When does water act as an acid and a base? water can donate a proton to become a hydronium ion or hydroxide ion. water can accept a proton to become a hydronium ion, or donate a proton to become a hydroxide ion. water can accept a proton to become a hydronium ion or a hydroxide ion. water can donate a proton to become a hydronium ion, or accept a proton to become a hydroxide ion.