An acid is a compound that can produce hydrogen ions (H+) in its aqueous solution. A H+ ion cannot exist independently; therefore, it combines with one water molecule to form a hydronium ion (H3O+). Thus, an acid is a compound that produces a hydronium ion (H3O+) as positive ion when dissolved in water.
CH3COOH + H2O ⇌ H3O+ + CH3COO-
Acetic acid Acetate ion