Defining Acids

According to the Bronsted-Lowry theory of acids and bases, an acid is a proton donor whilst a base is a proton acceptor. In water, an acid will donate a proton to the water molecule to form its conjugate base. Also, water is acting as a proton acceptor, that is, water is acting as a base and forming its conjugate acid.

HA_(aq) + H₂O_(l) -> H₃O⁺_(aq) + A^-_(aq)