• Pure water is a weak electrolyte and undergoes self-ionistion to a small extent.
• "The product of concentrations of H⁺ and OH^– ions in water at particular temperature is known as ionic product of water". It is designed as K_w.
• H₂O ⇌ H⁺ + OH^–; ΔH = +57.3 kJM^–1
• ; K[H₂O] = [H⁺][OH^–]; K_w = [H⁺][OH^–]
• The value of K_w increases with increase of temperature, i.e., the concentration of H⁺ and OH^– ions increases in temperature.
• The value of K_w at 25°C is 1×10^–14 mole/liter. Since pure water is natural in nature, H⁺ ions concentration must be equal to OH^– ion concentration.
• [H⁺] = [OH^–] = x or [ H⁺][OH^–] = x² = 1×10¹⁴ or x = 1×10^–7M or [H⁺] = [OH^–] = 1×10^–7 mole liter^–1
• This shows that 25°C, in liter only 10¹⁰ mole of water is in ionic form out of a total of approximately 55.5 moles.
• Thus when, [H⁺] = [OH^–]; the solution is natural
• [H⁺] > [OH^–]; the solution is acidic
• [H⁺] < [OH^–]; the solution is basic