Why do unsaturated fatty acids form liquid oils instead of solids?

Generally, the unsaturations of the fatty acids are of the cis type. This causes the arrangement of the molecule to be angled, with the apex at unsaturation. This angulation causes the melting points of unsaturated acids to be lower than those of their saturated counterparts and therefore are liquid at room temperature. Double bonds in trans distort little crystalline symmetry, which is very similar to that of saturated fatty acids.

Explanation:

They are long chain carboxylic acids, usually have no carbon pair (14 to 22), the most abundant have 16 and 18 carbons.

Fatty acids are saturated when they do not have double bonds, they are flexible and solid at room temperature.

Unsaturated or polyunsaturated if there are double or triple bonds in the chain, rigid at the level of the double bond being oily liquids.

Physical properties

- Solubility. They are bipolar or amphipathic molecules (from the Greek amphi, double). The head of the molecule is polar or ionic and therefore hydrophilic (-COOH). The chain is apolar or hydrophobic (-CH2- and -CH3 terminal groups).

- Melting point. In saturates, the melting point increases due to the number of carbons, showing a tendency to establish Van der Waals links between the carbonated chains.  Unsaturates have fewer interactions of this type due to the elbow of their chain.

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