Online Text

1.2 Classification of Matter

Matter can exist in one of three states of matter: a gas, a liquid, or a solid. A gas is highly compressible and will assume both the shape and the volume of its container. A liquid is not compressible and will assume the shape but not the volume of its container. A solid also is not compressible, and it has a fixed volume and shape of its own.




Matter can also be classified according to its composition. Most of the matter that we encounter exists in mixtures, which are combinations of two or more substances. Mixtures can be homogeneous or heterogeneous.

Mixtures can be separated into pure substances, and pure substances can be either compounds or elements.

A familiar example of a mixture is salt water. A sample of salt water has the same composition throughout. It can be separated into pure substances—water and ordinary table salt—by a physical process, such as distillation.



Pure water is collected in the flask on the right. When all of the water has been distilled from the mixture, pure salt—NaCl—will remain in the flask on the left. Both water and salt are pure substances. They cannot be further separated into simpler substances by any physical process. Each, however, can be decomposed into other substances by a chemical process, namely electrolysis. The following movie shows water being decomposed into oxygen and hydrogen.

Electrolysis of Water



The substances produced by the electrolysis of water cannot be further separated by any physical or chemical means. Oxygen and hydrogen are elements. When water is separated into its constituent elements, the relative amounts of those elements are always the same. Water is 11 percent hydrogen and 89 percent oxygen by mass. This is an example of the law of constant composition, also known as the law of definite proportions. Salt can also be separated into its constituent elements, sodium and chlorine, by electrolysis. Sodium chloride also has a constant composition, as do all pure substances. It is 39 percent sodium and 61 percent chlorine by mass.

In order to classify a sample of matter as a mixture or a pure substance, we must answer a series of questions.



Questions

Each of the following can be classified as a heterogeneous mixture, homogeneous mixture, compound, or element. How would you classify each?

 
Iced tea
heterogeneous mixture
homogeneous mixture
compound
element


 
Isopropyl alcohol
heterogeneous mixture
homogeneous mixture
compound

element