Due to the formation of insoluble calcium carbonate.
Explanation
Exhaled air contains a higher concentration of carbon dioxide ($CO_2$) as a byproduct of cellular respiration. Lime water is a saturated solution of calcium hydroxide [$Ca(OH)_2$]. The question tests the chemical reaction that occurs when these two substances interact.
Analysis of the Reaction
- Chemical Process: When carbon dioxide gas is passed through lime water, it reacts with calcium hydroxide to form calcium carbonate ($CaCO_3$).
- Cause of Milkiness: Calcium carbonate is insoluble in water. It forms a white precipitate that remains suspended in the solution, causing the clear lime water to turn milky or cloudy.
- Chemical Equation:
$Ca(OH)_2 (aq) + CO_2 (g) \rightarrow CaCO_3 (s) \downarrow + H_2O (l)$
(Lime Water + Carbon Dioxide $\rightarrow$ Calcium Carbonate + Water) - Analysis of Incorrect Options:
- Due to the formation of calcium hydroxide.: Calcium hydroxide is the reactant (lime water), not the product causing the change.
- Due to the dissolution of oxygen. & Due to the reaction with nitrogen.: Oxygen and nitrogen are present in exhaled air but do not react with lime water to form a precipitate.
Key Takeaway
The turning of lime water milky is the definitive chemical test for the presence of Carbon Dioxide. The milkiness is strictly due to the formation of insoluble Calcium Carbonate.