When small amount of acid is added to water at 25 °C then the Kw

Explanation

The ion product of water (Kw) is a constant at a given temperature, specifically 25 °C in this case. When an acid is added to water, the concentrations of H+ and OH- ions change, but the product of these concentrations, Kw, remains constant. This is because Kw is dependent solely on temperature and not on the presence of acids or bases. Therefore, the correct answer is that Kw remains constant. Options suggesting that Kw increases, decreases, or may vary are incorrect because they do not recognize the temperature-dependent nature of Kw.