Saturated and unsaturated

A saturated fatty acid chain has only single carbon-to-carbon bonds, while an unsaturated chain has one or more carbon-to-carbon double bonds (C=C). Malay: tepu dan tak tepu · Chinese: 饱和与不饱和.

The word saturated means the chain is “full” of hydrogen: every carbon holds as many hydrogen atoms as it can, because there are only single bonds. An unsaturated chain is not full, because a C=C double bond takes the place of two hydrogen atoms. This small structural difference decides the physical properties. Saturated fats pack together neatly, so they have higher melting points and are usually solid at room temperature and come mainly from animals. Unsaturated oils have kinks at their double bonds that stop them packing tightly, so they have lower melting points, are usually liquid, and come mainly from plants.

Example. Animal fat such as lard is saturated and solid; palm and other vegetable oils are unsaturated and liquid. If you shake each with bromine water, the unsaturated oil decolourises the orange bromine water because the double bonds add bromine across themselves, while the saturated fat leaves the colour unchanged.

Confusion to avoid. Do not mix up which one decolourises bromine water. Only the unsaturated substance does, because only it has the C=C double bonds needed for the addition reaction. Remember too that saturated does not mean “solid” by definition; it is the bonding that is described, and the solid state simply follows from it.

The saturated-unsaturated distinction runs through the whole oils-and-fats section, connecting structure, melting point and the bromine water test in one idea that SPM Chemistry questions return to again and again. It also explains why unsaturated oils turn rancid more easily, because the same double bonds that react with bromine also react with oxygen in the air. When a question gives you a melting point, a source, or a bromine water result, you can often deduce the rest, so learning the three properties as a set rather than in isolation is the habit that pays off in the exam.