Nitrate of which of the following elements are converted to their oxides on heating ?

Options

A. Li
B. Na
C. K
D. Mg

Explanation

To determine which nitrates are converted to their oxides upon heating, we can analyze the behavior of the nitrates of the given elements: lithium (Li), sodium (Na), potassium (K), and magnesium (Mg).

### Step-by-Step Solution:

1. **Identify the Elements and Their Nitrates**:
- The elements in question are lithium (Li), sodium (Na), potassium (K), and magnesium (Mg).
- Their respective nitrates are LiNO3, NaNO3, KNO3, and Mg(NO3)2.

2. **Understand Stability of Nitrates**:
- Nitrates of alkali metals (Li, Na, K) tend to be more stable compared to their oxides.
- In contrast, nitrates of alkaline earth metals (like Mg) can decompose upon heating.

3. **Analyze Lithium Nitrate (LiNO3)**:
- When heated, lithium nitrate decomposes as follows:
\[ 2 \text{LiNO}_3 \rightarrow \text{Li}_2\text{O} + 2 \text{NO}_2 + \frac{1}{2} \text{O}_2 \]
- This shows that lithium nitrate converts to lithium oxide (Li2O) upon heating.

4. **Analyze Magnesium Nitrate (Mg(NO3)2)**:
- When heated, magnesium nitrate decomposes as follows:
\[ \text{Mg(NO}_3\text{)}_2 \rightarrow \text{MgO} + 2 \text{NO}_2 + \frac{1}{2} \text{O}_2 \]
- This indicates that magnesium nitrate also converts to magnesium oxide (MgO) upon heating.

5. **Evaluate Sodium and Potassium Nitrates**:
- Sodium nitrate (NaNO3) and potassium nitrate (KNO3) are more stable and do not decompose to form their respective oxides upon heating.

6. **Conclusion**:
- From the analysis, we conclude that the nitrates of lithium and magnesium are converted to their oxides upon heating, while those of sodium and potassium are not.

### Final Answer:
The nitrates that are converted to their oxides on heating are those of **lithium (Li)** and **magnesium (Mg)**.