When ammonia is passed over heated copper oxide, the metallic coper is obtained. The reaction shows that ammonia is :

Options

A. A dehydrating agent
B. An oxidising agent
C. A reducing agent
D. A nitrating agent

Explanation

To solve the question, "When ammonia is passed over heated copper oxide, the metallic copper is obtained. The reaction shows that ammonia is:", we will analyze the reaction step by step.

### Step-by-Step Solution:

1. **Identify the Reaction**:
The reaction involves ammonia (NH₃) being passed over heated copper oxide (CuO). The products of this reaction are copper (Cu), nitrogen gas (N₂), and water (H₂O).

**Chemical Equation**:
\[ 3 \text{CuO} + 2 \text{NH}_3 \rightarrow 3 \text{Cu} + N_2 + 3 \text{H}_2O \]

2. **Determine Oxidation States**:
- **Copper Oxide (CuO)**:
- Copper (Cu) in CuO has an oxidation state of +2 (let's denote it as X).
- Oxygen (O) has an oxidation state of -2.
- Therefore, the oxidation state of Cu in CuO is +2.
- **Ammonia (NH₃)**:
- Nitrogen (N) in NH₃ has an oxidation state of -3 (let's denote it as Y).
- Each Hydrogen (H) has an oxidation state of +1.
- Therefore, the oxidation state of N in NH₃ is -3.

3. **Analyze the Changes in Oxidation States**:
- In the reaction, Cu is reduced from +2 in CuO to 0 in Cu (metallic copper).
- Nitrogen in NH₃ goes from -3 to 0 in N₂.
- This indicates that NH₃ is losing electrons (oxidation), while CuO is gaining electrons (reduction).

4. **Determine the Role of Ammonia**:
Since NH₃ is losing electrons and causing the reduction of CuO, it acts as a reducing agent. A reducing agent is defined as a substance that donates electrons to another substance, thereby reducing it.

5. **Conclusion**:
Based on the analysis, we can conclude that ammonia (NH₃) acts as a reducing agent in the reaction with heated copper oxide.

### Final Answer:
Ammonia is a reducing agent.