Milk of lime
Limewater is the common name for a diluted 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 calcium hydroxide in pure water and filtering off the excess undissolved Ca(OH)2. When excess calcium hydroxide is added to limewater, a suspension of calcium hydroxide particles results, giving it a milky aspect, in which case it has the common name of milk of lime. Milk of lime or a saturated solution of lime (limewater) has a pH of 12.3. It is basic in nature.
Carbon dioxide passed into limewater gives a milky solution. This is due to the insoluble suspension of calcium carbonate formed:
If excess CO2 is added, the following reaction takes place:
The milkiness disappears since calcium bicarbonate is water-soluble.
The above chemical properties are commonly used for testing the presence of carbon dioxide in gaseous samples in school laboratories, and refining of sugar in a process called carbonatation.
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:
Limewater is used in a process known as lime softening to reduce water hardness. It is also used as a neutralizing agent in municipal waste water treatment.
In buon fresco painting, limewater is used as the colour solvent to apply on fresh plaster. Historically, it is known as the paint whitewash.
Limewater is used in the preparation of maize for corn tortillas and other culinary purposes using a process known as nixtamalization. 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.
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