Glossary
chemical change
A chemical change makes one or more new substances; the evidence is a gas, a precipitate, a color change, or heat or light.
A chemical change (a chemical reaction) turns one set of substances into different ones. Rusting iron makes iron oxide; burning wood makes ash, carbon dioxide and water; milk souring makes new acids. Because new substances form, a chemical change is not reversed by simply undoing a step. You often see evidence: bubbles of a gas, a solid precipitate settling out, a color change, or heat and light released. A chemical property describes how a substance reacts — its flammability, or how it behaves with an acid — as opposed to a physical property like density. The test against a physical change is always the same question: has a new substance been made?
Taught in
Related terms
- physical change — A physical change alters a substance's form, state or shape without making a new substance — melting, boiling, dissolving or cutting.
Source: OpenStax Chemistry 1e and Chemistry: Atoms First 1e, first editions (CC BY 4.0). Each definition restates the Chemistry guide in this exam, which carries the per-source citations on its own page. (OpenStax 1e accessed 2026-10-08)