Question

Write the chemical name of –
(a) Lime water
(b) The white insoluble solid formed on reaction of carbon dioxide with lime water.

Answer 1

Hint: The reaction of solution of lime water with carbon dioxide is used in the qualitative analysis of carbon dioxide gas. By this reaction, formation of a white precipitate is observed and on passing excess gas the precipitate dissolves.

Complete solution
(a) The chemical name of lime water is calcium hydroxide. The formula of calcium hydroxide can be denoted by $ Ca{(OH)_2} $ .
(b) In the qualitative analysis of carbon dioxide, we add dilute hydrochloric acid in the given salt and the evolution of briskness of effervescence can be seen.
Then for the confirmation of the gas, the evolved gas is passed through lime water ( $ Ca{(OH)_2} $ ) and a white precipitate is formed which turns the lime water milky.
The following reaction will justify the statement:
 $ Ca{(OH)_2} + C{O_2} \to CaC{O_3} + {H_2}O $
The white insoluble precipitate formed is known as calcium carbonate. And the formula can be represented as $ CaC{O_3} $ .
But when excess of carbon dioxide is passed through the resulting solution, the milkiness disappears from the solution and a clear solution is obtained by the formation of calcium bicarbonate.
The following reaction will justify:
 $ CaC{O_3} + {H_2}O + C{O_2} \to Ca{(HC{O_3})_2}_{(aq)} $
Calcium bicarbonate is soluble salt of calcium thus the precipitate slowly gets dissolved in the solution. It is also formed when the temporary hardness of the water has to be removed.
So calcium bicarbonate is also used to remove the temporary hardness of water.
Additional information: Sulphur dioxide reacts with lime water and forms an oily viscous liquid, known as calcium sulfite ( $ CaS{O_3} $ ).
 $ \begin{gathered}
  Ca{(OH)_2} + S{O_2} \to CaS{O_3} + {H_2}O \\
  CaS{O_3} + {H_2}O + S{O_2} \to Ca{(HS{O_3})_2} \\
\end{gathered} $ .

Note:
Calcium sulphide formed is the cause of oily appearance of liquid. Also it turns the lime water milky due to the formation of this calcium sulphite which is further a precipitate of white colour. It is also not corrosive in nature.