Final answer:
Water can act as both an acid and a base. It can donate a proton to a base, forming hydronium ions. It can also accept a proton from an acid, forming hydroxide ions.
Step-by-step explanation:
Water is an amphoteric substance, which means it can act as both an acid and a base. As an acid, water can donate a proton (H+) to a base. For example, when reacting with a stronger base like NH3, water donates a proton and forms hydronium ions (H3O+). On the other hand, water can also act as a base by accepting a proton from a stronger acid. When reacting with a stronger acid like HCl, water accepts a proton and forms hydroxide ions (OH-).