What is the pH of pure water at 25°C, and which factor can change this value?

• A. pH 7.00; Changes with humidity
• B. pH 7.00; Changes with atmospheric pressure
• C. pH 7.00; Changes with dissolved salts
• D. pH 7.00; Changes with temperature

Answer: (D)

The pH of pure water at a given temperature is set by the ion product of water, Kw, which depends on temperature. At 25°C, Kw is 1.0 × 10^-14, so [H3O+] and [OH−] are both 1.0 × 10^-7 M, giving a neutral pH of 7.00. The factor that can change this value is temperature because Kw changes with temperature. As temperature increases, more water molecules ionize, increasing Kw. Since [H3O+] = [OH−] = sqrt(Kw), the hydronium concentration rises and the pH drops below 7; at lower temperatures, Kw decreases and the pH rises above 7. So temperature shifts the neutrality of water, making it the variable that changes the pH value.