When a substance gains electron, it is called :

Answer

To answer the question "When a substance gains electron, it is called:", we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Electron Gain**:
- When a substance gains electrons, it undergoes a change in its oxidation state. Electrons are negatively charged particles, and gaining them typically results in a decrease in the oxidation state of the substance.

2. **Defining Reduction**:
- The process of gaining electrons is known as reduction. In redox (reduction-oxidation) reactions, reduction is one half of the process where a substance is reduced by gaining electrons.

3. **Identifying the Role of the Substance**:
- In a redox reaction, the substance that gains electrons is reduced, while the substance that loses electrons is oxidized.

4. **Understanding Oxidizing Agents**:
- The substance that causes another substance to be oxidized (i.e., to lose electrons) is called an oxidizing agent. Therefore, when a substance gains electrons, it acts as an oxidizing agent in the context of the overall reaction.

5. **Conclusion**:
- Therefore, when a substance gains electrons, it is referred to as an "oxidizing agent."

### Final Answer:
When a substance gains electrons, it is called an **oxidizing agent**.

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