2.3 Chemical Properties
Reading Focus
Key Concepts
When can chemical properties be observed?- What observations might indicate that a chemical change has occurred?
- What is the difference between chemical and physical changes?
Vocabulary
chemical property
flammability?
reactivity
chemical change
precipitate
Reading Strategy
Relating Text and Visuals? Copy the table. Find examples of the clues for recognizing chemical changes in Figures 19 and 20.
Clue |
Example |
|---|---|
Change in color |
a. |
Production of gas |
b. |
Formation of precipitate |
c. |
How would you describe candles like the ones in Figure 16? Yo might describe their color or their hardness. Or, yo might observe that the candles float on water, and conclude that the density of candle wax is less than that of water. Color, hardness, and density are physical properties. But there is something else about the candles that may seem even more obvious to you: the fact that the candles are burning. The ability to burn is not a physical property because yo cannot observe burning without changing the composition of the material that is burning. As a candle burns, new substances form.
Observing Chemical Properties
Most candles are made from paraffin, which is a mixture of compounds containing carbon and hydrogen. As a candle burns, the compounds combine with oxygen in the air to form water and carbon dioxide. (Carbon dioxide is the gas that gives a carbonated beverage its fizz.) The ability to burn is a chemical property. A chemical property is any ability to produce a change in the composition of matter.
Chemical properties can be observed only when the substances in a sample of matter are changing into different substances. Flammability and reactivity are two examples of chemical properties.
Flammability Materials that burn can be used as fuel. Sometimes, people use burning newspapers to start a fire in a fireplace. Gasoline is the fuel burned in most automobiles. The chemical property that paper and gasoline share is their flammability. Flammability is a material's ability to burn in the presence of oxygen.
Figure 16 When candles burn, new substances form. The ability to burn is a chemical property.
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