Exploring Mixtures and their Separation

Mixtures and pure substances, solutions, suspensions and colloids, concentration, and how to separate the parts of a mixture.

Key ideas

A pure substance has one kind of particle; a mixture has two or more, in any proportion, and keeps their properties.
A solution is a uniform mixture with tiny particles that do not settle or scatter light.
A suspension has large particles that settle; a colloid has middle-sized particles that scatter a beam of light (Tyndall effect).
Solubility is the most solute that dissolves in 100 g of water at a given temperature; it usually rises with temperature for solids.
Compounds have fixed proportions and new properties; their parts can be separated only by chemical means.

Remember

Mass by mass concentration	mass of solute ÷ mass of solution × 100
Solution	solute + solvent
Separation methods	evaporation, centrifugation, separating funnel, sublimation, chromatography, distillation, crystallisation
Examples	camphor or ammonium chloride: sublimation; ink dyes: chromatography; cream from milk: centrifugation

Watch out

Dividing by the mass of the solvent instead of the solution.
Calling a colloid a solution because it looks clear.
Choosing evaporation to get pure crystals (crystallisation gives purer ones).

Worked example

Solution P has 20 g of sugar in 125 g of water; solution Q has 50 g of sugar in 175 g of water. (a) Find the concentration of each (mass by mass). (b) Which is more concentrated? (c) If they are mixed, what is the concentration of the mixture?

Working: mixture: (20 + 50) ÷ (145 + 225) × 100

Answer: (a) P: 13.79%, Q: 22.22%. (b) Q. (c) 18.92%