Solutions

About this unit

This unit studies solutions: homogeneous mixtures of two or more components. You learn how to express the amount of solute present, why gases and solids dissolve to different extents, how a dissolved solute changes the vapour pressure of a solvent, and how properties such as boiling point, freezing point and osmotic pressure depend only on the number of solute particles. These ideas are used to find molar masses and explain everyday effects such as salting roads or preserving food.

Key ideas

• Concentration terms: mass percentage, ppm, mole fraction, molarity and molality.
• Henry's law for the solubility of gases in liquids.
• Raoult's law, ideal and non-ideal solutions, positive and negative deviations, azeotropes.
• Four colligative properties: relative lowering of vapour pressure, elevation of boiling point, depression of freezing point and osmotic pressure.
• Osmosis and reverse osmosis.
• Abnormal molar masses and the van't Hoff factor i.

How to prepare

Memorise the formula for each concentration term and each colligative property, along with its units. Practise short numericals on molarity, molality, Raoult's law, ΔTb, ΔTf and π = CRT. Learn which concentration terms depend on temperature. Questions are often numerical or ask you to identify the type of deviation from a given example.