Introduction

We can survive weeks without food, but a close look at almost anything around us -- a bottle of mouthwash, sea water, the air we breathe, even a piece of jewellery -- shows it is rarely a pure substance. Most matter we handle is a mixture, and most of those mixtures are homogeneous: their composition and properties are uniform throughout, right down to the molecular scale. A homogeneous mixture of two or more substances is called a solution.

Three familiar examples make the point that solutions are not restricted to any one physical state:

• Sea water (a liquid solution) covers more than 70% of Earth's surface and carries many dissolved solids, chiefly NaCl.
• Air (a gaseous solution) is a homogeneous mixture of nitrogen, oxygen, carbon dioxide and trace gases.
• Brass (a solid solution) is a homogeneous mixture of copper and zinc.

What ties these three very different examples together is homogeneity -- uniform distribution of the components throughout the mixture, whatever physical state that mixture happens to be in. This unit develops the vocabulary and the quantitative laws needed to describe solutions: how to express their strength (concentration), how much solute a solvent can hold (solubility), how a dissolved gas behaves (Henry's law), how a dissolved solute changes the solvent's vapour pressure (Raoult's law), and the four special "colligative" properties that depend only on how many solute particles are present. …