Potassium reacting with water. Potassium (K) is a highly reactive metallic element. On contact with water it undergoes an exothermic (heat-producing) reaction to form potassium hydroxide (KOH) and hydrogen gas (H2). The reaction's equation is: 2 K + 2 H2O ----> 2 KOH + H2 The purple flame is produced as the high temperature causes the hydrogen gas to ignite. The purple colour is produced by small amounts of vaporised potassium in the flame. Potassium metal is stored under oil to prevent it from reacting in this way with atmospheric water.