An unknown gas is likely to be carbon dioxide if it turns limewater milky.
Option a
Calcium Hydroxide also known as lime [tex]Ca(OH)_2[/tex] is a sparingly soluble compound which when dissolved in water produces an alkaline solution known as lime water. When a gas such as carbon dioxide [tex]( CO_2 )[/tex] passes through this alkaline solution, it turns lime water milky due to the formation of an insoluble suspension of calcium carbonate [tex]( CaCO_3 )[/tex]. This can be understood by the following chemical reaction-
[tex]Ca(OH)_2 (aq) + CO_2 (g) \rightarrow CaCO_3 (s) + H_2O (l)[/tex]
If an excess amount of [tex]CO_2[/tex] gas is added in the solution, the following reaction takes place:
[tex]CaCO_3 (s) + H_2O (l) + CO_2 (g) \rightarrow Ca(HCO_3)_2 (aq)[/tex]
At this stage, the milkiness of the solution disappears since "calcium bicarbonate is soluble in water".
