`A overset("Calcination")rarr CaO + MgO + underset("Colourless gas")((B))`
When the colourless gas `(B)` is passed through lime water, initially a milky solution is obtained and on excess passage of gas `(B)` through the lime water, milkiness disappears.
Milkiness of lime water solution disappears due to the formation of :

Options

A. `CaCO_(3)`
B. `Ca(HCO_(3))_(2)`
C. `Ca(OH)_(2)`
D. `CaCl_(2)`

Explanation

To solve the problem step by step, let's analyze the information provided and the chemical reactions involved.

### Step 1: Identify the reaction taking place during calcination
- The question states that compound A undergoes calcination to produce calcium oxide (CaO), magnesium oxide (MgO), and a colorless gas (B).
- From the context, we can deduce that compound A is likely a carbonate, specifically dolomite (CaCO3·MgCO3), which decomposes upon heating.

### Step 2: Write the balanced equation for calcination
- The calcination of dolomite can be represented as:
\[ \text{CaCO}_3 \cdot \text{MgCO}_3 \xrightarrow{\text{heat}} \text{CaO} + \text{MgO} + \text{CO}_2 \uparrow \]
- Here, the colorless gas (B) is identified as carbon dioxide (CO2).

### Step 3: Understand the reaction of CO2 with lime water
- Lime water is a saturated solution of calcium hydroxide (Ca(OH)2). When carbon dioxide is passed through lime water, it reacts to form calcium carbonate (CaCO3), which is insoluble and gives a milky appearance to the solution:
\[ \text{Ca(OH)}_2 + \text{CO}_2 \rightarrow \text{CaCO}_3 \downarrow + \text{H}_2\text{O} \]

### Step 4: Analyze the effect of excess CO2
- When excess carbon dioxide is passed through the lime water, the calcium carbonate formed initially reacts with more carbon dioxide to form calcium bicarbonate (Ca(HCO3)2), which is soluble in water:
\[ \text{CaCO}_3 + \text{CO}_2 + \text{H}_2\text{O} \rightarrow \text{Ca(HCO}_3)_2 \]
- The formation of calcium bicarbonate leads to the disappearance of the milky solution.

### Conclusion
- Therefore, the milkiness of lime water solution disappears due to the formation of calcium bicarbonate (Ca(HCO3)2).

### Final Answer
The milkiness of lime water solution disappears due to the formation of calcium bicarbonate.

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