The idea
A pure substance has a definite composition and characteristic properties. Mixtures can vary in composition and can often be separated using physical methods such as filtration or distillation. Breaking a compound into simpler substances requires a chemical change. Molecules can contain one element, as in O₂, or more than one, as in H₂O.
Worked example
Classify oxygen gas, pure water and salt water.
- Oxygen gas is an element: each O₂ molecule contains only oxygen atoms.
- Pure water is a compound: hydrogen and oxygen are bonded in a fixed 2:1 atom ratio.
- Salt water is a mixture: the amount of dissolved salt can vary, and water can be recovered by distillation.
A mistake to avoid
A substance is not a compound just because its particles contain several atoms. The atoms must belong to different elements.
Check your understanding
Is air an element, compound or mixture?
Show the explanation
Air is a mixture of gases, including nitrogen, oxygen and smaller amounts of other gases.
Explore the lesson
This guide connects to What Is Matter Actually Made Of? in the IB Chemistry course. The first two published lessons per subject are free previews; further interactive lessons require an account.