Question

Lime water contains _________.
A. Calcium hydroxide
B. Calcium oxalate
C. Calcium hydride
D. Calcium sulfate

Answer 1

The term lime water has been used mostly in detecting the presence of carbon dioxide in the test tube or beaker as the carbon dioxide turns the lime water milky. This happens so because of the formation of calcium carbonate in the solution which forms a soluble white precipitate and can dissolve in the solution to give a milky white solution.

Complete step by step answer:
Limewater is the common name for a dilute aqueous solution of calcium hydroxide. Calcium hydroxide, $Ca{\left( {OH} \right)_2}$ , is sparsely soluble at room temperature in water ($1.5\;g/L{\text{ }}at{\text{ }}25\;^\circ C$ ). Pure (i.e. less than or fully saturated) limewater is clear and colorless, with a slight earthy smell and a bitter taste. It is basic in nature with a pH of $12.4$ .
When $C{O_2}$ is passed in the lime water, the lime water turns milky. The reaction is as follows:
$Ca{(OH)_{2(aq)}} + C{O_{2(g)}} \to CaC{O_{3(s)}} + {H_2}{O_{(l)}}$
The formation of calcium carbonate makes the lime water milky. If excess $C{O_2}$ is added then the following reaction takes place:
$CaC{O_{3(s)}} + {H_2}{O_{(l)}} + C{O_{2(g)}} \to Ca{(HC{O_3})_{2(aq)}}$
The milkiness of the solution disappears since calcium bicarbonate is water-soluble.

So, the correct answer is Option A .

Note:
Limewater may be prepared by mixing calcium hydroxide ($Ca{\left( {OH} \right)_2}$ ) with water and removing excess undissolved solute (e.g. by filtration). When excess calcium hydroxide is added (or when environmental conditions are altered, e.g. when its temperature is raised sufficiently), a milky solution results due to the homogeneous suspension of excess calcium hydroxide. This liquid has been known traditionally as milk of lime.