Final answer:
The chemical feature that distinguishes a saturated fatty acid from an unsaturated fatty acid is the presence of double bonds in unsaturated fatty acids, which cause a rigid bend or 'kink' in the carbon skeleton.
Step-by-step explanation:
In a saturated fatty acid, each carbon is bonded to two hydrogen atoms with single bonds. This means that the carbon atom in a saturated fatty acid is filled (or 'saturated') to capacity with hydrogen atoms. On the other hand, in an unsaturated fatty acid, there is at least one double bond between neighboring carbons, which causes a rigid bend or 'kink' in the carbon skeleton.
For example, palmitic acid is a saturated fatty acid containing only single bonds, and it is solid at room temperature. In contrast, oleic acid is an unsaturated fatty acid containing one double bond, and it remains liquid at room temperature.