The ionic product for water is the product of the molar concentrations of hydrogen ions (H+) and hydroxide ions (OH-) in water. It is expressed as Kw and is a measure of the acidity or basicity of water. At 25°C, the Kw for water is 1.0 x 10^-14. This means that the concentration of H+ ions and OH- ions in pure water at 25°C is equal to 1.0 x 10^-7 moles per liter.