Question

When excess carbon dioxide is passed through lime water, the milkiness first formed disappears due to-
[A] the reversible reaction taking place
[B] formation of water soluble calcium bicarbonate
[C] huge amount of heat evolved during the reaction
[D] formation of a water soluble complex of calcium.

Answer 1

Hint: Limewater is a compound which contains calcium and reacts with carbon dioxide at room temperature. It forms different products depending on the amount of carbon dioxide passed through it. In presence of $C{{O}_{2}}$, it forms a milky complex which is insoluble in water but in presence of excess carbon dioxide, it forms a different product.

Complete step by step solution:
Lime water or dilute aqueous solution of calcium hydroxide, is prepared by mixing calcium hydroxide with water and removing excess undissolved solute by filtration.
If carbon dioxide is passed through lime water, a milky solution is formed due to precipitation of insoluble suspension of calcium carbonate:
$Ca{{(OH)}_{2}}(aq)+C{{O}_{2}}(g)\to CaC{{O}_{3}}(s)+{{H}_{2}}O(l)$
Calcium hydroxide + carbon dioxide gives calcium carbonate + water
The milky solution is formed as calcium hydroxide is not soluble in water, it precipitates forming a homogenous mixture of liquid and solid which can be separated when the solid sediments at the bottom of the flask.
However, if excess carbon dioxide is bubbled through the lime water solution, calcium bicarbonate is formed instead of calcium carbonate. Calcium bicarbonate is soluble in water and the mixture appears clear. The reaction is-
$CaC{{O}_{3}}(s)+{{H}_{2}}O(l)+C{{O}_{2}}(g)\to Ca{{(HC{{O}_{3}})}_{2}}$
Due to the formation of water soluble calcium bicarbonate, the milkiness of the solution disappears.
Therefore, the correct option is [B] formation of water soluble calcium bicarbonate.

Additional information:
This property of lime water is used in school laboratories for testing presence of carbon dioxide in gaseous samples and also for refining sugar by a process called carbonatation.

Note:
It is important to remember here that the product is clear due to the water soluble calcium bicarbonate. Option [D] could have been the correct option too if option [B] was not mentioned in the question. As option [B] states an explained answer therefore it is the correct option.