When an acid reacts with water, it forms a hydronium ion plus the conjugate base of the original acid. Likewise, when a base reacts with water, it forms hydroxide ion plus the conjugate acid of the original base.
Using generic symbol HA for a weak acid, we can write the following balanced proton transfer reaction with water:
In the same way, we can write a balanced proton transfer reaction of a base,
For the same reasons as above, we say that