The water ion product constant, also known as the dissociation constant of water, is a measure of the equilibrium between the dissociation of water molecules into hydrogen ions (H+) and hydroxide ions (OH-). It is represented by the equation Kw = [H+][OH-], where the square brackets indicate the concentration of each ion in moles per liter.
Kw is a fundamental constant in water chemistry as it determines the relative concentrations of hydrogen ions and hydroxide ions in a solution. It also affects the pH of a solution, with a higher Kw value indicating a more basic solution and a lower Kw value indicating a more acidic solution.
Kw is affected by temperature, with higher temperatures resulting in a higher value for Kw. This is because the dissociation of water molecules into ions is an endothermic process, meaning it requires heat. As the temperature increases, more water molecules are able to dissociate, resulting in a higher concentration of ions and a higher Kw value.
At 25°C, the pH of pure water is 7. This is because at this temperature, the concentration of hydrogen ions and hydroxide ions is equal, resulting in a neutral solution. Therefore, the Kw value at 25°C is equal to 10-7 (since [H+] = [OH-] = 10-7 moles per liter), and the negative logarithm of this value gives a pH of 7.
The presence of other substances in water, such as acids or bases, can affect the value of Kw. This is because these substances can react with water molecules, altering the concentrations of hydrogen and hydroxide ions and therefore changing the Kw value. Additionally, the presence of dissolved salts can also affect the dissociation of water molecules and the resulting Kw value.