Chemicals are the basic types of matter. The elements, such as hydrogen and oxygen, are chemicals. So are the compounds, such as water, that are made when atoms of different elements join. Our bodies, food, and the clothes we wear are all made from chemicals.

The properties of a chemical describe what it does to other chemicals. Oxygen, for example, makes iron rust, or oxidize, so oxygen is an oxidizing agent.

Our bodies contain tens of thousands of chemicals that mix and react to give us energy and keep us alive. Toxic (poisonous) chemicals interfere with these reactions. Some chemicals are also corrosive—they eat through solid material.

A chemical reaction happens when two or more chemicals combine to make new chemical compounds. Many reactions release heat—for example, burning.

Biochemistry studies the molecules in living things, including plants, animals, and the human body. These molecules are all compounds of the element carbon.