TutorForAll

Chemistry 11–12 · Year 12

Testing organic unknowns for carbon-carbon double bonds, hydroxyl groups and carboxylic acids

Module 8: Applying Chemical Ideas

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

Three quick bench tests sort small organic molecules by functional group: bromine water for alkenes, an acidified oxidant for alcohols, and hydrogen carbonate for carboxylic acids.

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
  • cyclohexene and cyclohexane flammable and harmful
  • permanganate oxidiser; ethanoic acid corrosive vapour

Controls

  • microscale drops only
  • no flames; well-ventilated room
  • gloves and eye protection
  • 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.

What you need

  • Knowns in dropper bottles (1 mL each): cyclohexene, cyclohexane, ethanol, propan-2-ol, 2-methylpropan-2-ol, ethanoic acid, propanone; three coded unknowns from the same set. 2-methylpropan-2-ol melts at about 25 degrees Celsius (298.3 K in the NIST WebBook), so its bottle and any unknown that is solid stand in the warm water bath until liquid
  • Bromine water 0.02 mol/L, acidified potassium permanganate 0.01 mol/L, 0.5 mol/L sodium hydrogen carbonate, universal indicator paper, all in dropper bottles
  • Well plate, warm water bath at 60 degrees Celsius, test tubes, white card

How to do it

  1. Stand the 2-methylpropan-2-ol bottle, and any unknown that is solid, in the warm water bath until liquid. Bromine water: three drops of each known with three drops of bromine water; record immediate decolourisation (alkene) or not.
  2. Hydrogen carbonate: three drops of each known onto five drops of hydrogen carbonate solution; record fizzing (carboxylic acid); confirm with pH paper.
  3. Oxidant: three drops of each known with five drops of acidified permanganate, warmed two minutes; record decolourisation (primary or secondary alcohol, and alkene) or no change (tertiary alcohol, alkane, ketone).
  4. Make a results grid for the seven knowns and derive the minimum sequence of tests that identifies each functional group.
  5. Apply the sequence to the three unknowns and report each with the evidence; note which pairs the three tests cannot separate (ethanol from propan-2-ol) and what further test would.
  6. Write an equation for each positive test.

What you should see

Cyclohexene alone decolourises bromine water at once. Ethanoic acid alone fizzes with hydrogen carbonate and turns indicator paper red-orange. Permanganate decolourises with cyclohexene, ethanol and propan-2-ol, and stays purple with cyclohexane, 2-methylpropan-2-ol, propanone and (in the cold) ethanoic acid. The sequence bromine water, then hydrogen carbonate, then permanganate assigns every unknown to alkene, carboxylic acid, oxidisable alcohol or none of these; ethanol and propan-2-ol are separated only by the product test (acid from ethanol, ketone from propan-2-ol).

What changes

What you change
the organic compound tested
What you measure
result of each functional-group test
What you keep the same
  • drop volumes
  • reagent concentrations
  • bath temperature and time
  • fresh wells

Common misconceptions

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

  • A negative permanganate test means no functional group (tertiary alcohols and ketones carry groups that resist oxidation).
  • Fizzing with hydrogen carbonate happens with any acidic compound including alcohols (alcohols are far too weak to release carbon dioxide from hydrogen carbonate).
  • One test is enough to name an unknown (each test excludes possibilities; the sequence is what identifies).

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)CH12-15CH11/12-2CH11/12-3CH11/12-4
  • Chemistry Stage 6 Syllabus (2017), NESA; the current syllabus, taught in 2026 (codes read from the syllabus document); the Module 7 knowledge outcome, cited on a Module 8 entry because Module 7 carries the functional group and reaction content the unknowns are sorted byCH12-14
  • Chemistry 11-12 Syllabus (2025), NESA; implemented from 2028, not yet taughtCH-12-03CH-12WS-03CH-12WS-04
  • Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

  1. www.nsw.gov.au/education-and-training/nesa/curriculum/science/chemistry-stage-6-2017
  2. www.nsw.gov.au/sites/default/files/noindex/2025-03/chemistry-stage6-syllabus-word.docx
  3. curriculum.nsw.edu.au/learning-areas/science/chemistry-11-12-2025/outcomes
  4. edu.rsc.org/practical/qualitative-tests-for-organic-functional-groups-practical-videos-16-18-students/4014327.article
  5. edu.rsc.org/experiments/testing-for-unsaturation-with-bromine-on-a-microscale/552.article
  6. education.nsw.gov.au/content/dam/main-education/teaching-and-learning/curriculum/key-learning-areas/science/s-6/chemistry/Chemistry_module_8_depth__study.docx
  7. webbook.nist.gov/cgi/cbook.cgi?ID=C75650&Mask=4

All Concept Studio activities