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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.)
Figure 1 Reactivity with Oxygen
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
Figure 2 Physical Versus Chemical Properties
Physical property
Shape Bending an iron nail will change its shape.
Chemical property
Reactivity with Oxygen An iron nail can react with oxygen in the air to form iron oxide, or rust.
Comparing physical and Chemical Properties
How do you tell a physical property from a chemical property? You can observe physical properties without changing the identity of the substance. For example, you can find the density and hardness of wood without changing anything about the wood. Chemical properties, however, aren't as easy to observe. For example, you can see that wood is flammable only while it is burning. And you can observe that gold is nonflammable only when it won't burn. But a substance always has chemical properties. A piece of wood is flammable even when it's not burning. Figure 2 shows examples of physical and chemical properties.
The Right Stuff When choosing materials to use in manufac- turing, you must make sure their properties are suitable for their uses. For example, false teeth can be made from acrylic plastic, porcelain, or gold. According to legend, George Washington wore false teeth made of wood. Do research and find what Washington's false teeth were really made of. In your science journal, write a paragraph about what you have learned. Include information about the advan- tages of the materials used in modern false teeth.
Flammability Rubbing alcohol is able to burn easily.
State Rubbing alcohol is a clear liquid at room temperature.
Characteristic Properties
The properties that are most useful in identifying a substance are characteristic properties. These properties are always the same no matter what size the sample is. Characteristic properties can be physical properties, such as density and solubility, as well as chemical properties, such as flammability and reactiv- ity. Scientists rely on characteristic properties to identify and classify substances. Section 3 Chemical Properties 51
Figure 4 Each of the original ingredients has different physical and chemical properties than the final product, the cake, does!
What Happens During a Chemical Change?
Afun way to see what happens during chemical changes is to bake a cake. You combine eggs, flour, sugar, and other ingre- dients, as shown in Figure 4. When you bake the batter, you end up with something completely different. The heat of the oven and the interaction of the ingredients cause a chemical change. The result is a cake that has properties that differ from the properties of the ingredients.
Signs of Chemical Changes
Look back at Figure 3. In each picture, at least one sign indicates a chemical change. Other signs that indicate a chemical change include a change in color or odor, production of heat, fizzing and 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. When you cut the finished cake, you would see the air pockets made by gas bubbles that formed in the batter. These signs show that chemical changes have happened.
Matter and Chemical Changes
Chemical changes change the identity of the matter involved. So, most of the chemical changes that occur in your daily life, such as a cake baking, would be hard to reverse. Imagine trying to unbake a cake. However, some chemical changes can be reversed by more chemical changes. For example, the water formed in the space shuttle's rockets could be split into hydrogen and oxygen by using an electric current.
chemical change a change that occurs when one or more substances change into entirely new substances with different properties
For another activity related to this chapter, go to go.hrw.com and type in key- word HP5MATW.
Section 3 Chemical Properties 53
Change in Texture Grinding baking soda into a fine, powdery substance is a physical change.
Physical and Chemical Changes
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Reactivity with Vinegar Gas bubbles are produced when vinegar is poured into baking soda.
Acid Rain When fossil fuels are burned, a chemical change takes place. Sulfur from fos- sil fuels and oxygen from the air combine to produce sulfur dioxide, a gas. When sulfur dioxide enters the atmos- phere, it undergoes another chemical change by interact- ing with water and oxygen. Research this chemical reac- tion. Make a poster describing the reaction and showing how the final product affects the environment.
Physical Versus Chemical Changes
The most important question to ask when trying to decide if a physical or chemical change has happened is, Did the com- position change? The composition of an object is the type of matter that makes up the object and the way that the matter is arranged in the object. Figure 5 shows both a physical and a chemical change.
A Change in Composition Physical changes do not change the composition of a sub- stance. For example, water is made of two hydrogen atoms and one oxygen atom. Whether water is a solid, liquid, or gas, its composition is the same. But chemical changes do alter the composition of a substance. For example, through a process called electrolysis, water is broken down into hydrogen and oxygen gases. The composition of water has changed, so you know that a chemical change has taken place.
(^54) Chapter 2
Physical or Chemical Change?
- 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 a sheet 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. Record your observations. 4. Identify whether each change is a physical change or a chemi- cal change. Explain your answers.
The Properties of Matter
