Remember, water molecules can act as both acids and bases. And in fact, if you add two water molecules together, one will act as an acid and the second one will act as a base, creating a conjugate acid and a conjugate base. In other words, if this is our acid, it will donate the H, creating an oh ion. And if this is our base, it will accept that H creating a hydronium ion. So now, if we wanted to, we can also write the equilibrium equation for this reaction. In other words, kw, our ionization constant for water is equal to hydronium concentration times the hydroxide concentration.