Pure Substances and Mixtures

This lesson covers pure substances, mixtures and formulations as required by the Edexcel GCSE Combined Science specification (1SC0). You need to understand how purity is defined in chemistry, how melting and boiling points can be used to assess purity, and what a formulation is.

---

Purity in Chemistry

In everyday language, "pure" usually means "natural" or "nothing added" — for example, "pure orange juice." However, in chemistry, pure has a very specific meaning:

A pure substance in chemistry is one that consists of only a single element or a single compound. Nothing else is mixed with it.

Examples

Substance	Pure or Mixture?	Reason
Distilled water (H₂O)	Pure	Contains only water molecules
Tap water	Mixture	Contains dissolved minerals and salts
24-carat gold	Pure	Contains only gold atoms
18-carat gold	Mixture (alloy)	Contains gold, silver and copper
Clean air	Mixture	Contains nitrogen, oxygen, argon and other gases