Skip to content
Dabour HubLEARN · PRACTICE · EXPLORESign inStudent

3.3.5Organic Chemistry

Saturation Test — Edexcel AS Chemistry Revision Notes & Topical Questions

5 min read 5 parts Lesson 28 of 42

📚 Edexcel AS Chemistry Hub / Topical Revision
🩺
100% Curriculum Specification Match Lesson Doctor Verified
The only platform with 100% curriculum specification match and verified interactive question banks.
Curriculum Equation Key معادلة تفاعل تكوين الإستر (الأسترة) - Esterification Reaction EquationFast Exam Solution
R-COOH + R2-OH ⇌ (conc. H₂SO₄) R-COO-R2 + H₂O
Word Equation & Conditions: حمض كربوكسيلي + كحول (في وجود حمض الكبريتيك المركز كعامل حفاز ونازع للماء) ⇌ إستر + ماء
Location & Specifications: الكيمياء العضوية للثانوية العامة والتحصيلي و Edexcel Chemistry.
0%read

Part 1 of 5The Test

1
Add a few drops of orange bromine water to a small sample of the unknown hydrocarbon in a test tube.
2
Shake the mixture thoroughly so that the bromine comes into contact with the hydrocarbon.
3
If the orange colour disappears and the mixture becomes colourless, the hydrocarbon is unsaturated.
4
If the mixture stays orange, the hydrocarbon is saturated.
5
Carry out the test in a fume cupboard because bromine is toxic and corrosive.

Part 2 of 5Explaining the Results

Alkene
  • Alkene The C=C reacts by electrophilic addition, using up the bromine and removing the orange colour.
Equation
  • Equation C₂H₄ + Br₂ → CH₂BrCH₂Br, giving a colourless dihalogenoalkane.
Alkane
  • Alkane Alkanes have no C=C, and in the dark they do not react with bromine at room temperature, so there is no colour change.
Light
  • Light In bright ultraviolet light alkanes can react slowly by radical substitution, so keep the test away from strong light.
Result
  • Result Decolourisation of bromine water shows the presence of a double bond and cannot identify which alkene it is.

Part 3 of 5Using the Test

Using the Test
Distinguishing
  • Distinguishing The test separates saturated hexane from unsaturated hex-1-ene, which look the same.
Colour change
  • Colour change The key observation is orange to colourless, not colourless to clear or "disappears".
Sample
  • Sample A solution of bromine in an organic solvent can also be used, with the same colour change.
Safety
  • Safety Wear gloves and eye protection and use small quantities because bromine is hazardous.
Limit
  • Limit It shows that a molecule is unsaturated but does not say how many double bonds are present.

Part 4 of 5Worked Example

Question

  • A 1.40 g sample of an alkene of relative molecular mass 56.0 reacts completely with bromine.
  • Calculate the mass of bromine that reacts.
  • (Ar: Br 79.9.)

  1. Moles of alkene = 1.40 ÷ 56.0 = 0.0250 mol.
  2. Alkene + Br₂ → dibromoalkane, a 1 : 1 ratio, so moles of Br₂ = 0.0250 mol.
  3. Mr of Br₂ = 2 × 79.9 = 159.8.
  4. Mass = 0.0250 × 159.8 = 3.995 g.

Answer 4.00 g of Br₂ (3 significant figures)

Part 5 of 5Exam Checklist & Tips

3.3.5 – Key Points for the Exam

  • Saturated Containing only single carbon-carbon bonds.
  • Unsaturated Containing at least one carbon-carbon double or triple bond.
  • Bromine water An orange aqueous solution of bromine used to test for unsaturation.
  • Decolourisation Loss of colour, here from orange to colourless.
  • Addition reaction A reaction in which two reactants combine to give one product.

Common Mistakes

  • Writing that bromine water turns "clear" instead of orange to colourless.
  • Saying alkanes decolourise bromine water, when they react only slowly and only in ultraviolet light.
  • Concluding the test identifies the specific alkene, when it only shows the presence of C=C.
  • Leaving out the fume cupboard and safety precautions for bromine.

Last updated:

\n