Pure substances and mixtures explained

Use composition and observations to compare substances.

Purity is about composition

A pure substance consists of one chemical substance. Water can therefore be pure even though a water molecule contains two types of atom. Salt water contains water and dissolved salt: it is a mixture.

What can you see?

A solution can be clear. A suspension contains undissolved solid particles dispersed in a liquid. Filtration can remove those particles but does not remove dissolved salt. Melting or boiling measurements at a specified pressure can provide further evidence; one flat graph alone does not prove purity.

Worked example

Compare distilled water, salt solution and chalk in water.

Here we model distilled water as pure H₂O.
Salt solution is a clear mixture; chalk in water is a suspension.

Clear and pure do not mean the same thing.