JEE Main 2025 — 23 January, Evening Shift — Question 39

Auto-ionization of water: $\text{H}_2\text{O} \rightleftharpoons \text{H}^+ + \text{OH}^-$ is endothermic. Heating from 25°C to 80°C increases the equilibrium constant $K_w$. At 25°C: $K_w = 1 \times 10^{-14}$, $[\text{H}^+] = 10^{-7}$ M, pH = 7. At 80°C: $K_w > 1 \times 10^{-14}$, so $[\text{H}^+] = [\text{OH}^-] = \sqrt{K_w} > 10^{-7}$ M. pH = $-\log[\text{H}^+]$ decreases because $[\text{H}^+]$ increases. Thus, pH decreases when water is heated to 80°C.