1 · A double bond puts a bend in the chain
Each double bond puts a kink in the fatty acid chain, and the more kinks, the looser the packing, so the fat stays fluid at lower temperatures.A fatty acid is a carbon chain. The saturated kind is straight; the molecules pack tightly like chopsticks — which is why tallow is hard at room temperature.Each added double bond puts a fixed-angle kink in the chain. The more kinks, the less neatly it packs. Packing loosely means: at a lower temperature, it is still fluid.