Pure substances and testing purity
Chemistry · Key Stage 3 · The School
Pure means one substance
In chemistry a PURE substance contains only one element or one compound, with nothing else in it. That is stricter than everyday use: pure orange juice is a mixture of water, sugars, acids and pulp, and is not chemically pure at all. Getting this distinction straight prevents a great deal of confusion later.
The melting point test
A pure substance melts at a single, sharp temperature and boils at a single temperature. A mixture melts over a RANGE and usually at a lower temperature than the pure substance would. So measuring a melting point tells you two things at once: which substance it probably is, and whether it is pure.
Why it is useful
This test is why melting point is the standard purity check in a laboratory and why it matters in medicine: an impure drug may not work as intended, and the impurity may be harmful. It is also why salt is spread on icy roads — the salt makes a mixture whose melting point is below zero, so the ice melts.
Sample A melts sharply at 80 °C — over less than a degree. Sample B starts softening at 68 °C and is not fully liquid until 76 °C. A is pure; B is a mixture, and the impurity both lowers and SPREADS the melting point. Now notice the label on B says 'pure honey'. In everyday English that means nothing added; in chemistry it means one substance, and honey is dozens.