On heating to `400-500^(@)C`, relatively unstable hydrides and carbonates decompose. Which of the following will decompose when heated to `400-500^(@)C`?
`(I)LiH` `
(II)NaH` `
(III)Li_(2)CO_(3)` `
(IV)Na_(2)CO_(3)`

Options

A. `II, III`
B. `I, II, III`
C. `I, III`
D. `III, IV`

Answer

To determine which of the given compounds decompose when heated to 400-500°C, we will analyze the thermal stability of the hydrides and carbonates in the list.

### Step-by-Step Solution:

1. **Identify the Compounds**: The compounds given are:
- (I) LiH (Lithium Hydride)
- (II) NaH (Sodium Hydride)
- (III) Li₂CO₃ (Lithium Carbonate)
- (IV) Na₂CO₃ (Sodium Carbonate)

2. **Understand the Group Trends**:
- Both LiH and NaH are Group 1 hydrides.
- Both Li₂CO₃ and Na₂CO₃ are Group 1 carbonates.

3. **Analyze the Hydrides**:
- The thermal stability of Group 1 hydrides decreases as we move down the group. This means:
- LiH is more stable than NaH.
- Therefore, NaH is less stable and will decompose more readily when heated.

4. **Analyze the Carbonates**:
- The thermal stability of Group 1 carbonates increases as we move down the group. This means:
- Li₂CO₃ is less stable than Na₂CO₃.
- Therefore, Li₂CO₃ will decompose more readily when heated compared to Na₂CO₃.

5. **Conclusion**:
- From the analysis, we conclude that:
- (II) NaH will decompose when heated to 400-500°C.
- (III) Li₂CO₃ will also decompose when heated to 400-500°C.
- Thus, the compounds that will decompose are NaH and Li₂CO₃.

### Final Answer:
The compounds that will decompose when heated to 400-500°C are:
- (II) NaH
- (III) Li₂CO₃