Neutralizing an acid. Milk of magnesia, a milky white suspension of magnesium hydroxide (Mg(OH)2) in water, is added to a beaker with 0.1 M solution of hydrochloric acid (HCl). The solution has few drops of methyl orange indicator added. Initially color is red, indicating an acidic solution (left frame). When milk of magnesia dissolves, it reacts with the acid. Solution color turns yellow, indicating that acid has been neutralized (right frame). The reaction is HCl + Mg(OH)2 -> MgCl2 + H2O. Milk of magnesia is used medicinally as an antacid, to neutralize gastric acid (contains 0.5-1% of hydrochloric acid).