Question

Pork fat has a melting temperature of 40ºC and flaxseed oil has a melting temperature of -25ºC. What contributes to this ~65ºC difference in melting temperature?

A. Pork fat contains 40% palmitate and stearate, whereas flaxseed oil is 53% alpha-linolenate, the difference in average a melting temperature is about 65ºC, so it makes sense.
B. Pork fat is solid and clumpy, whereas flaxseed oil is liquid, so their melting temperatures reflect the difference in solids over liquids and the amount of enthalpic energy.
C. Animals eat high fatty food, whereas plants use photosynthesis in the hot sun, so the melting temperature differences reflect the lifestyle of the two organisms.
D. Melting temperature reflects the chemistry and physics of hydrogenated fatty acids compared to trans fat and polyunsaturated fats, so flaxseed is liquid and pork fat is not.

Answer 1

The difference in melting temperature between pork fat and flaxseed oil is mainly due to the higher proportion of long-chain saturated fatty acids in pork fat, which results in higher melting points, and the higher proportion of unsaturated fatty acids in flaxseed oil, leading to lower melting points.

Step-by-step explanation:

The approximately 65°C difference in melting temperature between pork fat and flaxseed oil is principally due to their fatty acid composition and the degree of unsaturation. Pork fat is mainly composed of higher proportions of long-chain saturated fatty acids, such as palmitate and stearate, which contribute to its higher melting temperature of around 40°C. These long chains with more carbon atoms increase intermolecular forces, known as London dispersion forces, resulting in a solid structure at room temperature. On the other hand, flaxseed oil is predominantly made up of alpha-linolenate, an unsaturated fatty acid, which has a lower melting temperature of around -25°C due to the presence of cis-double bonds that create kinks in the fatty acid chains, reducing the contact area between molecules and thus weakening the intermolecular forces.

This difference can also be seen more broadly between animal fats, typically solids at room temperature, and vegetable oils, which are usually liquid at room temperature due to a higher proportion of unsaturated fatty acids or short-chain saturated fatty acids. The melting point is not only influenced by the length of the carbon chain but also by the degree of saturation, where unsaturated fatty acids with cis-double bonds have significantly lower melting points than their saturated counterparts due to the molecular shapes and decreased intermolecular forces.