Pure water is a weak electrolyte; only a small amount ionizes into hydrogen and hydroxide ions. At any given temperature, the concentration of undissociated water is almost constant, so the ionic product of water is the product of the hydrogen and hydroxide ion concentrations, denoted as K_w. The square root of K_w gives the individual ion concentrations.

The ionic product of water varies with temperature, and its value is 1.0 x 10⁻¹⁴ at standard experimental conditions. Per Le Chatelier's principle, if the product of hydrogen and hydroxide ion concentrations becomes more than this value at any point, the excess ions will combine to form water molecules until it reaches equilibrium. Similarly, if the product of ionic concentrations falls below this value, water ionizes to form hydrogen and hydroxide ions to attain equilibrium.

The nature of an aqueous solution–neutral, acidic or alkaline–is defined by the concentrations of the hydrogen and hydroxide ions. In the case of acidic or basic solutions, K_w is still the product of the concentrations of the hydronium and hydroxide ions, but these two concentrations will clearly not be equal to each other.