Which of the following best describes Kw’s dependence on temperature?

• A. Kw is constant at a fixed temperature but changes with temperature.
• B. Kw is always 1 × 10^-14 regardless of temperature.
• C. Kw increases with temperature for all reactions.
• D. Kw decreases with temperature for all reactions.

Answer: (A)

Kw is the equilibrium constant for the autoionization of water, defined as Kw = [H+][OH−]. At any specific temperature, this product has a fixed value because the system sits at equilibrium. However, Kw is not a universal number; it depends on temperature. The value 1.0 × 10^−14 applies near 25°C, but Kw changes as temperature changes. For the autoionization of water, the process is endothermic, so increasing temperature shifts the equilibrium to produce more ions, making Kw larger at higher temperatures. Therefore, the correct description is that Kw is constant at a fixed temperature but changes with temperature. The other statements wrongly claim Kw is always the same number or that its temperature dependence applies universally to all reactions.