JEE Main · 2025 · 23 Jan 2025 · Shift 2 · Equilibrium

The self-ionisation of water, $\mathrm{H}{}_2\mathrm{O}\rightleftharpoons \mathrm{H}{}^{+}+\mathrm{OH}{}^{-}$, is endothermic, so raising the temperature shifts the equilibrium forward and increases $K_w$ above $1\times 10^{-14}$. Both $[\mathrm{H}{}^{+}]$ and $[\mathrm{OH}{}^{-}]$ rise equally, so the water remains neutral, but $[\mathrm{H}{}^{+}]$ is now larger than $10^{-7}\mathrm{M}$. Since $\mathrm{pH}=-\log [\mathrm{H}{}^{+}]$, a larger $[\mathrm{H}{}^{+}]$ gives a smaller pH — at 80 °C the pH drops to roughly 6.3–6.5 even though the water is still neutral. The pH decreases on heating.