Which of the following substance on being heated with give a gas that turns limewater milky?

Options

A. `Na_(2)CO_(3)`
B. `ZnCO_(3)`
C. `ZnSO_(3)`
D. `MgCO_(3)`

Answer

To determine which of the given substances, when heated, produces a gas that turns limewater milky, we need to analyze each option based on their thermal decomposition reactions.

### Step-by-Step Solution:

1. **Understanding Limewater**:
- Limewater is a dilute solution of calcium hydroxide (Ca(OH)₂). It turns milky when carbon dioxide (CO₂) or sulfur dioxide (SO₂) is bubbled through it, forming calcium carbonate (CaCO₃) or calcium sulfite (CaSO₃), both of which are white precipitates.

2. **Analyzing Each Substance**:
- **Option A: Na₂CO₃ (Sodium Carbonate)**:
- Sodium carbonate is thermally stable and does not decompose upon heating to release CO₂. Therefore, it will not turn limewater milky.

- **Option B: ZnCO₃ (Zinc Carbonate)**:
- Upon heating, zinc carbonate decomposes to form zinc oxide (ZnO) and carbon dioxide (CO₂):
\[ \text{ZnCO}_3 \rightarrow \text{ZnO} + \text{CO}_2 \uparrow \]
- The CO₂ produced will turn limewater milky.

- **Option C: ZnSO₃ (Zinc Sulfite)**:
- Heating zinc sulfite results in the formation of zinc oxide (ZnO) and sulfur dioxide (SO₂):
\[ \text{ZnSO}_3 \rightarrow \text{ZnO} + \text{SO}_2 \uparrow \]
- The SO₂ produced will also turn limewater milky.

- **Option D: MgCO₃ (Magnesium Carbonate)**:
- Magnesium carbonate decomposes upon heating to form magnesium oxide (MgO) and carbon dioxide (CO₂):
\[ \text{MgCO}_3 \rightarrow \text{MgO} + \text{CO}_2 \uparrow \]
- The CO₂ produced will turn limewater milky.

3. **Conclusion**:
- The substances that produce gases which turn limewater milky upon heating are:
- **ZnCO₃ (Zinc Carbonate)**
- **ZnSO₃ (Zinc Sulfite)**
- **MgCO₃ (Magnesium Carbonate)**

Thus, the correct options are **B, C, and D**.