Question

\[C{O_2}\] turns lime water milky, due to the formation of:
a.) \[Ca{\left( {HC{O_3}} \right)_2}\]
b.) \[CaC{O_3}\]
c.) \[Ca{(OH)_2}\]
d.) \[CaO\]

Answer 1

Hint: We should remember that this reaction of lime water turning milky in presence of Carbon dioxide is one of the surest tests for its presence. A white precipitate must form in this reaction which leads to the milkiness of lime water.

Complete step by step answer:
We already know that carbon dioxide reacts with lime water to form calcium carbonate, which precipitates out of the solution. This reaction between lime water, which is actually a solution of calcium hydroxide \[Ca{(OH)_2}\]and carbon dioxide, is found to result in the formation of an insoluble solid called calcium carbonate \[CaC{O_3}\]. The chemical reaction of the same is illustrated below:
 \[Ca{(OH)_2} + C{O_2} \to CaC{O_3} + {H_2}O\]
This solution will turn milky due to the fact that calcium carbonate is the white precipitate we were trying to determine.

But we should also note that if we continue to further bubble the carbon dioxide through the limewater another acid base reaction would occur which would result in the precipitate of calcium carbonate dissolving to give soluble calcium hydrogen carbonate.
This reaction is:
\[C{O_2} + {H_2}O \to CaC{O_3} + Ca{\left( {HC{O_3}} \right)_2}\]
This is also the reason why naturally occurring acid rain is able to chemically erode limestone resulting in cave formation.
When this solution evaporates in summers the reverse reaction occurs resulting in the formation of stalactites and stalagmites which are white in colour.

Hence, the correct answer is Option (B) \[CaC{O_3}\].

Note: While testing for Carbon dioxide, we must remember to perform the acidified Potassium dichromate test after the lime water test as both Sulphur dioxide and Carbon dioxide turns lime water milky but Sulphur dioxide solution turns acidified potassium chromate solution green whereas carbon dioxide shows no change.