Acting as either an acid or base: Amphoteric water

Water can act as either an acid or a base, depending on what it’s combined with. Substances that can act as either an acid or a base are called amphoteric. If you put water with an acid, it acts as a base, and vice versa. For instance, when acetic acid reacts with water, water acts as a base, or a proton acceptor. But in the reaction with ammonia, water acts as an acid, or a proton donor.

But can water react with itself? Yes, it can. Two water molecules can react with each other, with one donating a proton and the other accepting it:

This reaction is an equilibrium reaction. A modified equilibrium constant, called the Kw (which stands for water dissociation constant), is associated with this reaction. The Kw has a value of 1.0 × 10^–14 and has the following form:

1.0 × 10^–14 = Kw = [H₃O⁺] [OH^–]

In pure water, the [H₃O+] equals the [OH–] from the balanced equation, so [H₃O⁺] = [OH^–] = 1.0 × 10^–7. The Kw value is a constant. This value allows you to convert from [H⁺] to [OH^–], and vice versa, in any aqueous solution, not just pure water. In aqueous solutions, the hydronium ion and hydroxide ion concentrations are rarely going to be equal. But if you know one of them, Kw allows you to figure out the other one.

Take a look at the 2.0 M acetic acid solution problem in the section “Acetic acid and other weak acids,” earlier in this chapter. You find that the [H₃O⁺] is 6.0 × 10^–3. Now you have a way to calculate the [OH^–] in the solution by using the Kw relationship: