Reaction of water and calcium oxide. Water being added from a pipette to calcium oxide (white). Known as unslaked lime, the addition of water produces calcium hydroxide (hydrated lime or slaked lime). The reaction is vigorous and produces a lot of heat, and must be carried out with care. Here, steam is seen rising from the hot reaction mixture. Calcium oxide is obtained from limestone (calcium carbonate) by heating to liberate carbon dioxide. Both calcium oxide and calcium hydroxide are strong bases (alkalis).