When `CO_2` is passed through lime water , the milkiness first formed is due to the formation of ………………….

Options

A. the reversible reaction taking place
B. formation of water soluble calcium bicarbonate
C. huge amount of heat evolved during the reaction
D. formation of water soluble complex of calcium

Explanation

To solve the question regarding the formation of milkiness when carbon dioxide (CO₂) is passed through lime water, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Lime Water**: Lime water is a dilute solution of calcium hydroxide (Ca(OH)₂) in water. It is commonly used to test for the presence of carbon dioxide.

2. **Reaction with Carbon Dioxide**: When carbon dioxide gas is bubbled through lime water, a chemical reaction occurs. The carbon dioxide reacts with calcium hydroxide to form calcium carbonate (CaCO₃) and water (H₂O).

\[ \text{Ca(OH)}_2 (aq) + \text{CO}_2 (g) \rightarrow \text{CaCO}_3 (s) + \text{H}_2O (l) \]

3. **Formation of Calcium Carbonate**: Calcium carbonate is a solid that is not soluble in water. As it forms, it creates a suspension in the solution, which appears milky.

4. **Milkiness Due to Precipitate**: The milkiness observed in the lime water is due to the formation of this insoluble calcium carbonate precipitate. The solid particles scatter light, giving the solution a milky appearance.

5. **Further Reaction**: If excess carbon dioxide is added, the calcium carbonate can react with more CO₂ to form calcium bicarbonate (Ca(HCO₃)₂), which is soluble in water. However, the initial milkiness is specifically due to the formation of calcium carbonate.

### Final Answer:
The milkiness first formed when CO₂ is passed through lime water is due to the formation of **calcium carbonate (CaCO₃)**.

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