A solution of colourless salt on boiling with excess `NaOH` produces a non-flammable gas. The gas evolution ceases after sometime upon addition of `Zn` dust to the same solution, the gas evolution restarts. The colourless salt (s) is (are).

Options

A. `NH_4 NO_3`
B. `NH_4 NO_2`
C. `NH_4 Cl`
D. `(NH_4)_2 SO_4`

Answer

To solve the problem, we need to identify the colorless salt that produces a non-flammable gas when boiled with excess sodium hydroxide (NaOH) and whose gas evolution ceases upon the addition of zinc dust, but restarts afterward.

### Step-by-Step Solution:

1. **Identify the Non-Flammable Gas**:
- The problem states that a colorless salt produces a non-flammable gas upon boiling with NaOH. The most likely candidate for a non-flammable gas in this context is ammonia (NH3).

2. **Determine Possible Colorless Salts**:
- The colorless salts that can produce ammonia when treated with NaOH include ammonium nitrate (NH4NO3) and ammonium nitrite (NH4NO2).

3. **Reaction of Ammonium Nitrate with NaOH**:
- When ammonium nitrate is boiled with excess NaOH, the reaction can be represented as:
\[ NH_4NO_3 + NaOH \rightarrow NaNO_3 + NH_3 + H_2O \]
- Here, ammonia gas (NH3) is produced, which is non-flammable.

4. **Effect of Adding Zinc Dust**:
- When zinc dust is added to the solution containing sodium nitrate (NaNO3) formed from the previous reaction, a redox reaction occurs. The reaction can be represented as:
\[ 4NaNO_3 + 4Zn + 7NaOH \rightarrow 4Na_2ZnO_2 + 4NH_3 + 2H_2O \]
- This shows that the addition of zinc dust leads to the liberation of ammonia gas again.

5. **Reaction of Ammonium Nitrite with NaOH**:
- Similarly, when ammonium nitrite is boiled with excess NaOH, the reaction is:
\[ NH_4NO_2 + NaOH \rightarrow NaNO_2 + NH_3 + H_2O \]
- This also produces ammonia gas.

6. **Adding Zinc Dust to Ammonium Nitrite**:
- Upon adding zinc dust to the solution of sodium nitrite, the reaction can be represented as:
\[ 3NaNO_2 + 3Zn + 5NaOH \rightarrow 3Na_2ZnO_2 + 3NH_3 \]
- Again, ammonia gas is liberated.

7. **Conclusion**:
- Both ammonium nitrate (NH4NO3) and ammonium nitrite (NH4NO2) fit the criteria of the problem. They produce ammonia gas when boiled with NaOH, and the gas evolution ceases when zinc dust is added but restarts afterward.

### Final Answer:
The colorless salts are **ammonium nitrate (NH4NO3)** and **ammonium nitrite (NH4NO2)**.