Chemical substance is a type of substance that has a fixed chemical composition and characteristic properties. The components of a chemical substance cannot be separated by physical separation methods without breaking the chemical bonds. These substances can exist in solid, liquid, or gas forms.

Vapor and water are two different forms of the same chemical substance.

To create chemical substances, i.e., to purify them, it is usually necessary to remove mixtures. A chemical substance consists of molecules, formula units, and atoms that have the closest characteristic properties to the substance itself. A general example of a chemical substance would be water isolated from a river and water made from oxygen and hydrogen in a laboratory in the same proportions.

Some other chemical substances that are usually mixed with each other to form a pure substance include diamond (carbon), gold, table salt (sodium chloride), and refined sugar (sucrose). In addition to these chemical substances, naturally occurring simple or apparently pure substances can be a mixture of chemical substances. For example, tap water is a compound that contains small amounts of dissolved sodium chloride and other chemical substances such as iron and calcium.