Oils, fats and their properties

This page covers one Form 5 content standard from the Consumer and Industrial Chemistry chapter: oils, fats and their properties. The exam wants you to say what oils and fats are made of, to distinguish saturated from unsaturated fatty acids, to explain why fats are solid and oils are liquid, and to describe the test for unsaturation together with hydrogenation and rancidity. Get the saturated-versus-unsaturated idea exactly right, because almost every question in this standard turns on it.

What oils and fats are made of

Oils and fats belong to a family called lipids. Chemically, each molecule of an oil or fat is a triglyceride: an ester formed when one molecule of glycerol (propane-1,2,3-triol) reacts with three fatty acid molecules. Glycerol supplies three hydroxyl (–OH) groups, and each fatty acid supplies one carboxyl (–COOH) group, so three ester linkages form and three water molecules are released. Because they are esters, oils and fats can be hydrolysed back to glycerol and fatty acids, the reaction that later makes soap.

A fatty acid is a long-chain carboxylic acid, usually with an even number of carbon atoms. The nature of that chain, whether it is saturated or unsaturated, controls every physical property the exam asks about.

Saturated and unsaturated fatty acids

• A saturated fatty acid has only single bonds between its carbon atoms. Every carbon holds as many hydrogen atoms as possible, so the chain is “saturated” with hydrogen. The chains are straight, so molecules pack closely together.
• An unsaturated fatty acid contains one or more carbon-to-carbon double bonds (C=C). Each C=C puts a kink or bend in the chain, so the molecules cannot pack closely.

This single structural difference explains the physical behaviour below, so learn it as the root cause, not just a label.

Why fats are solid and oils are liquid

• Fats are obtained mainly from animals (for example butter and lard) and contain mostly saturated fatty acids. Their straight chains pack closely, the forces of attraction between molecules are stronger, the melting point is higher, and so a fat is a solid at room temperature.
• Oils are obtained mainly from plants (for example palm oil, corn oil and soybean oil) and contain mostly unsaturated fatty acids. The C=C kinks stop the chains from packing closely, the forces between molecules are weaker, the melting point is lower, and so an oil is a liquid at room temperature.

Notice the chain of reasoning the marking scheme rewards: type of fatty acid → how well the chains pack → strength of the forces between molecules → melting point → physical state.

Testing for unsaturation

Because unsaturated molecules contain C=C double bonds, they undergo addition reactions that saturated molecules do not. Two standard tests are used:

• Bromine water (reddish-brown) is decolourised by an unsaturated oil, because bromine adds across the C=C double bond. A saturated fat does not decolourise it (or does so only very slowly).
• Acidified potassium manganate(VII) (purple) is also decolourised by an unsaturated oil.

The more unsaturated a sample is, that is, the more C=C bonds per molecule, the more bromine water it decolourises. This lets you compare two samples.

Hydrogenation and rancidity

• Hydrogenation converts a liquid oil into a semi-solid fat. Hydrogen gas is added across the C=C double bonds using a nickel catalyst, removing the kinks. This is how margarine is manufactured from vegetable oil.
• Rancidity is the spoiling of an oil or fat. Oxygen in the air oxidises the fat, and the products have an unpleasant smell and taste. Antioxidants are added to food to slow this oxidation, and unsaturated oils turn rancid more readily because their C=C bonds are attacked by oxygen.

Worked example

Question. An unsaturated fatty acid molecule contains two carbon-to-carbon double bonds. Calculate the number of moles of hydrogen gas, H₂, needed to hydrogenate 0.5 mol of this fatty acid completely, and hence state what physical change you would expect in the sample.

Step 1, Relate H₂ to C=C bonds. In hydrogenation, one mole of H₂ adds across one mole of C=C double bonds. Each fatty acid molecule here has 2 C=C bonds, so 1 mol of the acid needs 2 mol of H₂.

Step 2, Scale to the amount given. Moles of H₂ = (moles of fatty acid) × (C=C bonds per molecule) = 0.5 mol × 2 = 1.0 mol.

Step 3, State the physical change. Removing the C=C bonds makes the chains straight, so they pack more closely; the melting point rises and the liquid oil becomes a semi-solid fat.

Answer. 1.0 mol of H₂ is required, and the oil is converted from a liquid into a semi-solid fat.

Practice question

Question. Two food samples, P and Q, are each shaken with a few drops of bromine water. Sample P decolourises the bromine water quickly; sample Q hardly changes its colour. (a) State which sample is more unsaturated. (b) State which sample is more likely to be a fat that is solid at room temperature, and explain why in terms of the arrangement of the molecules. (c) Name the catalyst used to convert sample P into a product like Q.

Answer. (a) Sample P is more unsaturated, because it decolourises the bromine water, bromine adds across its C=C double bonds. (b) Sample Q is more likely to be a solid fat. It contains mostly saturated fatty acids with straight chains that pack closely, so the forces between the molecules are stronger and the melting point is higher, keeping it solid at room temperature. (c) A nickel (Ni) catalyst is used, in the process called hydrogenation.

Exam tip

Always link the type of fatty acid to the physical state through the packing of the chains: saturated → straight chains → close packing → stronger forces → higher melting point → solid fat; unsaturated → kinked chains → loose packing → weaker forces → lower melting point → liquid oil. When you are asked for a test, name the reagent, the colour change, and the reason (bromine water is decolourised because it adds across the C=C bond). A one-word “bromine water” without the colour change rarely earns the full mark.

Where this fits

This is content standard 13.1 of the Consumer and Industrial Chemistry chapter, and it leads directly into soaps, because soap is made by hydrolysing these very oils and fats. Reinforce it with the chapter revision notes and drill the state-and-test questions in the worked examples. In our online 1-to-1 SPM Chemistry lessons, taught in English, from RM50/hr, with a paid one-hour trial, our teachers make sure your saturated-versus-unsaturated reasoning is complete, because that single chain of ideas carries most of the marks in this part of 4541.

Quick recap

• Oils and fats are triglycerides: esters of glycerol and three fatty acids.
• Saturated fatty acids have only single bonds and straight chains; unsaturated ones have C=C double bonds and kinked chains.
• Fats (mostly saturated) pack closely, melt higher and are solid; oils (mostly unsaturated) pack loosely, melt lower and are liquid.
• Unsaturation is tested with bromine water, which is decolourised.
• Hydrogenation (nickel catalyst) turns oil into fat; rancidity is oxidation, slowed by antioxidants.