Acting as either an acid or base Amphoteric water

Water can act eis either an acid or a base, depending on what it s combined with. Substances that can act as either an acid or a beise are called amphoteric. If you put water with an acid, it acts as a beise, 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. (See the eeirlier section Weak Ionizing partially for details on both reactions.) 

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 (which stands for water dissociation constant), is associated with this reaction. The has a value of 1.0 X 10 and has the following form  

In pure water, the [H3O ] equals the [OH ] from the balanced equation, so [H3O ] 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, K allows you to figure out the other one. 

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