Mixtures and Separation

A pure substance is made of only one kind of particle, so it has a fixed composition and fixed properties - like pure water or pure gold. A mixture contains two or more substances mixed together in any proportion, and each keeps its own properties. Air, sea water and milk are all mixtures. Mixtures are either homogeneous (uniform throughout, like salt dissolved in water) or heterogeneous (you can see the different parts, like sand in water).

A solution is a homogeneous mixture where one substance (the solute) dissolves completely in another (the solvent); the particles are so tiny you cannot see them or filter them out. A suspension is a heterogeneous mixture with large particles you can see, which settle down when left to stand and can be removed by filtering. A colloid sits in between: it looks uniform but its particles are big enough to scatter light, yet too small to settle or be filtered.

When a beam of light passes through a true solution, you cannot see its path - the particles are too small. But in a colloid, the particles are large enough to scatter the light sideways, making the beam visible. This scattering is called the Tyndall effect. It is why we can see a beam of sunlight in a misty forest or a torch beam in a dusty room. The Tyndall effect is a simple test to tell a colloid apart from a true solution.

Because the parts of a mixture keep their own properties, we can separate them using physical methods. Filtration removes insoluble solids; evaporation recovers a dissolved solid; distillation separates two liquids by their different boiling points; a separating funnel splits two liquids that do not mix; chromatography separates dissolved coloured substances; and a magnet picks out iron. Choosing the right method depends on the property that differs between the components.