2.13:
Solubility Equilibria: Ionic Product of Water
Water is a weak electrolyte and undergoes a small amount of self-ionization.
At any given temperature, the concentration of undissociated water is considered constant, so the ionic product of water Kw is the product of the concentrations of hydrogen and hydroxide ions.
Kw increases with increasing temperature. At 25 °C, the Kw of pure water is 10−14.
Under these conditions, the hydrogen and hydroxide ion concentrations of pure water are equal, so the concentration of individual ions equals the square root of Kw.
A neutral solution consists of equal concentrations of these ions.
Suppose the hydrogen ion concentration is greater than 10−7. In that case, the solution is considered acidic, whereas it is alkaline or basic below this value.
So, the solution is basic if the hydroxide ion concentration is more than 10−7, whereas it is acidic below this value.
2.13:
Solubility Equilibria: Ionic Product of Water
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 Kw. The square root of Kw 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, Kw 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.