Assertion: `AgNO_(3)` reacts with `KCN` to form white ppt. of AgCN. This white ppt. Disappears when excess KCN is added.
Reason: AgCN decomposes to form silver-carbide and evolve `N_(2)` gas.

Options

A. If both assertion and reason are true and the reason is the correct explanation of assertion
B. If both assertion and reason are true and the reason is not the correct explanation of assertion
C. If assertion is false but reason is true
D. If assertion is true but the reason is false

Answer

To analyze the given assertion and reason, we will break down the information step by step.

### Step 1: Understanding the Assertion
The assertion states that AgNO₃ reacts with KCN to form a white precipitate of AgCN, which disappears upon the addition of excess KCN.

- **Chemical Reaction**:
- When silver nitrate (AgNO₃) reacts with potassium cyanide (KCN), the reaction can be represented as:
\[ \text{AgNO}_3 + \text{KCN} \rightarrow \text{AgCN (s)} + \text{KNO}_2 \]
- **Observation**:
- The formation of AgCN results in a white precipitate.

### Step 2: Effect of Excess KCN
The assertion also mentions that the white precipitate disappears when excess KCN is added.

- **Explanation**:
- When excess KCN is added, it forms a soluble complex, potassium argentocyanide (K[Ag(CN)₂]), which does not precipitate.
- The reaction can be represented as:
\[ \text{AgCN (s)} + 2 \text{KCN} \rightarrow \text{K[Ag(CN)}_2] + \text{KNO}_2 \]
- **Conclusion**:
- This confirms that the assertion is correct.

### Step 3: Understanding the Reason
The reason states that AgCN decomposes to form silver carbide and evolves N₂ gas.

- **Analysis**:
- The decomposition of AgCN does not lead to the formation of silver carbide (Ag₂C₂) under normal conditions.
- Instead, AgCN can decompose at high temperatures (around 400 K) to produce silver (Ag) and nitrogen gas (N₂), but not silver carbide.
- The reaction can be represented as:
\[ 2 \text{AgCN} \rightarrow 2 \text{Ag} + \text{N}_2 + \text{C} \]
- **Conclusion**:
- The reason provided is incorrect because AgCN does not decompose to form silver carbide.

### Final Conclusion
- The assertion is true, but the reason is false. Therefore, the correct answer is that the assertion is correct while the reason is incorrect.

### Summary
- **Assertion**: True
- **Reason**: False