Final answer:
Saturated fatty acids have only single bonds, while unsaturated fatty acids have double bonds that create a bend in the molecule. This causes unsaturated fatty acids to have weaker intermolecular attractions and lower melting points, making them liquids at room temperature.
Step-by-step explanation:
Saturated fatty acids contain only single bonds along the length of the carbon chain, while unsaturated fatty acids have one or more double bonds. The presence of double bonds in unsaturated fatty acids creates a bend or kink in the molecule, preventing them from stacking closely together like saturated fatty acids. As a result, unsaturated fatty acids have weaker intermolecular attractions and lower melting points, making them liquids at room temperature.