Question

Explain why?
Lime water turns milky, when carbon dioxide gas is passed into it.

Answer 1

Hint- In order to understand the reason behind the turning white of lime water, we will first see the chemical composition of lime water and then we will see the reaction between the lime water and the carbon dioxide and the nature of the chemical compound formed to understand the nature and chemical reason behind it.

Complete answer:

The chemical name of lime water is calcium hydroxide \[Ca{\left( {OH} \right)_2}\]
The reaction between lime water, which is a solution of calcium hydroxide \[Ca{\left( {OH} \right)_2}\] , and carbon dioxide $C{O_2}$ will result in the formation of an insoluble solid called calcium carbonate $CaC{O_3}$ .
\[Ca{\left( {OH} \right)_2}\left( {aq} \right) + C{O_2}\left( {aq} \right) \to CaC{O_3}\left( s \right) \downarrow + {H_2}O\left( l \right)\]
The solution will turn milky due to the fact that calcium carbonate is a white precipitate. This reaction is actually used to test for the presence of carbon dioxide.
If you continue to bubble the carbon dioxide through the limewater another acid - base reaction occurs which results in the precipitate dissolving to give soluble calcium hydrogen carbonate:
$C{O_2}\left( g \right) + {H_2}O\left( l \right) + CaC{O_3}\left( s \right) \to Ca{\left( {HC{O_3}} \right)_2}\left( {aq} \right)$

Note- An unknown gas is bubbled through a solution of calcium hydroxide - if the solution turns cloudy, then the unknown gas is carbon dioxide. This is used for testing of carbon dioxide. The process of formation of calcium hydrogen carbonate or bicarbonate is how naturally occurring acid rain is able to chemically erode limestone resulting in cave formation.