The melting point of unsaturated fatty acids

Options

A. Increases with increase in double bonds
B. Decreases with increase in double bonds
C. Rises in some and fells in others
D. These is no relationship between unsaturation and melting point.

Answer

### Step-by-Step Solution:

1. **Understanding Fatty Acids**:
- Fatty acids can be classified into two categories: saturated and unsaturated.
- Saturated fatty acids have no double bonds between carbon atoms, while unsaturated fatty acids contain one or more double bonds.

2. **Physical State at Room Temperature**:
- Unsaturated fatty acids are typically liquid at room temperature due to the presence of double bonds that prevent tight packing of the molecules.
- In contrast, saturated fatty acids are solid at room temperature because their straight chains allow for tighter packing.

3. **Effect of Double Bonds**:
- The presence of double bonds in unsaturated fatty acids leads to weaker intermolecular forces compared to saturated fatty acids.
- This is because double bonds introduce kinks in the fatty acid chains, preventing them from aligning closely together.

4. **Hydrogenation Process**:
- When hydrogen is added to unsaturated fatty acids (a process known as hydrogenation), the double bonds can be converted to single bonds.
- This process increases the saturation of the fatty acids and typically raises their melting points, as the molecules can pack more closely together.

5. **Melting Point Trend**:
- The melting point of unsaturated fatty acids decreases as the number of double bonds increases.
- This is due to the fact that more double bonds result in weaker intermolecular interactions, leading to lower melting points.

6. **Conclusion**:
- Therefore, the melting point of unsaturated fatty acids decreases with an increase in the number of double bonds.

### Final Answer:
The melting point of unsaturated fatty acids decreases with an increase in the number of double bonds.

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