the drain cleaner is added into water & a drain cleaner consists of sodium hydroxide and aluminum ->  sodium hydroxide, aluminum, and water are combined  sodium hydroxide, aluminum, and water are combined & hydrogen is produced ->  hydrogen is produced when sodium, aluminum, and water are combined  sodium hydroxide, aluminum, and water are combined & the solution becomes really hot ->  heat energy is released when sodium hydroxide, aluminum, and water are combined  hydrogen is produced when sodium, aluminum, and water are combined & heat energy is released when sodium hydroxide, aluminum, and water are combined -> a chemical reaction happened when sodium hydroxide, aluminum, and water are combined