The key chemical feature that distinguishes a saturated fatty acid from an unsaturated fatty acid is the presence or absence of double bonds between carbon atoms in the hydrocarbon chain.

Here's the breakdown:

* Saturated fatty acids have no double bonds between carbon atoms in their hydrocarbon chain. This means each carbon atom in the chain is bonded to the maximum number of hydrogen atoms possible.

* Unsaturated fatty acids have one or more double bonds between carbon atoms in their hydrocarbon chain. This means some carbon atoms are not bonded to the maximum number of hydrogen atoms.

Consequences of the double bond:

* Saturated fatty acids are generally solid at room temperature due to the straight, tightly packed chains.

* Unsaturated fatty acids are generally liquid at room temperature due to the bends in the chains caused by the double bonds, which prevents close packing.

Examples:

* Saturated fatty acid: Palmitic acid (found in palm oil)

* Unsaturated fatty acid: Oleic acid (found in olive oil)

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