When ammonium chloride is heated in a test tube and red litmus is placed at the mouth of the test tube. It first turns blue and then red because

Options

A. the vapour of ammonium chloride are basic as well as acidic
B. it first liberates `NH_3` and then HCl gas
C. It first liberates `Cl_2` and then `NH_3` gas
D. It first liberates HCl gas and then `NH_3`

Explanation

**Step-by-Step Solution:**

1. **Understanding the Reaction**: When ammonium chloride (NH4Cl) is heated, it decomposes into ammonia (NH3) and hydrochloric acid (HCl) gases. The reaction can be represented as:
\[ \text{NH}_4\text{Cl (s)} \rightarrow \text{NH}_3\text{ (g)} + \text{HCl (g)} \]

2. **Behavior of Litmus Paper**: Litmus paper is used to test the acidity or basicity of a solution. Red litmus paper turns blue in the presence of a base, while blue litmus paper turns red in the presence of an acid.

3. **Sequence of Events**:
- When ammonium chloride is heated, ammonia gas is released first. Ammonia is a basic gas, which will turn red litmus paper blue.
- After some time, hydrochloric acid gas is released, which is acidic and will turn blue litmus paper red.

4. **Conclusion**: Therefore, when red litmus paper is placed at the mouth of the test tube, it first turns blue due to the presence of ammonia (a base), and then it turns red due to the presence of hydrochloric acid (an acid).

5. **Correct Option**: Based on the sequence of gas release, the correct answer is:
- **Option D**: It first liberates hydrochloric gas and then ammonia.