_________ acts as a catalyst during the hydrogenation of vegetable oils.

Answer

**Step-by-Step Solution:**

1. **Understanding Hydrogenation**: Hydrogenation is a chemical reaction where unsaturated compounds (those containing double or triple bonds between carbon atoms) react with hydrogen to become saturated (where only single bonds exist between carbon atoms).

2. **Identifying Unsaturated Compounds**: Unsaturated compounds have double or triple bonds between carbon atoms. For example, vegetable oils are typically unsaturated fats.

3. **The Role of a Catalyst**: In chemical reactions, a catalyst is a substance that speeds up the reaction without being consumed in the process. It facilitates the reaction without undergoing any permanent change itself.

4. **Choosing the Catalyst for Hydrogenation**: During the hydrogenation of vegetable oils, a catalyst is necessary to make the reaction occur at a practical rate.

5. **Identifying the Catalyst Used**: The most commonly used catalyst for the hydrogenation of vegetable oils is nickel. Nickel helps in speeding up the reaction between the unsaturated vegetable oils and hydrogen.

6. **Conclusion**: Therefore, the answer to the question is that **nickel** acts as a catalyst during the hydrogenation of vegetable oils.

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