Pure Substances and Mixtures

All the matter around you — anything that has mass and takes up space — is either a pure substance or a mixture.

• A pure substance has a fixed composition throughout: every part is the same. Distilled water and pure copper are pure substances.
• A mixture is two or more substances physically combined, in any ratio, that keep their own properties. Sea water, air and a cup of rooibos are mixtures.

Mixtures are further split by how uniform they are:

• Homogeneous (a solution): one visible phase, evenly mixed — you cannot see the parts. Salt fully dissolved in water; the air you breathe.
• Heterogeneous: you can see (or separate out) the different parts. Sand in water; a bowl of chakalaka.