Classification of Mattersection E) Properties of Matter

What You'll Learn• to identify substances

using physical properties• differences between

physical andchemical changes

• how to identifychemical changes

• the law of conservationof mass

Make Flash Cards For each boldheading in this section, make aflash card. The flash card shouldcontain the main point of theparagraphs below the headingwritten on it. When you finish thesection, review the flash cards.

• Before You ReadWhen you see someone, how do you identify that person asa friend or a stranger? How do you identify a friend on thephone? What are some things about people that help yourecognize them? On the .lines below, list some things you useto identify people.

...• Read to Learn .. Physical Properties

You can stretch a rubber band, but you can't stretch a pieceof string very much. You can bend a piece of wire, but youcan't bend a matchstick easily.The rubber band and the wirechange shape, but the substances that they are made of do notchange. The ability to stretch or bend is a physical property. Aphysical property is a feature or characteristic that describesan object or substance, Some examples of physical propertiesare color, shape, size, density, melting point, and boiling point.

(;) Compare and Contrast How do physical properties describe appearance?Make.the following Foldable to " How.would you describe' a tennis ball? You could describehelp you understand how physi- some of its physical properties, such as shape and color. Youcal and chemical properties are , : could say it is a solid, not a liquid or a gas. For example, youdifferent, and how physical and might describe a tennis ball as a brightly colored, hollowchemical changes are different. '. sphere. You could also measure some physical properties of

the ball. You could measure its diameter with a ruler. Youcould measure its mass with a balance.

To describe a soft drink in a cup, you could start by sayingit is a brown liquid. You could measure the volume and thetemperature of the soft drink. Each of these characteristics isa physical property of that soft drink. .

258 CHAPTER 1 S Classification of Matter

How do physical properties describe behavior?Some physical properties describe the behavior of a mate-

rial or substance. You know that a magnet attracts objects thatcontain iron, such as.a safety pin. Attraction to a magnet is a ,physical property of iron. Every substance has physical prop-erties, that make it useful for certain tasks;

Some metals, such as copper, are useful because they bendeasily and can be drawn out into wires. Other metals, such asgold, are useful because they easily can be pounded into sheetsas thin as 0.1 micrometers. That's about four-millionths of aninch. This property of gold makes it useful for decoratingobjects. Gold that has been flattened this way is called gold leaf.

Think again about the soft drink. If you knock over thecup, the drink will spill. If you knock over a jar of honey;however, it does not flow as quickly. The ability to flow is aphysical property of liquids. M

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How are physical properties used toseparate materlalsz '

,You can use the physical properties of size and hardness toseparate some substances. Removing the seeds from ,a water-melon is easy.The seeds are small and hard, and the flesh of thelarge watermelon is soft' and juicy. You may have used a screenor sifter to remove pebbles from soil. The small pieces of soilquickly fall through the sifter, leaving the larger pebbles behind.

Look at the figure. Tile dish contains sand mixed with ironfilings. You probably would not be able to sift out the iron fil-ings. They are about the same size as the sand particles. Whatyou could do is pass a magnet through the mixture. The ,mag-net attracts only the iron filings: A magnet does ngt attractsand. This is an example using the physical property of mag-netism to separate substances in a mixture, Recycling cornpa-nies use magnetism to separate scrap iron from other metals.

Using Magnetism to Separate Materials ,,\'

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1. Explain Why does honeyflow more slowly thanwater?

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2. Apply Sifting probablyworks best for separatingwhat type of materials?

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Picture This3~ Explain Why would it be

difficult to sift the ironfilings from the sand?

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4. Explain What affect doesan energy change have onthe identity of a substance?

Picture Thiss. Observe Where is the solid.

material left behind in thedistillation process?

260 CHAPTER lS Classification of Matter

Physical ChangeWhen you break a stick of chewing gum, you change its size

and shape. You do not change the identity of the materialsthat make up the gum. f.;".. , ,,'Why does the identity of a substance staythe same?

When a substance freezes, boils, evaporates, or condenses, itundergoes a physical change. A physical change is a change insize, shape, or state of matter. Heat might be added or removedduring a physical change. Changes in energy do not change theidentity of the substance being heated or cooled. All substanceshave distinct properties that are constant, or never change. Theproperties of density, specific heat, boiling point, and meltingpoint are constant for substances. These properties can be used

.to identify unknown substances in a mixture.Iron is a substance that changes states when it absorbs or

releases energy. At high temperatures, iron melts. However,iron has the same physical properties that identify it as iron,whether it is in the liquid or solid state: MWhat is distillation?

Distillation is the process of separating substances in amixture by evaporating a liquid and condensing its vapor ..Alaboratory distillation process is shown below. The liquid isheated until it vaporizes. Then, the vapors are cooled untilthey condense. A solid material is left behind. Distillation isused to make drinking water out of salt water.

Thermometer..,;1 Cooling

water outiiJi )Ii

Condenser

Coolingwater in

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Liquids with different boiling points also can be distilled..Themixture is heated slowly,until it begins to boil. Vapors of the liq-uid with the lowest boiling point form first, They are condensedand collected. As temperature increases the second liquid boils.Its vapors are condensed and collected. Distillation is used oftenill industry. For instance, natural oils such as mint are distilled.

Chemical Properties and Changes , 'A chemical property, is a characteristic of a substance that

indicates whether. it can undergo a change that results in a newsubstance. A warning on a can of liquid paint thinner or lighterfluid states that the liquid is flammable (FLAmuh buhl). Ifa 'substance is flammable, it can burn. When a substance burns, itproduces new substances during a chemical change. Therefore,whether or not a substance is flammable is a chemical property.Knowing which substances are flammable. helps you to usethem safely.

Detecting Chemical Ch~n_eYour senses tell you when a chemical change has happened.

Leave a pot of chili cooking· on the stove too long and you willsmell it burning ..The smell tells you a new substance formed.

How does the identity of a substance change?You.smell a rotten egg or see rust on abike. These are signs

that a chemical change has taken place. A chemical change is achange of one substance to another. When fireworks explode, .they produce heat, light, and sound. Heat, light, and sound areoften signs of a rapid energy release and of a chemical change.

The only proof that a chemical change has takenplaceis 'that a new substance isformed. For example, when hydrogenandoxygen combine in ~ rocket engine, they use chemicalchanges to produce heat, light, and sound. But there are nosuch clues when iron combines with oxygen to form rust.Rust forms slowly. The only clue that iron has changed to a'new substance is the presence of rust. Burning and rusting arechemical changes because new substances are formed. ~ .

How can a chemical change be used toseparate substances?-

You can separate substances using-a chemical change. Oneexample is cleaning silver.:Silver chemically reacts with sulfurcompounds in the air to form silver sulfide, or tarnish. A dif-ferent chemical reaction changes the tarnish back to silverusing warm water, baking soda; and aluminum foil.

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6. Infer How can knowing achemical property such asflammability help you touse a product safely?

-'"7. Explain What is the onlyproofthat a chemicalchange has taken place?

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<ilThink it ave!8. Apply How are shifting

sand dunes an example ofphysical weathering?

~ .Applying Math9. Calculate If a 2-kg log is

burned, what is the massof the ash, smoke, and

. carbon dioxide producedby the chemical change?

262 CHAPTER 15 Classification of Matter

Weathering-Chemical or Physical Change?The forces of nature continuously shape Earth's surface ..

Rocks split, rivers carve deep canyons, sand dunes shift, andinteresting formations develop in caves. These chafrges ~reknown as weathering. Do you think weathering changes arephysical or chemical? Weathering changes are physical andchemical changes.

What is physical weathering?As a stream cuts through rock to form a canyon, small

particles of rock are carried downstream. The large rocksand the small particles of rock have the same characteristics.Because the particles are not changed, this type of weatheringis physical.

What is chemical weathering?Limestone is made up mostly of a chemical called calcium

carbonate. Calcium carbonate does not dissolve easily in water.But if the water is even slightly acidic, calcium carbonate reacts.A new substance, calcium hydrogen carbonate, is formed. Thissubstance dissolves in water. This change in limestone is achemical change. The calcium carbonate changes to calciumhydrogen carbonate in the chemical reaction. Rainwater can dis-solve limestone because of this reaction. This chemical changeleads to weathering. Chemical changes like this one create cavesandthe rock formations that are found in them.

The Conservation of MassA chemical property of wood is that it burns. After you

burn a log, all that i~left is a small pile of ash. Because you sawsmoke and light, felt heat, and saw a change in appearance asthe log was burning, you know a chemical change took place.

If you look closely at the ashes, you will see that the asheshave less mass than the log. Did the rest of the matter thatmade up the log disappear? No, it did not. Suppose you couldburn a log in a sealed container. The container would have tohold enough oxygen for the fire and contain all the smokethefire produced. If you measured the mass in the container beforeand after the fire, you would find that both are the same.

No mass is lost during a fire because no mass is gained orlost in a chemical change. According to the law of conservationof mass, the mass of all substances that are present before achemical change equals the mass of all the substances that

,: remain after the change ...........................................................................................

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• After You ReadMini Glossarychemical change: a change of one substance to anotherchemical property: a characteristic of a substance that indi-

cates whether it can undergo a certain chemical change 'distillation: the process of separating substances in a mixture

by evaporating a liquid and condensing its vapor

law of conservation of mass: the mass,~m all substancesthat are present before a chemical chang~ equals the massof all the substances that remain after the change

physical change: a change in size, shape, or state of matterphysical property: a feature or characteristic that describes

an object or substance

1. Review the terins and their definitions in the Mini Glossary. What is the main differencebetween a physical change and a chemical change? .

2. Complete the table below by giving an example of the property or change.

Physical property Example:

" Chemical property Example:

Physical change Example:

Chemical change Example: ~

Separation uslnq Example:physical change p~

.Separation using Example:chemical change

3. Imagine explaining physical and chemical changes to a group of elementary school students.Describe some items around your house to use as examples of physical and chemical changes.

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