pH of Neutral Water at 65°C: Why It Drops Below 7 with Rising Kw

Kw Temperature Dependence

Water dissociates according to the equilibrium:

H₂O ⇌ H₃O⁺ + OH⁻

This dissociation is endothermic, meaning that as temperature rises, the equilibrium shifts toward increased ionization. As a result, Kw increases. For instance, at 65°C,
Kw ≈ 1.2 × 10⁻¹³, which is significantly higher than its value at 25°C.

Calculating pH at 65°C

For neutral water, the hydronium and hydroxide ion concentrations are equal:

[H₃O⁺] = [OH⁻] = √(Kw)

Substituting Kw = 1.2 × 10⁻¹³ gives:

[H₃O⁺] = 3.46 × 10⁻⁷ M

Therefore, pH = −log(3.46 × 10⁻⁷) ≈ 6.46 — which is slightly less than 7.
This means that even though pure water remains neutral ([H₃O⁺] = [OH⁻]), the pH value drops below 7 as temperature increases.

Option Analysis

• a. Slightly less than 14: Incorrect. A pH of 14 would imply an extremely low [H₃O⁺] concentration, not consistent with neutral conditions.
• b. Equal to 7: Incorrect at elevated temperatures. pH = 7 applies only when Kw = 1.0 × 10⁻¹⁴ (25°C).
• c. Slightly less than 7: Correct. Higher Kw increases both [H₃O⁺] and [OH⁻] equally, yielding a lower pH while maintaining neutrality (e.g., pH ≈ 6.56 at 65°C).
• d. Slightly more than 7: Incorrect. Rising temperature decreases, not increases, the pH of neutral water.