Question

What happens when carbon dioxide gas is bubbled through lime water:
(i) In small amounts?
(ii) In excess?

Answer 1

Hint: The reaction which occurs when carbon dioxide is passed through lime water in a small amount is nullified when excess amount of carbon dioxide is passed. This will provide the basis for the compounds formed in both these reactions. The compound formed when small amounts of carbon dioxide is passed through lime water is water-insoluble, while the compound formed when excess amounts of carbon dioxide is passed is water-soluble.

Complete answer:
Lime water is an aqueous solution of calcium hydroxide $\left( {Ca{{\left( {OH} \right)}_2}} \right)$ which is colourless and clear liquid. When carbon dioxide is passed through lime water, the amount of gas passed matters as different compounds are formed. The interaction between carbon dioxide and lime water can be represented as follows: -
(i) When carbon dioxide is passed in small amounts:
 On bubbling Carbon dioxide in clear lime water in small amounts, the clear solution turns milky due to the formation of calcium carbonate $\left( {CaC{O_3}} \right)$ . It becomes a suspension, as calcium carbonate is the white precipitate formed which is insoluble in water.
\[Ca{\left( {OH} \right)_2}(aq)\; + {\text{ }}C{O_2}(g)\; \to {\text{ }}CaC{O_3}(s)\; + {\text{ }}{H_2}O(l)\]
(ii) When carbon dioxide is passed in excess:
Now, on passing excess of carbon dioxide through the milky suspension, the solution turns clear due to the formation of calcium carbonate \[Ca{\left( {HC{O_3}} \right)_2}\] , which is soluble in water.
\[CaC{O_3}(s)\; + {\text{ }}{H_2}O(l)\; + {\text{ }}C{O_2}(g)\; \to \;Ca{\left( {HC{O_3}} \right)_2}(aq)\]

Note:
The interaction between carbon dioxide and lime water is an important one as it is used for testing the presence of carbon dioxide gas in organic qualitative analysis and so on. The change in the appearance of the solution is very noticeable, and hence it is a good test.