Potassium metal reacting explosively as it is dropped in water. Potassium reacts with water to form soluble potassium hydroxide and hydrogen gas, which ignites with a lilac flame due to the heat of the reaction. The water turns pink due to the presence of the indicator phenolphthalein, which turns pink in alkaline solutions, such as that of potassium hydroxide. Potassium is an alkali earth metal, all of which are highly reactive due to their having an unpaired electron in their outer shell.