3.4.4Physical Chemistry
Test for Unsaturation — AQA AS Chemistry Revision Notes & Topical Questions
5 min read 5 parts Lesson 79 of 102
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 5Saturated and Unsaturated Compounds
Saturated and Unsaturated Compounds
Saturated
- Saturated A saturated compound has only single bonds between carbon atoms, as in the alkanes.
Unsaturated
- Unsaturated An unsaturated compound contains at least one carbon–carbon double bond, as in the alkenes.
Reactivity
- Reactivity The pi bond of C=C is electron-rich, so alkenes react with electrophiles while alkanes do not.
Why test
- Why test A quick test lets a chemist decide whether an unknown hydrocarbon contains C=C before further analysis.
Part 2 of 5Carrying Out the Bromine Water Test
1
Place about 1 cm³ of the unknown liquid in a test tube, or bubble the gas into a tube of reagent.
2
Add a few drops of bromine water, which is orange or yellow-brown, to the sample.
3
Stopper the tube and shake it so the two substances mix thoroughly.
4
Look carefully at the colour of the mixture straight away and after a short time.
5
Record the colour change precisely, using the words orange to colourless for a positive result.
Part 3 of 5Interpreting the Result
Positive result
- Positive result With an alkene the bromine water changes from orange to colourless, because bromine is used up in an addition reaction.
Negative result
- Negative result With an alkane the mixture stays orange, because no reaction occurs in the dark at room temperature.
Chemistry
- Chemistry Bromine adds across the C=C bond, for example ethene forming colourless 1,2-dibromoethane.
Equation
- Equation C₂H₄ + Br₂ → CH₂BrCH₂Br, so the coloured Br₂ is removed from solution.
| Sample | Observation with bromine water | Conclusion |
|---|---|---|
| Cyclohexene | Orange to colourless | Unsaturated, C=C present |
| Hexane | Stays orange | Saturated, no C=C |
Part 4 of 5Writing Good Answers
Writing Good Answers
Use colourless
- Use colourless Examiners require the word colourless, because clear only means transparent and a clear solution can still be coloured.
Give the direction
- Give the direction State the starting colour and the end colour, orange to colourless, never the reverse.
Observation first
- Observation first Describe what is seen and then make the deduction that a double bond is present.
Limits of the test
- Limits of the test The test shows that a double bond is present but not which alkene it is.
Part 5 of 5Exam Checklist & Tips
3.4.4 – Key Points for the Exam
- Unsaturated A compound that contains at least one carbon–carbon double or triple bond.
- Saturated A compound containing only single bonds between its carbon atoms.
- Bromine water An orange aqueous solution of bromine used as the standard test for C=C.
- Addition reaction A reaction in which two molecules join to form one product with no other products.
Common Mistakes
- Saying the bromine water goes clear instead of colourless.
- Describing a negative result as nothing happens; write that the solution stays orange.
- Writing carbon dioxide or a similar deduction when the question asks for an observation.
- Stating that the alkane decolourises bromine water slowly in the dark; no reaction takes place.
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