"The Bronsted and Lowry's acid theory says that an acid loses a H+ ion that recieved by a base to make the conjugate of the acid and the base. By the theory when acid is introduced to water the disassociation reaction is really an acid-base reaction. Water becomes the hydronium ion that is acidic and acid becomes it's own conjugate base because the minus ion is basic. But when a base disassociates in water the water acts like the acid because it gives a hydrogen ion to the base turning it into acid and the water becomes a basic OH- ion. So the water can act as both an acid or a base according to the theory."