Solubility Equilibria: Ionic Product of Water

The ionic product of water, denoted Kw, is the product of hydrogen and hydroxide ion concentrations in aqueous solutions. Water is a weak electrolyte that undergoes self-ionization, producing these ions in equilibrium. At 25°C, Kw equals 10−14, providing a fundamental constant for determining solution acidity or basicity and understanding chemical equilibria in aqueous systems.

The ionic product of water increases with increasing temperature because higher thermal energy promotes water self-ionization. At 25°C, Kw is 1.0 × 10−14, but this value changes at different temperatures. This temperature dependence is critical when performing analytical work outside standard conditions, as it directly affects hydrogen and hydroxide ion concentrations and solution pH.

In pure water at 25°C, hydrogen and hydroxide ion concentrations are equal. Since their product equals Kw (10−14), each ion concentration is the square root of Kw, which equals 10−7 M. This equal concentration defines a neutral solution. In acidic or basic solutions, the concentrations differ, but their product still equals Kw, allowing you to calculate one ion concentration from the other.

Solution nature depends on hydrogen and hydroxide ion concentrations relative to 10−7 M at 25°C. A neutral solution has equal concentrations of both ions at 10−7 M. If hydrogen ion concentration exceeds 10−7 M, the solution is acidic; if hydroxide ion concentration exceeds 10−7 M, the solution is basic. These thresholds directly relate to the ionic product of water and enable classification of aqueous solutions.

Per Le Chatelier's principle, if the product of hydrogen and hydroxide ion concentrations exceeds Kw, excess ions combine to form water molecules until equilibrium is restored. Conversely, if the product falls below Kw, water ionizes to produce more ions. This self-correcting mechanism maintains the ionic product of water constant at a given temperature, ensuring equilibrium in aqueous solutions.

Water is a weak electrolyte because only a small amount of water molecules ionize into hydrogen and hydroxide ions at any given temperature. The concentration of undissociated water remains nearly constant, allowing Kw to be expressed simply as the product of ion concentrations. This limited ionization distinguishes water from strong electrolytes, which completely dissociate in solution.

Kw remains constant at a given temperature across all aqueous solutions, including acidic, basic, and neutral ones. In neutral solutions, hydrogen and hydroxide concentrations are equal at 10−7 M. In acidic or basic solutions, these concentrations are unequal, but their product still equals Kw. This constant product allows determination of one ion concentration from the other in any solution.