The ionization (dissociation) of water is described by an equilibrium ex- pression in which the concentration of the parent substance is in the denom- inator and the concentrations of the dissociated products are in the numerator: [2-1] K is the dissociation constant (here and throughout the text, quantities in square brackets symbolize the molar concentrations of the indicated sub- stances, which in many cases are only negligibly different from their activities; Section 1-3D). Because the concentration of the undissociated H 2 O ([H 2 O]) is so much larger than the concentrations of its component ions, it can be considered constant and incorporated into K to yield an expression for the ionization of water, [2-2] The value of K w , the ionization constant of water, is 10 ⫺14 at 25°C