Experiment: Testing for unsaturation with bromine water

Testing for unsaturation with bromine water is a short but important Form 5 practical in the Carbon Compounds chapter. It is the standard way to tell an unsaturated hydrocarbon from a saturated one, and it appears in both written and practical papers. This guide sets out the aim, the apparatus, a numbered procedure, the observations to expect, and the science process skills the practical papers reward.

Aim

To test hydrocarbon samples for unsaturation using bromine water, and to distinguish an unsaturated compound (an alkene) from a saturated compound (an alkane) by whether the bromine water is decolourised.

Apparatus and materials

• Test tubes and test-tube rack
• Dropper or teat pipette
• Stopper for the test tube
• An alkene sample, for example a cyclohexene or hexene liquid, or ethene gas collected earlier
• An alkane sample, for example a cyclohexane or hexane liquid
• Bromine water (a dilute, reddish-brown aqueous solution of bromine)

Procedure

1. Label two test tubes: one for the alkene and one for the alkane.
2. Using a dropper, add a few drops of bromine water to each test tube.
3. To the first tube add the alkene sample; to the second tube add the same amount of the alkane sample.
4. Stopper each tube and shake it gently to mix the layers.
5. Observe and record the colour of the bromine water in each tube immediately and again after shaking.
6. Keep both tubes away from strong sunlight during the test, so that any change is due to the compound and not to light.
7. If a gas is being tested instead of a liquid, bubble or shake the gas with bromine water in a stoppered tube and observe the colour.

Expected observations

In the tube with the alkene, the reddish-brown colour of the bromine water fades quickly and the mixture becomes colourless after shaking. In the tube with the alkane, the reddish-brown colour of the bromine water stays unchanged after shaking, as long as the tubes are kept out of bright light. The rapid loss of colour in the first tube, and the persistence of colour in the second, is the key contrast to record.

Inference and conclusion

The alkene decolourises bromine water because its carbon-carbon double bond adds bromine across it in an addition reaction at room conditions, so the coloured bromine is removed. The alkane does not decolourise bromine water in the dark because it has only single carbon-carbon bonds and does not undergo this addition. The conclusion is that decolourising bromine water at room conditions is a test for unsaturation: a compound that decolourises it contains a carbon-carbon double (or triple) bond, while one that does not is saturated.

Science process skills (Paper 3 style)

Stating a hypothesis. A suitable hypothesis is: if a hydrocarbon is unsaturated, then it will decolourise bromine water at room conditions, because the double bond reacts with bromine by addition.

Identifying variables. The manipulated variable is the type of hydrocarbon (unsaturated or saturated); the responding variable is whether the bromine water is decolourised; the controlled variables are the volume of bromine water, the amount of sample, and keeping the tubes away from light.

Making an inference. From the observation that a sample decolourises bromine water, infer that the sample is unsaturated and contains a carbon-carbon multiple bond.

Operational definition. Define “unsaturated” operationally: a hydrocarbon is unsaturated if it decolourises reddish-brown bromine water at room conditions in the absence of sunlight.

Safety precautions

• Wear safety goggles, because bromine water is corrosive and irritating to skin and eyes.
• Handle bromine water in a fume cupboard or well-ventilated area, so that bromine vapour is not inhaled.
• Stopper the test tubes before shaking, so that the liquids do not splash out.
• Keep the hydrocarbon samples away from flames, because many of them are flammable.
• Wash any splashes off the skin at once with plenty of water, to limit irritation from the bromine.

Common errors

• Testing in bright sunlight. In strong light an alkane can slowly react with bromine and lose colour, giving a false positive; keep the tubes shaded so only unsaturation causes the change.
• Adding too little sample. Using far too little alkene may leave some bromine unreacted and the colour only partly faded; add enough sample to see a clear result.
• Confusing decolourising with dilution. The colour must be removed by reaction, not merely paled by adding water; compare against the untouched alkane tube.
• Not shaking to mix. If the layers are not mixed, the reaction is slow and the change is unclear; stopper and shake gently.
• Reporting a colour change wrongly. State the colour before and after, reddish-brown to colourless for the alkene, unchanged for the alkane, rather than a vague “it changed”.

How our teachers use this experiment

In one-to-one SPM Chemistry lessons, taught in English from RM50 per hour, we make sure students can explain not just what happens but why, why the double bond removes the colour, and why light must be excluded so the alkane result is fair. The bromine water test is one of the most frequently examined ideas across SPM Chemistry, linking the preparation of ethene, the reactions of alkenes and the difference between saturated and unsaturated compounds, and the same reasoning is rewarded in the Paper 3 practical assessment (Paper 3 is a practical test assessing science process skills).