Mixtures and Pure Substances

This lesson covers mixtures, pure substances and the key differences between them, as required by the Edexcel GCSE Combined Science specification (1SC0). You need to understand what makes a substance pure in chemistry, how mixtures differ from compounds, and how to use melting and boiling points to test for purity.

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Pure Substances in Chemistry

In everyday language, "pure" often means "nothing added" (e.g. "pure orange juice"). In chemistry, the word pure has a very specific meaning:

A pure substance is one that consists of only a single element or a single compound. It is not mixed with anything else.

Examples of Pure Substances

Substance	Type	Why It Is Pure
Distilled water	Compound (H₂O)	Contains only water molecules
Pure iron	Element (Fe)	Contains only iron atoms
Pure sodium chloride	Compound (NaCl)	Contains only sodium chloride

Testing for Purity — Melting and Boiling Points

A pure substance has a sharp, fixed melting point and a sharp, fixed boiling point.