What are oils and fats?
Oils and fats are natural esters formed when one molecule of glycerol (propane-1,2,3-triol) combines with three molecules of fatty acids. Because each glycerol carries three ester linkages, these molecules are called triglycerides. The general reaction is glycerol + 3 fatty acids → triglyceride + 3H2O.
The difference between a fat and an oil is only physical state at room temperature: fats are solid while oils are liquid. This depends on the degree of saturation of the fatty-acid chains.
Saturated versus unsaturated
A saturated fatty acid has only C–C single bonds; its straight chains pack closely, giving stronger forces and a higher melting point, so animal fats (butter, lard) are solid. An unsaturated fatty acid contains one or more C=C double bonds; the kinks stop close packing, so vegetable oils (palm, corn, sunflower) are liquid.
| Property | Fats | Oils |
|---|---|---|
| Source | Mostly animal | Mostly plant |
| State (25°C) | Solid | Liquid |
| C=C bonds | Few (saturated) | Many (unsaturated) |
Key idea
Unsaturated oils decolourise bromine water because Br2 adds across each C=C bond: C=C + Br2 → C(Br)–C(Br). Saturated fats do not react. This is the standard test for unsaturation.
Hydrogenation and rancidity
Liquid oils are turned into semi-solid fats (margarine) by hydrogenation: C=C + H2 → C–C, using a nickel catalyst at about 180°C. Adding hydrogen removes C=C bonds, raising the melting point. On long storage, oils undergo oxidation of their C=C bonds, producing short-chain acids with an unpleasant smell — this is rancidity.
Worked example
One mole of a fatty acid contains two C=C bonds. How many moles of Br2 react with it? Each C=C adds one Br2, so 2 C=C need 2 mol Br2.
Remember
- Fats = solid, mostly saturated; oils = liquid, mostly unsaturated.
- Bromine water test distinguishes unsaturated from saturated.
- Excess saturated fat raises blood cholesterol.