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In 1864 two Norwegian chemists, Peter Waage and Cato Guldberg showed empirically, that for a chemical reaction at equilibrium, there is a mathematical expression that has a constant value, provided the temperature does not change. This is called Keq (where the eq subscript represents the fact that this is an equilibrium system). Because the equilibrium constant expression is so widely used in chemistry, it is often just written as K. At other times, the subscript is changed to represent a particular type of equilbrium. For example, you will see the following K's:
| Ksp | solubility product constant | |
| Kw | water dissociation constant | |
| Ka | acid dissociation constant | |
| Kc | concentration equilibrium constant | |
| Kp | pressure equilibrium constant |
So, what is this K expression? Let's develop it "experimentally" by using a hydrogen iodide simulation.
