What is meant by hardening of oils?

Explanation

### Step-by-Step Solution:

1. **Definition of Hardening of Oils**:
- Hardening of oils refers to the process of converting liquid oils into solid or semi-solid fats. This transformation is primarily achieved through the addition of hydrogen to the unsaturated bonds present in the oils.

2. **Understanding Unsaturated Bonds**:
- Oils typically contain unsaturated fatty acids, which have double or triple bonds between carbon atoms. These unsaturated bonds contribute to the liquid state of the oil.

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