An acid is a proton (H⁺) donor.

A base is a proton (H⁺) acceptor.


A proton is the same as a hydrogen ion. A good way to think about that is to realise that a hydrogen atom is just one proton and zero neutrons surrounded by only one electron. If that atom becomes an ion by the removal of the electron, then only one proton is left.


When sulfuric acid reacts with copper (II) oxide (CuO):

Cu²⁺O²⁻ (s)         +         H₂SO₄ (aq)         →         Cu²⁺ (aq)         +         SO₄²⁻ (aq)         +         H₂O (l)

H₂SO₄ is an acid. It donates protons (H⁺) to CuO, the base.