If hydrogen is added in a reaction, it is called ..................... reaction.

Answer

To answer the question, "If hydrogen is added in a reaction, it is called ..................... reaction," we need to identify the appropriate term that describes this process.

### Step-by-Step Solution:

1. **Understanding the Concept of Addition of Hydrogen**:
- When hydrogen is added to a substance, it often leads to a change in the oxidation state of that substance. This is a key concept in redox (reduction-oxidation) reactions.

2. **Identifying the Reaction Type**:
- The addition of hydrogen typically results in the reduction of another substance. This is because reduction is defined as the gain of electrons or the addition of hydrogen to a compound.

3. **Example of a Reaction**:
- Consider the reaction where hydrogen is added to nitrogen to form ammonia (NH₃). In this reaction:
- Nitrogen (N₂) has an oxidation state of 0.
- In ammonia (NH₃), nitrogen has an oxidation state of -3.
- Here, nitrogen is reduced because its oxidation state decreases.

4. **Conclusion**:
- Since the addition of hydrogen leads to the reduction of the other reactant, we conclude that when hydrogen is added in a reaction, it is called a **reduction reaction**.

### Final Answer:
If hydrogen is added in a reaction, it is called a **reduction reaction**.

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