Q. pH of water is 7 at 25°C. If water is heated to 80°C., it's pH will :
A) Decrease
B) H⁺ concentration increases, OH⁻ concentration decreases
C) Remains the same
D) Increase
Explanation
[Solution by Dr. Abubacker] Ionisation of water is endothermic, so heating increases $K_w$. [H⁺] rises, and since pH = −log[H⁺] the pH falls below 7 (although water remains neutral, as [H⁺] = [OH⁻]). Answer (1).
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.