Lime water is the common name for saturated calcium
hydroxide solution. It is sparsely soluble. Its chemical formula is Ca(OH)2.
Since calcium hydroxide is only sparsely soluble, i.e. ca. 1.5 g per liter at
25 °C,[1] there is no visible distinction to clear water (homogeneous).
Attentive observers will notice a slightly earthy smell. It is clearly
distinguishable by the alkaline taste of the calcium hydroxide. The term lime
refers to the mineral, rather than the fruit. When exposed with carbon dioxide,
limewater turns into a milky solution.
While limewater is a clear solution, milk of lime on the
other hand is a suspension of calcium hydroxide particles in water. These
particles give it the milky aspect. It is commonly produced by reacting quicklime
(calcium oxide) with an excess of water - usually 4 to 8 times the amount of
water to the amount of quicklime. Reacting water with quicklime is sometimes
referred to as "slaking" the lime. The calcium oxide will convert to
the hydroxide according to the following reaction scheme:
CaO + H2O → Ca(OH)2
This reaction is strongly exothermic and will generate
enough heat to bring the suspension to a scalding temperature. At a ratio of 2
parts water to 1 part lime (by weight), the generated heat is sufficient to
bring the suspension, i.e. the water in it, to boil.
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