Which of the following can act as an oxidising agent as well as a reducing agent?
1. `H_(2)O_(2)`, 2. `H_(2)S`, 3. `SO_(2)`, 4. `HNO_(2)`

Options

A. `1,2,3`
B. `2,3,4`
C. `1,3,4`
D. All

Explanation

To determine which of the given compounds can act as both an oxidizing agent and a reducing agent, we need to analyze the oxidation states of the elements in each compound. An oxidizing agent is a substance that gains electrons (is reduced), while a reducing agent is a substance that loses electrons (is oxidized). Let's evaluate each compound step by step.

### Step 1: Analyze H₂O₂ (Hydrogen Peroxide)

1. **Determine the oxidation states**:
- In H₂O₂, the oxidation state of oxygen is -1 (since it is in the peroxide form).
- The oxidation state of hydrogen is +1.

2. **Identify possible changes**:
- Oxygen can be oxidized to O₂ (oxidation state 0) or reduced to H₂O (oxidation state -2).

3. **Conclusion**:
- Since oxygen in H₂O₂ can either lose electrons (be oxidized) or gain electrons (be reduced), H₂O₂ can act as both an oxidizing agent and a reducing agent.

### Step 2: Analyze H₂S (Hydrogen Sulfide)

1. **Determine the oxidation states**:
- In H₂S, the oxidation state of sulfur is -2.

2. **Identify possible changes**:
- Sulfur can be oxidized to a higher oxidation state (maximum +6 in SO₄²⁻) but cannot be reduced further than -2.

3. **Conclusion**:
- H₂S can only act as a reducing agent since it can be oxidized but cannot be reduced.

### Step 3: Analyze SO₂ (Sulfur Dioxide)

1. **Determine the oxidation states**:
- In SO₂, the oxidation state of sulfur is +4.

2. **Identify possible changes**:
- Sulfur can be oxidized to +6 (in SO₄²⁻) or reduced to -2 (in H₂S).

3. **Conclusion**:
- SO₂ can act as both an oxidizing agent and a reducing agent since it can be oxidized or reduced.

### Step 4: Analyze HNO₂ (Nitrous Acid)

1. **Determine the oxidation states**:
- In HNO₂, the oxidation state of nitrogen is +3.

2. **Identify possible changes**:
- Nitrogen can be oxidized to +5 (in HNO₃) or reduced to -3 (in NH₃).

3. **Conclusion**:
- HNO₂ can also act as both an oxidizing agent and a reducing agent since it can be oxidized or reduced.

### Final Conclusion:
Based on the analysis:
- **H₂O₂**: Can act as both an oxidizing and reducing agent.
- **H₂S**: Can only act as a reducing agent.
- **SO₂**: Can act as both an oxidizing and reducing agent.
- **HNO₂**: Can act as both an oxidizing and reducing agent.

Thus, the compounds that can act as both oxidizing and reducing agents are **H₂O₂, SO₂, and HNO₂**.