The pH of neutral water at 25°C is 7.0. As the temperature increases, ionisation of water increases, however, the concentration of H⁺ ions and OH^– ions are equal. What will be the pH of pure water at 60°C?

Question

The pH of neutral water at 25°C is 7.0. As the temperature increases, ionisation of water increases, however, the concentration of H⁺ ions and OH^– ions are equal. What will be the pH of pure water at 60°C?

Philoid · Accepted Answer

• The formation of hydrogen ions (hydroxonium ions) and hydroxide ions from water is an endothermic process. Using the simpler version of the Equilibrium :

H₂O (l) ⇋ H⁺ (aq.) + OH^- (aq.)

Then the ionic product of water K_w = [H⁺] [OH^-]

Again, pK_w = pH + pOH (by taking negative log ; -logx=px)

o Hence at 25°C [H⁺] =[OH^-] = 10⁷ (as pH=-log[H+] = 7 )

K_w = [H+] [OH-] = 10^-14

When temperature increases K_w increases at 60°C K_w >10^-14

Therefore, [H⁺] [OH^-] >10^-14

Or [H⁺] > 10^-7

Or pH <7.

The pH of neutral water at 25°C is 7.0. As the temperature increases, ionisation of water increases, however, the concentration of H+ ions and OH– ions are equal. What will be the pH of pure water at 60°C?

The pH of neutral water at 25°C is 7.0. As the temperature increases, ionisation of water increases, however, the concentration of H⁺ ions and OH^– ions are equal. What will be the pH of pure water at 60°C?