Q.How pH of pure water vary with temperature ? Explain.

Step 1. Water's own ionization, $H_2O(l)+H_2O(l)\rightleftharpoons H_3O^+(aq)+OH^-(aq)$, is an equilibrium, and like any equilibrium its constant Kw is temperature-dependent.

Step 2. This ionization is an endothermic process (it requires energy to break an O-H bond and separate the resulting ions), so by Le Chatelier's principle, raising the temperature shifts the equilibrium further toward the ionized (H3O+ + OH-) side, increasing Kw.

Step 3. Reference values illustrate the trend: Kw is roughly $0.11\times10^{-14}$ at 273 K but climbs to about $5.47\times10^{-14}$ by 323 K -- a nearly fifty-fold increase.

Step 4. Since in pure water $[H_3O^+]=[OH^-]=\sqrt{K_w}$, a larger Kw means a larger [H3O+], and since $pH=-\log_{10}[H_3O^+]$, a larger [H3O+] means a SMALLER (lower) pH. So pure water's pH actually drops below 7 as it is heated. …

Apply Le Chatelier's principle to water's endothermic autoionization equilibrium, then translate the resulting r …

• Assuming pure water's pH is always exactly 7 regardless of temperature -- 7 is specific to 298 K, where Kw = 1.0e-14. …