Introduction

Everyday life is full of chemical and physical changes. A banana ripens over a few days, a piece of silver slowly tarnishes, and iron rusts -- and all three of these changes proceed steadily in one direction only, never reversing themselves. Not every change behaves this way, though. Consider how oxygen is transported by haemoglobin in the blood: in the lungs, haemoglobin combines with oxygen to form oxyhaemoglobin, but oxyhaemoglobin also has a tendency to break back down into haemoglobin, releasing the oxygen again. In the lungs, haemoglobin, oxygen and oxyhaemoglobin are all present together at the same time -- this is a genuinely reversible change.

A handful of chemical reactions proceed in only one direction (irreversible reactions), but a great many proceed in both directions -- these are called reversible reactions. In an ordinary (one-directional) reaction, the concentration of reactants keeps falling and the concentration of products keeps rising as time passes. In a reversible reaction, the story is more interesting: at first the reaction runs mostly forward, building up product. But as product accumulates, the reverse reaction (product turning back into reactant) speeds up too. Eventually a particular stage is reached at which the rate of the reverse reaction becomes exactly equal to the rate of the forward reaction -- this is the state of equilibrium. …