When carbon dioxide is passed through lime water, it turns milky due to the formation of:

The correct option is

Calcium Carbonate

Explanation

The chemical reaction between a base (lime water) and an acidic oxide (carbon dioxide). This is a standard laboratory test used to detect the presence of carbon dioxide gas.

Detailed Analysis:
Lime water is a saturated solution of Calcium Hydroxide [Ca(OH)₂]. When carbon dioxide (CO₂) gas is passed through this solution, a chemical reaction occurs forming Calcium Carbonate (CaCO₃).

The chemical equation for the reaction is:
Ca(OH)₂ (aq) + CO₂ (g) → CaCO₃ (s) + H₂O (l)

Calcium Carbonate is insoluble in water and forms a white precipitate. The suspension of these white solid particles in the solution gives it a milky or cloudy appearance.

Distinction from Calcium Bicarbonate:
If carbon dioxide is passed through the lime water for an extended period (in excess), the milkiness disappears. This happens because the insoluble Calcium Carbonate reacts further with the excess CO₂ and water to form Calcium Bicarbonate [Ca(HCO₃)₂], which is soluble in water.

Key Takeaway:
The "milky" appearance of lime water upon contact with carbon dioxide is caused by the formation of the insoluble precipitate Calcium Carbonate (CaCO₃).