Will the pH of pure water be the same at $4^\circ ext{C}$ and $25^\circ ext{C}$? Select the correc

Question

Will the pH of pure water be the same at $4^\circ	ext{C}$ and $25^\circ	ext{C}$? Select the correct statement with the appropriate reason.

Professor PC Thomas Classes · Answer

The ionization of water, $	ext{H}_2	ext{O}(l) ightleftharpoons 	ext{H}^+(aq) + 	ext{OH}^-(aq)$, is an endothermic process ($\Delta H > 0$).
According to Le Chatelier's principle, as temperature decreases from $25^\circ	ext{C}$ to $4^\circ	ext{C}$, the equilibrium shifts backward, meaning the degree of ionization decreases.
Thus, at lower temperatures, the concentration of $	ext{H}^+$ decreases.
Since $K_w = [	ext{H}^+][	ext{OH}^-]$ decreases at $4^\circ	ext{C}$, $[	ext{H}^+]$ becomes less than $10^{-7}	ext{ M}$.
Therefore, $	ext{pH} = -\log[	ext{H}^+]$ will be greater than $7$ at $4^\circ	ext{C}$. The pH of pure water is not the same at these two temperatures.