JEE Chemistry 2025 Question with Solution

Given: pH of water is $7$ at $25^\circ \text{C}$, and water is heated to $80^\circ \text{C}$.

Find: How the pH changes on heating.

Water dissociates as

At $25^\circ \text{C}$, the concentrations of $\text{H}^+$ and $\text{OH}^-$ are each $10^{-7} \, \text{mol/L}$, so the pH is $7$.

As temperature increases, the ionization of water increases. Therefore, more $\text{H}^+$ and more $\text{OH}^-$ ions are produced. Since the concentration of $\text{H}^+$ increases, the numerical value of pH decreases.

This does not mean the water becomes acidic; neutrality still means equal concentrations of $\text{H}^+$ and $\text{OH}^-$.

Therefore, the pH of water decreases when heated to $80^\circ \text{C}$.

The correct option is A.

Given: Pure water is neutral at $25^\circ \text{C}$ with pH $7$.

Find: The effect of raising temperature to $80^\circ \text{C}$ on pH.

The key idea is that dissociation of water increases with temperature. So at higher temperature, both $\text{H}^+$ and $\text{OH}^-$ concentrations increase equally.

Because pH is related to $\text{H}^+$ concentration, an increase in $\text{H}^+$ causes pH to shift to a lower value.

Hence, the pH will decrease, even though the water remains neutral because $[\text{H}^+] = [\text{OH}^-]$ still holds.

Therefore, the correct option is A.