Chemical Properties How would you describe a piece of wood before and after it is burned? Has it changed color? Does it have the same texture? The original piece of wood changed, and physical properties alone can't describe what happened to it. Chemical Properties Physical properties are not the only properties that describe matter. Chemical properties describe matter based on its ability to change into new matter that has different properties. For example, when wood is burned, ash and smoke are created. These new substances have very different properties than the original piece of wood had. Wood has the chemical property of flammability. Flammability is the ability of a substance to burn. Ash and smoke cannot burn, so they have the chemical property of nonflammability. Another chemical property is reactivity. Reactivity is the ability of two or more substances to combine and form one or more new substances. The photo of the old car in Figure 1 illustrates reactivity and nonreactivity. What does the term reactivity mean? (See the Appendix for answers to Reading Checks.) Reactivity with Oxygen Figure 1 Objectives Describe two examples of chemical properties. Explain what happens during a chemical change. Distinguish between physical and chemical changes. Terms to Learn chemical property chemical change chemical property a prop- erty of matter that describes a substance's ability to participate in chemical reactions Reading Organizer As you read this section, create an outline of the section. Use the headings from the section in your outline. The iron used in this old car has the chemi- cal property of reactivity with oxygen. When iron is exposed to oxygen, it rusts. The bumper on this car still looks new because it is coated with chromium. Chromium has the chemi- cal property of nonreac- tivity with oxygen. 50 Chapter 2 The Properties of Matter
Welcome message from author
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
Chemical PropertiesHow would you describe a piece of wood before and afterit is burned? Has it changed color? Does it have the sametexture? The original piece of wood changed, and physicalproperties alone can't describe what happened to it.
Chemical PropertiesPhysical properties are not the only properties that describematter. Chemical properties describe matter based on its abilityto change into new matter that has different properties. Forexample, when wood is burned, ash and smoke are created.These new substances have very different properties than theoriginal piece of wood had. Wood has the chemical propertyof flammability. Flammability is the ability of a substance toburn. Ash and smoke cannot burn, so they have the chemicalproperty of nonflammability.
Another chemical property is reactivity. Reactivity is theability of two or more substances to combine and form oneor more new substances. The photo of the old car in Figure 1
illustrates reactivity and nonreactivity.
~ What does the term reactivity mean? (See theAppendix for answers to Reading Checks.)
Reactivity with OxygenFigure 1
ObjectivesDescribe two examples of chemicalproperties.
Explain what happens during achemical change.
Distinguish between physical andchemical changes.
Terms to Learnchemical propertychemical change
chemical property a property of matter that describes asubstance's ability to participatein chemical reactions
Reading Organizer As you read thissection, create an outline of the section.Use the headings from the section inyour outline.
The iron used in this old car has the chemical property of reactivity with oxygen. Wheniron is exposed to oxygen, it rusts.
The bumper on this carstill looks new because itis coated with chromium.Chromium has the chemical property of nonreactivity with oxygen.
50 Chapter 2 The Properties of Matter
Figure 2 Physical Versus Chemical Properties
Physical property
Shape Bending an ironnail will change its shape.
Chemical property
Reactivity with OxygenAn iron nail can reactwith oxygen in the air toform iron oxide, or rust.
Comparing physical and Chemical PropertiesHow do you tell a physical property from a chemical property?You can observe physical properties without changing theidentity of the substance. For example, you can find thedensity and hardness of wood without changing anythingabout the wood.
Chemical properties, however, aren't as easy to observe.For example, you can see that wood is flammable only whileit is burning. And you can observe that gold is nonflammableonly when it won't burn. But a substance always has chemicalproperties. A piece of wood is flammable even when it's notburning. Figure 2 shows examples of physical and chemicalproperties.
The Right StuffWhen choosing
materials to use in manufacturing, you must make suretheir properties are suitable fortheir uses. For example, falseteeth can be made from acrylicplastic, porcelain, or gold.According to legend, GeorgeWashington wore false teethmade of wood. Do researchand find what Washington'sfalse teeth were really madeof. In your science journal,write a paragraph about whatyou have learned. Includeinformation about the advantages of the materials used inmodern false teeth.
Flammability Rubbingalcohol is able to burneasily.
State Rubbing alcoholis a clear liquid at roomtemperature.
Characteristic PropertiesThe properties that are most useful in identifying a substanceare characteristic properties. These properties are always the sameno matter what size the sample is. Characteristic propertiescan be physical properties, such as density and solubility, aswell as chemical properties, such as flammability and reactivity. Scientists rely on characteristic properties to identify andclassify substances.
Section 3 Chemical Properties 51
Changing Change "1. Place a folded paper ....".
towel in a small pieplate.
2. Pour vinegar into thepie plate until the entire ~paper towel is damp. ~
3. Place three shiny pennieson top of the paper towel.
4. Put the pie plate in a safeplace. Wait 24 hours.
5. Describe and explain thechange that took place.
Chemical Changes and New SubstancesA chemical change happens when one or more substances arechanged into new substances that have new and differentproperties. Chemical changes and chemical properties are notthe same. Chemical properties of a substance describe whichchemical changes will occur and which chemical changes willnot occur. But chemical changes are the process by whichsubstances actually change into new substances. You can learnabout the chemical properties of a substance by looking at thechemical changes that take place.
You see chemical changes more often than you may think.For example, a chemical reaction happens every time a batteryis used. Chemicals failing to react results in a dead battery.Chemical changes also take place within your body when thefood you eat is digested. Figure 3 describes other examples ofchemical changes.
~~ How does a chemical change differ from achemical property?
Figure 3 Examples of Chemical Changes
Effervescent tabletsbubble when the citricacid and baking sodain them react in water.
The Statue of Liberty ismade of orange-browncopper but it looks greenfrom the metal's interactionwith moist air. New coppercompounds formed and thesechemical changes made thestatue turn green over time.
Soured milk smells badbecause bacteria have formednew substances in the milk.
The hot gas formed whenhydrogen and oxygen join tomake water helps blast thespace shuttle into orbit.
52 Chapter 2
Figure 4 Each of theoriginal ingredients hasdifferent physical andchemical properties than thefinal product, the cake, does!
What Happens During a Chemical Change?Afun way to see what happens during chemical changes is tobake a cake. You combine eggs, flour, sugar, and other ingredients, as shown in Figure 4. When you bake the batter, youend up with something completely different. The heat of theoven and the interaction of the ingredients cause a chemicalchange. The result is a cake that has properties that differ fromthe properties of the ingredients.
Signs of Chemical ChangesLook back at Figure 3. In each picture, at least one sign indicatesa chemical change. Other signs that indicate a chemical changeinclude a change in color or odor, production of heat, fizzingand foaming, and sound or light being given off.
In the cake example, you would smell the cake as it baked.You would also see the batter rise and begin to brown. Whenyou cut the finished cake, you would see the air pockets madeby gas bubbles that formed in the batter. These signs showthat chemical changes have happened.
Matter and Chemical ChangesChemical changes change the identity of the matter involved.So, most of the chemical changes that occur in your dailylife, such as a cake baking, would be hard to reverse. Imaginetrying to unbake a cake. However, some chemical changescan be reversed by more chemical changes. For example, thewater formed in the space shuttle's rockets could be split intohydrogen and oxygen by using an electric current.
chemical change a change thatoccurs when one or more substanceschange into entirely new substanceswith different properties
For another activity relatedto this chapter, go togo.hrw.com and type in keyword HP5MATW.
Section 3 Chemical Properties 53
Change in TextureGrinding bakingsoda into a fine,powdery substanceis a physical change.
Physical and Chemical Changes
o
50
Reactivitywith VinegarGas bubblesare producedwhen vinegaris poured intobaking soda.
Acid Rain When fossil fuelsare burned, a chemical changetakes place. Sulfur from fos-sil fuels and oxygen from theair combine to produce sulfurdioxide, a gas. When sulfurdioxide enters the atmosphere, it undergoes anotherchemical change by interacting with water and oxygen.Research this chemical reaction. Make a poster describingthe reaction and showing howthe final product affects theenvironment.
Physical Versus Chemical ChangesThe most important question to ask when trying to decide ifa physical or chemical change has happened is, Did the composition change? The composition of an object is the type ofmatter that makes up the object and the way that the matteris arranged in the object. Figure 5 shows both a physical anda chemical change.
A Change in CompositionPhysical changes do not change the composition of a substance. For example, water is made of two hydrogen atoms andone oxygen atom. Whether water is a solid, liquid, or gas, itscomposition is the same. But chemical changes do alter thecomposition of a substance. For example, through a processcalled electrolysis, water is broken down into hydrogen andoxygen gases. The composition of water has changed, so youknow that a chemical change has taken place.
54 Chapter 2
Physical or Chemical Change?1. Watch as your teacher places a burning wooden stick into a
test tube. Record your observations.
2. Place a mixture of powdered sulfur and iron filings on asheet of paper. Place a bar magnet underneath the paper,and try to separate the iron from the sulfur.
3. Drop an effervescent tablet into a beaker of water. Recordyour observations.
4. Identify whether each change is a physical change or a chemical change. Explain your answers.
The Properties of Matter
Reversing Changes
Can physical and chemical changes be reversed?Many physical changes are easily reversed. Theydo not change the composition of a substance. Forexample, if an ice cube melts, you could freeze theliquid water to make another ice cube. But composition does change in a chemical change. So, mostchemical changes are not easily reversed. Look atFigure 6. The chemical changes that happen whena firework explodes would be almost impossible toreverse, even if you collected all of the materialsmade in the chemical changes.
Figure 6 This display of fireworks representsmany chemical changes happening at thesame time.
at Skills
Chemical propertiesdescribe a substancebased on its ability tochange into a new substance that has differentproperties.Chemical properties canbe observed only whena chemical change mighthappen.Examples of chemicalproperties are flammability and reactivity.New substances formas a result of a chemicalchange.Unlike a chemicalchange, a physicalchange does notalter the identity ofa substance.
1. In your own words, write a definition for each of the following terms: chemical property andchemical change.
Unde standing Key Ideas2. Rusting is an example of a
a. physical property.b. physical change.c. chemical property.d. chemical change.
. 3. Which of the following is acharacteristic property?
a. densityb. chemical reactivityc. solubility in waterd. All of the above
4. Write two examples of chemical properties and explain whattheyare.
5. The Statue of Liberty was originally a copper color. After beingexposed to the air, she turneda greenish color. What kind ofchange happened? Explain youranswer.
6. Explain how to tell the difference between a physical and achemical property.
7. The temperature of an acidsolution is 25°C. A strip ofmagnesium is added, and thetemperature rises 2°C each minute for the first 3 min. Afteranother 5 min, the temperaturehas risen two more degrees.What is the final temperature?
Critica Ihinking8. Making Comparisons Describe
the difference between physicaland chemical changes in termsof what happens to the matterinvolved in each kind of change.
9. Applying Concepts Identifytwo physical properties and twochemical properties of a bag ofmicrowave popcorn beforepopping and after.
For a variety of links related to thischapter, go to www.SCilinks.org~
Topic: Chemical ChangesSciLinks code: HSM0266
Related Documents
