Glossary
chemical change
A chemical change makes a new substance with new properties — burning, rusting and digesting food are chemical changes.
A chemical change makes a new substance, one whose properties differ from what went in. Burning paper, iron rusting, milk souring and food being digested are chemical changes: the atoms are rearranged into different compounds, and no amount of cooling or stirring turns the products back into the starting material. Several signs point to one — a gas that is not just boiling, a color change that is not just mixing two colors, a solid appearing in a clear solution, heat or light given off without an outside source, or a new smell. No single sign is proof on its own; a burning candle shows almost all of them, while a color change in a solution can be only a dilution. The reliable statement is the composition one: a chemical change cannot be undone by a physical method, because what is there afterward is a different substance.
Taught in
Related terms
- physical change — A physical change alters a substance's form, size or state without making a new substance — melting, cutting and dissolving are the usual examples.
- matter — Matter is anything with mass that takes up space, and it exists as a solid, a liquid, a gas or a plasma.
Source: NIST periodic table and reference data for the atomic masses and half-life figures, NIH PubChem for the substance data, and NIST Special Publication 811 for the unit conventions — the same sources the chemistry guide cites. All three are US federal government material, not subject to copyright in the United States. Every definition, example and calculation above was written for this course; no dictionary or textbook text is reproduced. (NIST and PubChem accessed 2026-10-26; definitions written 2026-10-27)