Now let’s throw some of that hydrogen chloride we made earlier into water. What happens? The chlorine runs off with all the electrons, that’s what—after stealing them from hydrogen to fill out the outer shell, it’s not about to let them go now. We end up with a chloride ion and a hydrogen ion. The hydrogen ion, in reality, binds to the water to form a water molecule with an extra hydrogen—the hydronium ion. This hydronium ion, though, quickly breaks up into water and hydrogen ion, only to reform again and again. For all practical purposes, we have water, chloride and a bunch of homeless hydrogen ions, so the reaction is usually written as in Figure13(b).

Such a solution of hydrogen chloride in water is called hydrochloric acid. The acid part is the extra proton, donated by the HCl. There are many such hydrogen donors—molecules that donate hydrogen to ionize water. Acids are hydrogen donors.