Question

When ${\rm{C}}{{\rm{O}}_2}$ is passed into lime water, white turbidity appears which dissolves in excess of ${\rm{C}}{{\rm{O}}_2}$.
Answer whether the above statement is true or false.
(A) True
(B) False

Answer 1

As we know that calcium hydroxide is also known as lime water and it is formed from calcium oxide and water with exothermic reaction. So, when ${\rm{C}}{{\rm{O}}_2}$ is passed into lime water, it forms colloids and is insoluble in water.

Complete answer:
Lime water is represented by the formula ${\rm{Ca}}{\left( {{\rm{OH}}} \right)_{\rm{2}}}$ and is known as calcium hydroxide. When ${\rm{C}}{{\rm{O}}_2}$ is passed into lime water, it reacts and forms calcium carbonate as we can see in the below reaction:
${\rm{Ca}}{\left( {{\rm{OH}}} \right)_{\rm{2}}}\,{\rm{(aq)}}\,{\rm{ + }}\,{\rm{C}}{{\rm{O}}_{\rm{2}}}\,{\rm{(g)}}\, \to \,{\rm{CaC}}{{\rm{O}}_{\rm{3}}}{\rm{(s)}}\,{\rm{ + }}\,{{\rm{H}}_{\rm{2}}}{\rm{O}}\,{\rm{(l)}}$

The colour of calcium carbonate is white and it is insoluble in water hence the solution becomes milky and turbidity appears in the solution. The size of the particle ranges in colloidal state.

When excess of ${\rm{C}}{{\rm{O}}_2}$ is passed to calcium hydroxide or lime water then it forms calcium bicarbonate which is soluble in water and has colourless solution. As we can see in the below reaction-
${\rm{Ca}}{\left( {{\rm{OH}}} \right)_{\rm{2}}}\,{\rm{(aq)}}\,{\rm{ + }}\,{\rm{C}}{{\rm{O}}_{\rm{2}}}\,{\rm{(excess)}}\,{\rm{(g)}}\, \ to \,{\rm{Ca(HC}}{{\rm{O}}_{\rm{3}}}{{\rm{)}}_2}{\rm{(aq)}}\,$

Hence the correct answer is option (A).

Note: The hardness of water is also due to the presence of bicarbonates of calcium and magnesium and as the concentration of calcium and magnesium increases the hardness increases. Hard water interferes with the action of soaps and detergents. There are a number of methods which are used to remove hardness of water.