Stearic acid and linoleic acid each have 18 carbon atoms. Why does stearic acid melt at $69°\mathrm{C}$ but linoleic acid melts at $-5{}^{\circ }\mathrm{C}$ ?

Stearic acid and linoleic acid each have 18 carbon atoms.

Does stearic acid melt at $69°\mathrm{C}$but linoleic acid melts at $-5$${}^{\circ }\mathrm{C}$

Fatty acids are a component of fat in our body and diet. During digestion, the body converts lipids into fatty acids, which are absorbed into the bloodstream. Fatty acid molecules usually combine in three groups to form a molecule known as a triglyceride.

Stearic acid is a saturated fatty acid, while linoleic acid is an unsaturated acid. Saturated acids are closely bound and as long chains increase these intermolecular forces, so more energy is needed to break this force and more temperature to melt, while in the case of unsaturated acids, double bonds create some kink, disturbing the regular structure and weakening it, so less melting temperature.