Scientists classify matter in several ways that depend on what it is made of and how it behaves. For example, matter that has the same composition and properties throughout is called a substance. Elements, such as a bar of gold or a sheet of aluminum, are substances. When different elements combine, other substances are formed.
The elements hydrogen and oxygen exist as separate, colorless gases.However, these two elements can combine, as shown in Figure 1, to form the compound water, which is different from the elements that make it up. A compound is a substance whose smallest unit is made up of atoms of more than one element bonded together. Compounds often have properties that are different from the elements that make them up. Water is distinctly different from the elements that make it up. It is also different from another compound made from the same elements. Have you ever used hydrogen peroxide (H2O2) to disinfect a cut? This compound is a different combination of hydrogen and oxygen and has different properties from those of water. Water is a nonirritating liquid that is used for bathing, drinking, cooking, and much more. In contrast, hydrogen peroxide carries warnings on its labels such as Keep Hydrogen Peroxide Out of the Eyes. Although it is useful in solutions for cleaning contact lenses, it is not safe for your eyes as it comes from the bottle.
When two or more substances (elements or compounds) come together but don’t combine to make a new substance, a mixture results. Unlike compounds, the proportions of the substances in a mixture can be changed without changing the iden-tity of the mixture. For example, if you put some sand into a bucket of water, you have a mixture of sand and water. If you add more sand or more water, it’s still a mixture of sand and water. Its identity has not changed. Air is another mixture. Air is a mixture of nitrogen, oxygen, and other gases, which can vary at different times and places. Whatever the proportion of gases, it is still air. Even your blood is a mixture that can be separated, as shown in Figure 2 by a machine called a centrifuge.