pH of water is 7 at 25° C. If water is heated to 80° C., it's pH will :

Solution

Chemistry ma’am · 4 steps

Heat pushes water to ionise more, so [H⁺] rises and pH falls below 7.

1. Ionisation is endothermic. Breaking water into ions, H₂O ⇌ H⁺ + OH⁻, absorbs heat. By Le Chatelier's principle, heating shifts this equilibrium to the right, so K_w increases with temperature.

   K_w = [H⁺][OH⁻] = 1.0 × 10⁻¹⁴ at 25°C, larger at 80°C

2. Both ions rise equally. Each water molecule that ionises gives one H⁺ and one OH⁻. So [H⁺] = [OH⁻] = √K_w, and this value is now more than 10⁻⁷ mol L⁻¹.
3. Effect on pH. A larger [H⁺] means a smaller pH = -log[H⁺]. At 80°C the pH of pure water is about 6.3, yet the water is still neutral because [H⁺] = [OH⁻].
4. So. On heating to 80°C, the pH of water decreases, which is option (a).

The idea to keep. pH 7 means neutral only at 25°C. Neutral always means [H⁺] = [OH⁻], and since K_w rises with temperature, the neutral pH falls when water is heated.

Answer from the NTA final answer key. Solution by PrepWiser.