The next few topics will require some basic knowledge about how chemical reactions work, so we will pause here to provide some elementary background. Consider, for example two hypothetical substances A and B which can react to form a product C. The reaction is written as
The double arrow symbol, is there for an important reason: it reminds us that chemical reactions proceed in both directions. When a molecule of A encounters one of B, with a certain probability it will react to form C. However, from time to time, a molecule C will also break up into its components A and B. The net reaction depends on the competition between the forward reaction and the back reaction, and when the two rates are equal the system is in chemical equilibrium and concentrations of the substances do not evolve. As an example, in any glass of common liquid water, the following reaction is taking place:
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