Chemistry 11–12 · Year 12

Testing for carbon-carbon double bonds with bromine water and acidified permanganate on a microscale

Module 7: Organic Chemistry

PracticalMedium risk

School laboratory, not for home

In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

The idea

An alkene adds bromine across its double bond and decolourises bromine water at once, while an alkane needs light and time to substitute; the difference identifies unsaturation.

Safety card

Medium riskLearners carry it out, with a teacher supervising

Setting: In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

Hazards

bromine water irritates eyes and skin and its vapour is irritant
cyclohexene and cyclohexane are flammable and harmful
permanganate stains and is an oxidiser

Controls

microscale drops only; bromine water in a dropper bottle
no flames; well-ventilated room
gloves and eye protection
wells rinsed into an organic waste container

Note

NSW Department of Education Chemical Safety in Schools (CSIS) package, 2021 Technical Update: Section 1.7 (risk assessment) and Volume 2 Appendix D (generic assessment advice and DoE chemical categories); record a RiskAssess (riskassess.com.au) risk assessment before the lesson and check the school's hazardous chemical register (CSIS Section 1.9) for local restrictions. Liquid bromine is never handled in the classroom; CSIS Appendix F advises buying bromine water ready-made when that is all that is needed.

What you need

Bromine water (about 0.02 mol/L), bought ready-made as CSIS Appendix F advises or prepared by the technician in a fume cupboard, 5 mL in a dropper bottle
Acidified potassium permanganate 0.01 mol/L, 5 mL in a dropper bottle
Cyclohexene 1 mL, cyclohexane 1 mL, sunflower oil 1 mL, ethanol 1 mL, in dropper bottles
Well plate, plastic pipettes, white card, dark cupboard and a sunny windowsill (or a lamp) for the alkane comparison

How to do it

Put three drops of each organic liquid in separate wells; add three drops of bromine water to each and swirl; record the colour after ten seconds.
Repeat with acidified permanganate in fresh wells; record the colour after ten seconds and after two minutes.
For the cyclohexane wells, note where the orange colour sits (it moves into the upper hydrocarbon layer), then cover one plate and leave it in the dark and put an identical plate under the lamp for ten minutes; record any fading.
Write equations for the addition to cyclohexene (dibromocyclohexane) and the substitution in cyclohexane, and explain the role of light.

What you should see

Cyclohexene decolourises bromine water on the first swirl and decolourises the purple acidified permanganate within seconds (colourless manganese(II) ions form in acid; brown manganese(IV) oxide appears only if the acid runs short). Sunflower oil (unsaturated fatty acid chains) also decolourises both. Cyclohexane does not change either reagent in the dark: the orange colour moves into the upper cyclohexane layer and stays; under the lamp it fades slowly, the sign of light-driven substitution. Bromine water stays orange with ethanol, while ethanol slowly decolourises the permanganate (oxidation of the alcohol, not addition).

What changes

What you change: the organic compound tested (saturated or unsaturated)
What you measure: whether bromine water or permanganate is decolourised, and how fast
What you keep the same: drop volumes
reagent concentrations
lighting (dark against lamp)
temperature

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

Bromine water tests for any hydrocarbon (it distinguishes unsaturated from saturated; alkanes do not decolourise it quickly).
The colour disappears because bromine evaporates (it is consumed by addition; the alkane well keeps its colour).
Oils are unsaturated because they are liquid (they are liquid because they are unsaturated; the test shows the double bonds).

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Chemistry Stage 6 Syllabus (2017), NESA; the current syllabus, taught in 2026 (codes read from the syllabus document)CH12-14 CH11/12-5
Chemistry Stage 6 Syllabus (2017), NESA; the current syllabus, taught in 2026 (codes read from the syllabus document); the Module 8 knowledge outcome, cited on a Module 7 entry because Module 8 carries the qualitative tests for carbon-carbon double bonds, hydroxyl groups and carboxylic acidsCH12-15
Chemistry Stage 6 Syllabus (2017), NESA; the current syllabus, taught in 2026 (codes read from the syllabus document); a Working Scientifically outcome not among those the syllabus targets in Module 7, which it allows in any moduleCH11/12-3
Chemistry 11-12 Syllabus (2025), NESA; implemented from 2028, not yet taughtCH-12-03 CH-12WS-03
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

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