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Limewater is the common name for a saturated solution of calcium hydroxide. Calcium hydroxide, Ca(OH)2 is sparsely soluble in water (1.5 g/L at 25 °C.) Pure limewater is clear and colorless, with a slight earthy smell and an alkaline bitter taste of calcium hydroxide. The term lime refers to the alkaline mineral, and is unrelated to the acidic fruit.
Limewater is prepared by stirring excess calcium hydroxide in pure water, and filtering off the excess insoluble Ca(OH)2. When excess calcium hydroxide is added to limewater, a suspension of calcium hydroxide particles remains, giving it a milky aspect, in which case it has the common name of milk of lime. Milk of lime is an alkaline solution with a pH of 12.3.
Carbon dioxide passes through limewater, giving a milky solution. This is due to the insoluble suspension of calcium carbonate formed:
If excess CO2 is added, the following reaction takes place:
Waste gases from industries containing sulfur dioxide can be cleaned by bubbling through limewater, a process called sulfation, in which the toxic sulfur dioxide is trapped as a precipitate:
It is also used in industry as a neutralizing agent in municipal waste water treatment.
Limewater is widely used by marine aquarists as a primary supplement of calcium and alkalinity for reef aquariums. Corals of order Scleractinia build their endoskeletons from aragonite (a polymorph of calcium carbonate). When used for this purpose, limewater is usually referred to as Kalkwasser. It is also used in tanning and making parchment. The lime is used as a dehairing agent based on its alkaline properties.