`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.
`(A)` is :

Options

A. `CaCO_(3)`
B. `MgCO_(3)`
C. `CaCO_(3).MgCO_(3)`
D. `CaSO_(4).MgCO_(3)`

Explanation

To solve the question, we need to identify the substance (A) that undergoes calcination to produce CaO, MgO, and a colorless gas (B). We also need to analyze the behavior of gas (B) when passed through lime water.

### Step-by-Step Solution:

1. **Understanding Calcination**:
- Calcination is a thermal treatment process where a substance is heated to high temperatures in the absence of air or limited air. This process is commonly used to convert ores into oxides.

2. **Identifying the Products**:
- The products of calcination given in the question are CaO (calcium oxide) and MgO (magnesium oxide).

3. **Determining the Source of CaO and MgO**:
- Calcium oxide (CaO) is typically produced from the calcination of limestone (CaCO3), and magnesium oxide (MgO) can be produced from the calcination of magnesium carbonate (MgCO3).
- Therefore, the starting material (A) could be a mixture of calcium carbonate and magnesium carbonate.

4. **Identifying the Colorless Gas (B)**:
- When calcium carbonate (CaCO3) is heated, it decomposes to produce CaO and carbon dioxide (CO2) gas.
- Similarly, magnesium carbonate (MgCO3) also decomposes to produce MgO and carbon dioxide (CO2).
- Thus, the colorless gas (B) produced in this reaction is carbon dioxide (CO2).

5. **Analyzing the Reaction with Lime Water**:
- Lime water is a dilute solution of calcium hydroxide (Ca(OH)2). When carbon dioxide (CO2) is passed through lime water, it reacts to form calcium carbonate (CaCO3), which is insoluble and causes the solution to turn milky.
- The reaction is:
\[ Ca(OH)2 + CO2 \rightarrow CaCO3 (s) + H2O \]
- When excess CO2 is passed through the lime water, the milkiness disappears because the calcium carbonate formed reacts with more carbon dioxide to form soluble calcium bicarbonate:
\[ CaCO3 + CO2 + H2O \rightarrow Ca(HCO3)2 \]

6. **Conclusion**:
- Based on the above analysis, the substance (A) that undergoes calcination to produce CaO, MgO, and the colorless gas (B) is a mixture of calcium carbonate and magnesium carbonate.

### Final Answer:
(A) is a mixture of calcium carbonate (CaCO3) and magnesium carbonate (MgCO3).

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