Testing for SaturationOxford AQA IGCSE Chemistry: Revision notes
Section 1
What is the difference between saturated and unsaturated hydrocarbons?
Hydrocarbons can be classified by whether they contain a carbon-carbon double bond:
- Saturated hydrocarbons (alkanes) contain only single carbon-carbon bonds
- Unsaturated hydrocarbons (alkenes) contain at least one carbon-carbon double bond (C=C)
- The first two alkenes are ethene (C2H4) and propene (C3H6), with general formula CnH2n
Key termssaturatedunsaturatedethenepropene
Section 2
How does bromine water test for unsaturation?
Bromine water is an orange solution used to test whether a hydrocarbon is saturated or unsaturated.
- Add a sample of the hydrocarbon to bromine water and shake
- If a carbon-carbon double bond is present (an alkene), the bromine water turns from orange to colourless
- If the hydrocarbon is saturated (an alkane), the bromine water stays orange - no reaction occurs
- The colour change happens because the double bond reacts with the bromine, whereas a saturated molecule cannot react in this way
Key termsbromine water
Exam tip
Always state both the starting colour (orange) and the result (colourless) for full marks - 'it changes colour' alone is not specific enough.
Section 3
Why is this test useful?
- It lets chemists identify unknown hydrocarbons as either alkanes or alkenes without needing any other equipment
- It is used to check the products of cracking, since cracking produces a mixture including alkenes
- Alkanes give a negative result because they have no reactive double bond for the bromine to react with
Example
An unknown hydrocarbon is shaken with bromine water and it turns colourless - this identifies it as an alkene, an unsaturated hydrocarbon.
Must Know
- Saturated hydrocarbons (alkanes) contain only single C-C bonds; unsaturated hydrocarbons (alkenes) contain at least one C=C double bond
- Ethene (C2H4) and propene (C3H6) are the first two alkenes, general formula CnH2n
- The test for unsaturation is to shake with bromine water
- Orange to colourless = unsaturated (alkene, contains a double bond)
- Stays orange = saturated (alkane, no double bond)
That's the notes covered.
Carry on to the next subtopic.