JEE Main 2025 Jan 23 Shift 2, Chemistry Q16: Ionization Of Acids And Bases

The self-ionisation of water, $H_{2}O\rightleftharpoons H^{+}+O{H}^{-}$, is endothermic. Raising the temperature shifts the equilibrium forward and increases $K_w$.

In pure water $[H^{+}]=[O{H}^{-}]=\sqrt{K_w}$. Since $K_w$ grows with temperature, $[H^{+}]$ also grows, and therefore pH $=-\log [H^{+}]$ decreases.

Note that the water remains neutral because $[H^{+}]$ and $[O{H}^{-}]$ rise together; only the numerical value of pH drops below 7.