Which of the following reagent is helpful in distinguishing between ethene and ethyne?

Options

A. `Br_2 `
B. `KOH " " KMnO_4`
C. Ammoniacal `AgNO_3`
D. Bromine water

Explanation

To distinguish between ethene (C2H4) and ethyne (C2H2), we can use ammoniacal silver nitrate (AgNO3 in ammonia solution) as a reagent. Here’s a step-by-step explanation of the process:

### Step-by-Step Solution:

1. **Identify the Compounds**:
- Ethene is an alkene with the formula C2H4.
- Ethyne is an alkyne with the formula C2H2.

2. **Select the Reagent**:
- The reagent we will use is ammoniacal silver nitrate (AgNO3 in ammonia solution).

3. **Reaction with Ethyne**:
- When ethyne is bubbled through ammoniacal silver nitrate, it reacts to form a yellow-white precipitate of silver acetylide (Ag2C2).
- The reaction can be represented as:
\[ C2H2 + 2AgNO3 \rightarrow Ag2C2 + 2HNO3 \]
- This indicates that ethyne reacts with the reagent.

4. **Reaction with Ethene**:
- Ethene does not react with ammoniacal silver nitrate. Therefore, there will be no precipitate formed when ethene is treated with this reagent.

5. **Conclusion**:
- The formation of a yellow-white precipitate when using ammoniacal silver nitrate indicates the presence of ethyne, while the absence of a precipitate indicates ethene.

### Final Answer:
The reagent that is helpful in distinguishing between ethene and ethyne is **ammoniacal silver nitrate (AgNO3 in ammonia solution)**.

---