Pure Substance
Matter with a fixed chemical composition and a definite set of physical properties, comprising either a single element or a single compound.
Unit
Dimensionless
In depth
A pure substance melts and boils at a sharp, reproducible temperature, which is why melting-point determination is a classic purity test. Any impurity depresses and broadens the melting range. Purity in the laboratory is quoted as a percentage or as a grade such as analytical reagent (AR).
Examples in the real world
Distilled water, 99.999% pure copper, crystalline sucrose, and a cylinder of high-purity argon.