Substance: definition in chemistry
Substance: Matter with a uniform, definite composition and fixed properties — either an element or a compound. Also called a pure substance.
A pure substance has the same composition in every sample and melts and boils at sharp, fixed temperatures at a given pressure, which is one way chemists test for purity. Mixtures, by contrast, are combinations of substances and usually melt or boil over a range of temperatures. In everyday speech any material may be called a substance, but in chemistry the word usually means a pure substance.
Examples
- Distilled water (H₂O) — a compound
- Pure copper (Cu) — an element
- Table sugar (sucrose, C₁₂H₂₂O₁₁) — a compound
- Oxygen gas (O₂) — an element made of diatomic molecules
On the periodic table
All 118 elements on the periodic table are pure substances; compounds are the other kind.