Question

Which of the following turns lime water milky?
a) $CO$
b) Carbon dioxide
c) Sulphur trioxide
d) None of the above

Answer 1

The chemical formula of lime water should be known. The reaction of gas with lime water and the formula of the product is necessary. The chemical property of the product formed is important.

Complete step by step solution:
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:
$Ca{(OH)_2} + C{O_2} \to CaC{O_3} + {H_2}O$
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.
Carbon monoxide does not react with lime water.
Sulphur trioxide also do not react with lime water.
Thus only carbon dioxide turns lime water milky.
Hence,The correct option is (c).

Note: Sulphur trioxide does not react with lime water but sulphur dioxide reacts with lime water and forms an oily viscous liquid.
$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} \\\$
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.