Question

Which of the following turns lime water milky?
A. CO
B. Carbon dioxide
C. Sulphur trioxide
D. None of the above

Answer 1

Hint: Limewater is the common name given for a dilute aqueous solution of calcium hydroxide, that is, \[\text{Ca(OH}{{\text{)}}_{2}}\] and it is sparsely soluble at room temperature in water. The reaction of this compound with a specific gas results in formation of calcium carbonate which has a turbid precipitate.

Complete Solution :
- Carbon dioxide is the gas that reacts with calcium hydroxide solution, limewater, to produce a white precipitate of calcium carbonate. Limewater is a solution of calcium hydroxide as mentioned above. When carbon dioxide is bubbled through the limewater solution, the limewater turns milky or cloudy white. Carbon dioxide which is also commonly known by its chemical formula $C{{O}_{2}}$, is one of the major building blocks of life on earth. $C{{O}_{2}}$ is consumed by plants in the process of photosynthesis and is produced by animals during respiration. Carbon dioxide is all around us and it is also one of the greenhouse gases which maintain the temperature of the earth. Following is the reaction which takes place when it is added to lime water.
\[\text{Ca(OH}{{\text{)}}_{2}}+\text{C}{{\text{O}}_{\text{2}}}\to \text{CaC}{{\text{O}}_{\text{3}}}\]

- When excess of the gas is passed through the solution the equilibrium shifts backwards and the solution turns colourless again.
So, the correct answer is “Option B”.

Note: In addition to carbon dioxide, sulphur dioxide also forms a milky precipitate with lime water. Here’s how we can differentiate between them. The mixture of carbon dioxide and sulphur dioxide is passed into an acidified potassium dichromate solution. Sulphur dioxide reacts with acidified potassium dichromate solution to form green coloured solution as potassium dichromate is reduced to chromic sulphate by the sulphur dioxide gas. When carbon dioxide gas reacts with potassium the solution turns reddish pink. Carbon dioxide gas is not affected by potassium dichromate.