internet.

• Magnetism is a physical property. Are all metals magnetic? Find out at www.youtube.com/watch?v=iK-5-BRLn78 .

• You can view a video of the melting points of various substances at www.youtube.com/watch?v=NYb2iTNFCsI .

• Do you think metals like lead melt easily? Check out this alloy containing lead melting in boiling water: www.youtube.com/watch?v=JI9qKbQCukg .

• The physical property density can be observed by floating bubbles on carbon dioxide. Watch the video atwww.youtube.com/watch?v=h3EWYrxrsns .

Extensive and Intensive Properties

Some properties of matter depend on the size of the sample, while some do not. An extensive property is a propertythat depends on the amount of matter in a sample. The mass of an object is a measure of the amount of matter thatan object contains. A small sample of a certain type of matter will have a small mass, while a larger sample will

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have a greater mass. Another extensive property is volume. The volume of an object is a measure of the space thatis occupied by that object.

An intensive property is a property of matter that depends only on the type of matter in a sample and not onthe amount. For example, the electrical conductivity of a pure substance is a property that depends only on thetype of substance. Silver, gold, and copper are excellent conductors of electricity, while glass and plastic are poorconductors. A larger or smaller piece of glass will not change this property. Other intensive properties include color,temperature, density, and solubility.

For more information on extensive and intensive properties, visit www.chem.tamu.edu/class/majors/tutorialnotefiles/intext.htm .

States of Matter

Water is a very common substance that we frequently encounter in all three states of matter ( Figure 2.2). Whenwater is in the solid state, we call it ice, while water in the gaseous state is referred to as steam or water vapor. Thephysical state of matter is a physical property because the identity of a pure substance does not change when it ismelted, frozen, or boiled.

FIGURE 2.2Water is the same substance in any ofits three states. (A) A frozen waterfall inHungary. (B) The Nile River in Egypt. (C)A steam powered train in Wales.

Solid

A solid is a form of matter that has a definite shape and volume. The shape of a solid does not change if it istransferred from one container to another. The particles of a solid are packed tightly together in fixed positions,usually in an orderly arrangement ( Figure 2.3). Solids are almost completely incompressible, meaning that solidscannot be squeezed into a smaller volume. When a solid is heated, it expands only slightly.

Liquid

A liquid is a form of matter that has a definite volume, but an indefinite shape. As water is poured from one containerinto another, it adopts the shape of its new container. However, the volume of the sample does not change becausethe water molecules are still relatively close to one another in the liquid state. Its ability to flow means that water isconsidered a fluid. Unlike a solid, the arrangement of particles in a liquid is not rigid and orderly. Liquids are alsoincompressible, and the volume of water expands slightly when heated.

Gas

A gas is a form of matter that has neither a definite shape nor a definite volume. A gas takes up the shape andvolume of its container. This is because the particles of a gas are very, very far apart from one another compared to

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FIGURE 2.3The particles of a gas are very far apartcompared to the particles of a liquid or asolid.

the particles that make up solids and liquids. Gases, like liquids, are fluids. Gases are easily compressed because ofthe large spaces in between gas particles. The term gas is most often used to refer to a substance that is found in thegas state at room temperature, such as hydrogen and nitrogen. However, substances that are solids or liquids at roomtemperature can also evaporate into the gaseous state. A vapor is the gaseous state of a substance that is a solid orliquid at room temperature. Examples include water vapor or sodium vapor ( Figure 2.4).

FIGURE 2.4Sodium vapor lamps glow with a distinc-tive yellow color.

For a summary of the phases of matter, visit www.chem.purdue.edu/gchelp/atoms/states.html .

A fun, interactive simulation about phases of matter can be downloaded at www.concord.org/activities/states-matter.

Another interactive simulation that will help you understand solids, liquids, and gases can be found at http://phet.colorado.edu/en/simulation/states-of-matter . A variety of activity guides can be found by scrolling down.

NASA provides diagrams and explanations of states of matter at http://www.grc.nasa.gov/WWW/k-12/airplane/state.html

NASA explains properties of gases in an informative website at http://www.grc.nasa.gov/WWW/k-12/airplane/gasprop.html .

Physical Changes

As an ice cube melts and it acquires the ability to flow, its shape changes. However, its composition does not change.Melting is an example of a physical change. A physical change is a change to a sample of matter in which someproperties of the material change, but the identity of the matter does not. Physical changes can further be classified asreversible or irreversible. The melted ice cube may be refrozen, so melting is a reversible physical change. Physicalchanges that involve a change of state are all reversible. Other changes of state include vaporization (liquid to gas),

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freezing (liquid to solid), and condensation (gas to liquid). Dissolving is also a reversible physical change. Whensalt is dissolved in water, the salt is said to have entered the aqueous state. Boiling off the water will cause the saltto reform in its solid state.

An example of an irreversible physical change would be grinding a piece of wood into sawdust. Such a change isirreversible because the sawdust cannot be reconstituted into the same piece of wood that it was before. Cutting thegrass or pulverizing a rock would be other irreversible physical changes.

Some other examples of physical changes can be seen on the web:

• The melting and freezing of tert-butanol can be viewed at these two links:

– Melting: www.dlt.ncssm.edu/core/Chapter2-Phys_and_Chem_Prop/melt_tbutanol-lg.htm– Freezing: www.youtube.com/watch?v=lQnrRIDpPlU

• Iodine undergoes an interesting physical change called sublimation. Click www.youtube.com/watch?v=4E0sy-FN2M8 to watch!

• Metals expand when heated. This physical change can be seen using a ball and ring at www.youtube.com/watch?v=QNoE5IoRheQ .

Lesson Summary

• A pure substance is a sample of matter with a fixed composition and identifiable physical properties that arethe same for every sample of a particular substance.

• Extensive properties depend on the size of a sample, while intensive properties do not.• Solid, liquid, and gas are the three common states of matter.• Physical changes do not alter the identity of a substance.

Lesson Review Questions

Reviewing Concepts

1. Explain why water is considered a pure substance, while milk is not.2. List at least three physical properties of silver.3. Give an example of an extensive property and an intensive property of an iron nail. Explain.4. Compare the arrangements of particles in the three states of matter.5. The odor of gasoline is very strong and distinctive. Can gasoline in the gaseous state be referred to as a vapor?

Explain.6. Describe how the sharpening of a pencil is a different type of physical change than dissolving some sugar in

water.

Problems

Use the Table 2.1 to answer the following questions.

TABLE 2.1: Properties of Substances

Substance Color Melting Point (°C) Boiling Point (°C)Bromine red-brown -7 59

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TABLE 2.1: (continued)

Substance Color Melting Point (°C) Boiling Point (°C)Chlorine yellow-green �101 �34Gold yellow 1064 2856Iron silver 1538 2862Mercury silvery-white �117 78Nitrogen colorless �210 �196Oxygen colorless �219 �183Sodium chloride white 801 1413Water colorless 0 100

7. What is the physical state of each of the following substances at room temperature?

a. oxygenb. ironc. mercury

8. What is the name of the physical change that occurs in each of the following instances?

a. Oxygen is cooled from �150°C to �200°C.b. Iron is cooled from 1600°C to 1500°C.c. Mercury is heated from �130°C to -100°C.

9. Which colorless substance is a solid at �50°C?10. Which substance is a liquid at 1000°C?11. Which substance condenses from a gas to a liquid as the temperature is cooled from 20°C to �50°C?

Further Reading / Supplemental Links

• Take a quiz on properties of matter at www.chem.tamu.edu/class/majors/tutorialnotefiles/intext.htm .

Points to Consider

A mixture is different from a pure substance because it does not have a uniform and definite composition. Puresubstances include both elements and compounds.

• Can mixtures be further classified into more than one type?• How are compounds different than elements?

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2.2 Classification of Matter

Lesson Objectives

• Define a mixture and understand why mixtures are different than pure substances.• Classify mixtures as homogeneous or heterogeneous.• Describe several ways to separate mixtures.• Distinguish between elements and compounds.

Lesson Vocabulary

• chemical change• chemical formula• chemical symbol• compound• distillation• element• filtration• heterogeneous mixture• homogeneous mixture• mixture• phase

Check Your Understanding

Recalling Prior Knowledge

• What is a pure substance?• What are some examples of physical changes, and what happens to the identity of a substance when it

undergoes a physical change?

In the previous lesson, “Properties of Matter,” we learned about the physical properties of pure substances and aboutthe physical changes that matter can undergo. However, there are many examples of matter that cannot be classifiedas pure substances because they have a composition that is variable rather than fixed.

Mixtures

Ordinary table salt is called sodium chloride. It is considered a pure substance because it has a uniform and definitecomposition. All samples of sodium chloride are chemically identical. Water is also a pure substance. Salt easily

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dissolves in water, but salt water cannot be classified as a pure substance because its composition can vary. You maydissolve a small amount of salt or a large amount into a given sample of water. A mixture is a physical blend oftwo or more components, each of which retains its own identity and properties. Only the form of the salt is changedwhen it is dissolved into water. It retains its composition and chemical properties.

An activity that makes it easy to understand the differences between elements, compounds, and mixtures canbe found at www.bbc.co.uk/schools/ks3bitesize/science/chemical_material_behaviour/compounds_mixtures/activity.shtml .

Homogeneous Mixtures

A homogeneous mixture is a mixture in which the composition is uniform throughout the mixture. The salt waterdescribed above is homogeneous because the dissolved salt is evenly distributed throughout the entire salt watersample. It is often easy to confuse a homogeneous mixture with a pure substance because they are both uniform.The difference is that the composition of the pure substance is always the same. Air is a mixture because thedifferent gases that make up air may be present in varying amounts. A troublesome aspect of modern society is thatthe burning of fossil fuels in large amounts has increased the amount of carbon dioxide in our atmosphere comparedto the levels that were present hundreds of years ago before the industrial revolution. Carbon dioxide acts as agreenhouse gas, trapping heat in our atmosphere. The increase in carbon dioxide levels has led to a correspondingincrease in average global temperatures ( Figure 2.5).

FIGURE 2.5Carbon dioxide levels in Earth’s atmo-sphere (measured in parts per million)have increased dramatically since 1900.The average global temperature has alsoseen an increase, as seen on the left-hand axis.

Heterogeneous Mixtures

A heterogeneous mixture is a mixture in which the composition is not uniform throughout the mixture. Vegetablesoup is a heterogeneous mixture. Any given spoonful of soup will contain varying amounts of the different vegetablesand other components of the soup.

A phase is any part of a sample that has a uniform composition and properties. By definition, a pure substance ora homogeneous mixture consists of a single phase. A heterogeneous mixture consists of two or more phases. Whenoil and water are combined, they do not mix evenly ( Figure 2.6). Instead, they form two separate layers, each ofwhich is considered a phase.

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FIGURE 2.6Oil and water do not mix. Instead, theyform two distinct layers called phases.The oil phase is less dense than the waterphase, so the oil floats on top of thewater.

Separation of Mixtures

A pizza is a heterogeneous mixture. If you have a pizza slice with green peppers on it and you don’t like greenpeppers, you could easily pick them off before eating the slice. Other mixtures may not be as easy to separate. Theoil and water mixture can be separated by carefully pouring off the oil layer in a process called decanting. Thecomponents of all mixtures can be separated by some physical means.

Filtration

Sand does not dissolve in water. In order to separate sand from water, the mixture can be poured through a filter. Thetiny holes of a filter, called pores, allow the water to pass through while trapping the sand grains behind. Filtrationis a technique that separates solids from liquids in a heterogeneous mixture ( Figure 2.7).

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FIGURE 2.7This filtering apparatus uses suction to pull the water through a fine glassfilter, leaving behind a yellow-colored solid.

Distillation

Distillation is a method of separating a mixture based on differences in boiling points. A distillation apparatus ispictured in the Figure 2.8.

FIGURE 2.8A small-scale laboratory distillation ap-paratus is used to separate substancesbased on differences in their boilingpoints.

For example, the impure liquid could be a mixture of alcohol and water. As the mixture is heated, the alcohol beginsto boil before the water does because of its lower boiling point. The alcohol vapors rise up in the flask and intothe long cylindrical tube, which is called a condenser. Cold water is continually pumped into the outside of thecondenser, which causes the alcohol vapors to condense back into the liquid state. The purified alcohol then dripsinto the flask labeled “distilled liquid,” while the water is left behind in the original flask.

Distillation is commonly used to purify water by separating it from the various dissolved minerals that are found innatural water sources.

You can view the distillation of methanol to separate it from a dye at www.youtube.com/watch?v=74rOJ1GFbC0(1:16).

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MEDIAClick image to the left or use the URL below.URL: http://www.ck12.org/flx/render/embeddedobject/54704

Pure Substances

As described in the lesson, “Properties of Matter,” pure substances have a definite and fixed composition. Puresubstances can further be classified as either elements or compounds.

Elements

An element is the simplest form of matter that has a unique set of properties. Examples of well-known elementsinclude oxygen, iron, and gold ( Figure 2.9). Elements cannot be broken down into simpler substances. Additionally,one element cannot be chemically converted into a different element.

FIGURE 2.9Chemical elements are the simplest of pure substances. (A) An oxygen tank like this is used by people with aneed for breathing assistance. (B) A simple skillet can be made from cast iron. (C) Gold bars are formed andused for monetary purposes.

Compounds

A compound is a pure substance containing two or more elements that have been chemically combined in a fixedproportion. The elements carbon and hydrogen combine to form many different compounds. One of the simplestis called methane, which always has four times as many hydrogen particles as carbon particles. Methane is a puresubstance because it always has the same composition. However, it is not an element because it can be broken downinto simpler substances (the elements carbon and hydrogen). The process of breaking down a compound into itselements is sometimes called decomposition.

Recall that the components of a mixture can be separated from one another by physical means. This is not true fora compound. Table salt is a compound consisting of equal parts of the elements sodium and chlorine. Salt cannot

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be separated into its two elements by filtering, distillation, or any other physical process. Salt and other compoundscan only be decomposed into their elements by a chemical process. A chemical change is a change that producesmatter with a different composition. Many compounds can be decomposed into their elements by heating. Whensugar is heated, it decomposes to carbon and water. Water is also a compound, but it cannot be broken down intohydrogen and oxygen by heating. However, the passage of an electrical current through water will produce hydrogenand oxygen gases.

The properties of compounds are generally very different than the properties of the elements from which thecompound is formed. Sodium is an extremely reactive soft metal that cannot be exposed to air or water. Chlorine isa deadly gas. The compound sodium chloride is a mostly unreactive white solid that is essential for all living things( Figure 2.10).

FIGURE 2.10(A) Sodium is so reactive that it must be stored under oil. (B) Chlorine is a poisonous yellow-green gas. (C) Saltcrystals are a compound of sodium and chlorine.

Distinguishing Between Pure Substances and Mixtures

The flowchart pictured in the Figure 2.11 summarizes the types of matter discussed in this chapter.

FIGURE 2.11The flowchart describes the classificationof matter.

Matter can be divided into two categories: pure substances and mixtures. Pure substances can be either elements orcompounds. Mixtures can be either homogeneous or heterogeneous. Another term for a homogeneous mixture is a

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solution. Sometimes it can be difficult to tell the difference between a compound and a homogeneous mixture basedsolely on appearance. Consider gasoline. It is certainly homogeneous and seems a likely candidate for a compound,meaning that it would have a fixed composition. However, as you may know, gasoline can be purchased at a stationin a variety of grades, such as regular or premium grade. Premium grades have a higher octane rating and cost morethan regular grades. Gasoline is thus a blend of different compounds, so it is a mixture.

Chemical Symbols and Formulas

Chemists use symbols and formulas as an abbreviated way of writing chemical reactions and the elements andcompounds involved in them. A chemical symbol is a unique one- or two-letter designation of an element. Someexamples of chemical symbols are O for oxygen, Zn for zinc, and Fe for iron. The first letter of a symbol is alwayscapitalized. If the symbol contains two letters, the second letter is lower case. The majority of elements have symbolsthat are based on their English names. However, some of the elements that have been known since ancient timeshave maintained symbols that are based on their Latin names ( Table 2.2).

TABLE 2.2: Symbols and Latin Names for Various Elements

Chemical Symbol Name Latin NameNa Sodium NatriumK Potassium KaliumFe Iron FerrumCu Copper CuprumAg Silver ArgentumSn Tin StannumSb Antimony StibiumAu Gold AurumPb Lead Plumbum

As stated earlier, compounds are combinations of two or more elements. A chemical formula is an expression thatshows the elements in a compound and the relative proportions of those elements. For example, water is composedof hydrogen and oxygen in a two to one ratio. The chemical formula for water is H2O. Sulfuric acid is one of themost widely produced chemicals in the Unites States and is composed of the elements hydrogen, sulfur, and oxygen.The chemical formula for sulfuric acid is H2SO4.

Lesson Summary

• Mixtures are physical blends of two or more pure substances and can either be homogeneous or heterogeneous.Mixtures can be separated into their components by physical means such as filtration or distillation.

• Pure substances may be either elements or compounds. Elements are the simplest types of matter, while acompound is a chemical combination of two or more elements. A compound may only be decomposed intoits elements by a chemical process.

• Chemical symbols and formulas are used to represent elements and compounds.

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Lesson Review Questions

Reviewing Concepts

1. Why can’t a mixture be classified as a pure substance?2. Classify each of the following as a homogeneous or heterogeneous mixture.

a. saladb. iced teac. soild. cooking oil

3. When is filtration used to separate a mixture? When is distillation used?4. What is wrong with the following explanation of an experiment? “When it was heated, the sample of carbon

decomposed.”5. Identify the following as either a chemical symbol or a chemical formula.

a. COb. Coc. Wd. Bee. HI

Problems

6. Describe a way to separate a mixture of fine sand and table salt without having to directly sort tiny crystalsusing tweezers.

7. Name the elements found in each of the following compounds.

a. ammonia (NH3)b. saltpeter (KNO3)c. glucose (C6H12O6)d. millerite (NiS)

8. Identify each of the following items as an element, compound, or mixture. If it is a mixture, further classify itas homogeneous or heterogeneous.

a. gasolineb. calciumc. carbon dioxided. blood

Further Reading / Supplemental Links

• Another method for separating a mixture is chromatography. You can see how bomb specialists use chro-matography to identify the chemical components that were used in the making of the bomb. There is also achromatography activity that you can do at home! Go to www.pbs.org/wgbh/nova/bombingactivity/chromatog.html .

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Points to Consider

In a chemical reaction, one or more substances are changed into different substances.

• How are chemical changes different from physical changes?• What are the visual clues that a chemical reaction is taking place?

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2.3 Changes in Matter

Lesson Objectives

• Identify the chemical properties of a substance.• Describe chemical changes and differentiate them from physical changes.• Use various visual clues to identify whether a chemical reaction is taking place.

Lesson Vocabulary

• chemical property• chemical reaction• precipitate• product• reactant

Check Your Understanding

Recalling Prior Knowledge

• What is a chemical change?• What are the three common states of matter?

As you have seen, physical changes are those that do not alter the identities of the substances involved. In contrast,chemical changes produce substances with different compositions. In this lesson, you will learn how to recognizechemical changes.

Chemical Changes

When exposed to air, an object made of iron will eventually begin to rust ( Figure 2.12).

As the rust forms on the surface of the iron, it flakes off to expose more iron, which will continue to rust. Rust isclearly a substance that is different from iron. Rusting is an example of a chemical change.

A chemical property describes the ability of a substance to undergo a specific chemical change. A chemicalproperty of iron is that it is capable of combining with oxygen to form iron oxide, the chemical name of rust. A moregeneral term for rusting and other similar processes is corrosion. Other terms that are commonly used in descriptionsof chemical changes are burn, rot, explode, decompose, and ferment. Chemical properties are very useful as a wayof identifying substances. However, unlike physical properties, chemical properties can only be observed as thesubstance is in the process of being changed into a different substance.

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FIGURE 2.12Rust (iron oxide) forms on an unprotectediron surface.

A chemical change is also called a chemical reaction. A chemical reaction is a process that occurs when one ormore substances are changed into one or more new substances. Zinc (Zn) is a silver-gray element that can be groundinto a powder. If zinc is mixed at room temperature with powdered sulfur (S), a bright yellow element, the result willsimply be a mixture of zinc and sulfur. No chemical reaction occurs. However, if energy is provided to the mixturein the form of heat, the zinc will chemically react with the sulfur to form the compound zinc sulfide (ZnS). Picturedbelow are the substances involved in this reaction ( Figure 2.13).

FIGURE 2.13Zinc (A) and sulfur (B) are two elementsthat undergo a chemical reaction whenheated to from the compound zinc sulfide(C).

The reaction between zinc and sulfur can be depicted in something called a chemical equation. In words, we couldwrite the reaction as:

zinc + sulfur! zinc sulfide

A more convenient way to express a chemical reaction is to use the symbols and formulas of the substances involved:

Zn + S! ZnS

The substance(s) to the left of the arrow in a chemical equation are called reactants. A reactant is a substancethat is present at the start of a chemical reaction. The substance(s) to the right of the arrow are called products. Aproduct is a substance that is present at the end of a chemical reaction. In the equation above, zinc and sulfur arethe reactants that chemically combine to form zinc sulfide as a product.

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Recognizing Chemical Reactions

How can you tell if a chemical reaction is taking place? Certain visual clues indicate that a chemical reaction islikely (but not necessarily) occurring, including the following examples:

1. A change of color occurs during the reaction.2. A gas is produced during the reaction.3. A solid product, called a precipitate, is produced in the reaction.4. A visible transfer of energy occurs in the form of light as a result of the reaction.

Mercury(II) oxide is a red solid. When it is heated to a temperature above 500°C, it easily decomposes into mercuryand oxygen gas. The red color of the reactant, mercury oxide, is gradually replaced by the silver color of the product,mercury. The color change is one sign that this reaction is occurring. Watch this decomposition take place at http://www.youtube.com/watch?v=_Y1alDuXm6A (1:12).

MEDIAClick image to the left or use the URL below.URL: http://www.ck12.org/flx/render/embeddedobject/59229

When zinc reacts with hydrochloric acid, the reaction bubbles vigorously as hydrogen gas is produced ( Figure2.14). The production of a gas is also an indication that a chemical reaction may be occurring.

FIGURE 2.14Zinc reacts with hydrochloric acid to pro-duce bubbles of hydrogen gas.

When a colorless solution of lead(II) nitrate is added to a colorless solution of potassium iodide, a yellow solidcalled a precipitate is instantly produced ( Figure 2.15). A precipitate is a solid product that forms from a reaction

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and settles out of a liquid mixture. The formation of a precipitate may also indicate the occurrence of a chemicalreaction.

FIGURE 2.15A yellow precipitate of solid lead(II) io-dide forms immediately when solutions oflead(II) nitrate and potassium iodide aremixed.

All chemical changes involve a transfer of energy. When zinc reacts with hydrochloric acid, the test tube becomesvery warm as energy is released during the reaction. Some other reactions absorb energy. While energy changes area potential sign of a chemical reaction, care must be taken to ensure that a chemical reaction is indeed taking place.Physical changes may also involve a transfer of energy. A solid absorbs energy when it melts, and the condensationof a gas releases energy. The only way to be certain that a chemical reaction has occurred is to test the compositionof the substances after the change has taken place to see if they are different from the starting substances.

Combustion is a chemical change. Watch as magnesium reacts with oxygen to form magnesium oxide. View thislab-scale fireworks display at www.youtube.com/watch?v=EZ3JT2nWfMA (1:31).

MEDIAClick image to the left or use the URL below.URL: http://www.ck12.org/flx/render/embeddedobject/54705

Lesson Summary

• The composition of matter changes during a chemical reaction. The ability of a substance to react with othersubstances is reflected in its chemical properties.

• In a chemical reaction, the reacting substances are called reactants, and the substances that are produced bythe reaction are called products. A chemical equation shows reactants becoming products.

• Four clues to a possible chemical reaction include a color change, the production of a gas, the formation of aprecipitate, and an observable transfer of energy.

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Lesson Review Questions

Reviewing Concepts

1. What is the main difference between a physical change and a chemical change?2. How must the chemical properties of a given substance be observed?3. Give an example of a situation where a color change occurs, but not because of a chemical reaction.

Problems

4. Classify the following as physical properties or chemical properties of the element nickel.

a. It is gray in color.b. It melts at 1455°C.c. It reacts with sulfuric acid.d. It is a conductor of electricity.e. It forms a compound with oxygen.

5. Combustion, also called burning, is a reaction with oxygen gas. When propane is combusted, carbon dioxideand water are formed. Identify the reactants and the products of this combustion reaction.

6. Classify each of the following as a chemical change or a physical change.

a. Sugar dissolves in water.b. A peach rots.c. Icicles melt in the warm sunlight.d. A baking cake rises in the oven.e. A leaf changes its color in the fall.f. Food coloring is added to a glass of water.

7. Describe clues that you might observe during the following situations that would indicate that a chemicalreaction is likely occurring.

a. A log burns in the fireplace.b. A mixture of baking soda and vinegar fizzes.c. A silver fork tarnishes.d. A plumbing pipe becomes clogged with lime.

Points to Consider

A chemist must do more than simply make observations about chemical changes that are occurring in his or herexperiments. He or she must also take measurements in order to fully understand what is taking place duringchemical reactions.

• What types of measurements are routinely made by chemists?• How does a chemist indicate the degree of confidence in a measurement?• How are different numerical measurements manipulated when performing calculations?

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www.ck12.org Chapter 2. Matter and Change

2.4 References

1. (A) Ranveig Thattai; (B) Alicia Nijdam. (A) http://commons.wikimedia.org/wiki/File:Kutthuvilakku_wall_closeup.jpg; (B) http://www.flickr.com/photos/anijdam/2585351373/ . (A) Public Domain; (B) CC BY 2.0

2. (A) User:Rodrigo/Wikimedia Commons; (B) Christine und David Schmitt (Flickr:cheesy42); (C) StevenWhateley. (A) http://commons.wikimedia.org/wiki/File:Lillafured_icedwaterfall_wman.jpg; (B) http://commons.wikimedia.org/wiki/File:Nile_river_at_Luxor_2007.jpg; (C) http://commons.wikimedia.org/wiki/File:Steam_Train.JPG . (A) CC BY 2.5; (B) CC BY 2.0; (C) Public Domain

3. Christopher Auyeung. CK-12 Foundation . CC BY-NC 3.04. User:Proton02/Wikimedia Commons. http://commons.wikimedia.org/wiki/File:LPS_Lamp_35W_running.jpg

. Public Domain5. Courtesy of NOAA/NCDC. http://www.ncdc.noaa.gov/indicators/ . Public Domain6. Flickr:Yortw. http://www.flickr.com/photos/yortw/5470226807/ . CC BY 2.07. User:Smokefoot/Wikimedia Commons. http://commons.wikimedia.org/wiki/File:FilterFunnelApparatus.png

. Public Domain8. Pearson Scott Foresman. http://commons.wikimedia.org/wiki/File:Distillation_%28PSF%29.png . Public

Domain9. (A) Leon Brocard (Flickr:acme); (B) User:Evan-Amos/Wikimedia Commons; (C) Agnico-Eagle Mines Lim-

ited. (A) http://www.flickr.com/photos/acme/8532536/; (B) http://commons.wikimedia.org/wiki/File:Cast-Iron-Pan.jpg; (C) http://commons.wikimedia.org/wiki/File:Gold_Bars.jpg . (A) CC BY 2.0; (B) PublicDomain; (C) Public Domain

10. (A) Hi-Res Images of Chemical Elements/Jumk.de Webprojects; (B) User:Greenhorn1/Wikimedia Commons;(C) User:Miansari66/Wikimedia Commons. (A) http://images-of-elements.com/sodium.php; (B) http://commons.wikimedia.org/wiki/File:Chlorine2.jpg; (C) http://commons.wikimedia.org/wiki/File:Salt_,_Lake_salt.JPG . (A) CC BY 3.0; (B) Public Domain; (C) Public Domain

11. Christopher Auyeung. CK-12 Foundation . CC BY-NC 3.012. Duff Axsom (Flickr:duff_sf). http://www.flickr.com/photos/sfbear/472100458/ . CC BY 2.013. (A) Ben Mills (User:Benjah-bmm27/Wikimedia Commons); (B) User:Vineyard/Wikimedia Commons; (C)

Courtesy of A. Eisen/NOAA. (A) http://commons.wikimedia.org/wiki/File:Zinc-sample.jpg; (B) http://commons.wikimedia.org/wiki/File:SulfurReagent.jpg; (C) http://oceanexplorer.noaa.gov/explorations/03windows/logs/jul30/media/zincsulfideprecipitate.html . (A) Public Domain; (B) CC BY 3.0; (C) Public Domain

14. User:Chemicalinterest/Wikimedia Commons. http://commons.wikimedia.org/wiki/File:Zn_reaction_with_-HCl.JPG . Public Domain

15. Paige Powers (Flickr:paigggeyy). http://www.flickr.com/photos/paigggeyy/5533819494/ . CC BY 2.0

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