Reactions 2
Chemistry: AQA A Level, topic 5.6.2
Testing for Alkenes
We can test a solution to see if the alkene function group is present.
The test
- Add bromine water to the solution.
- If an alkene is present, the solution will lose its orange colour and become colourless.
- If an alkene is not present, it will NOT decolourise and the solution remains orange.
How the test works
- The test works because:
- If the alkene is present, it can undergo an addition reaction with the bromine.
- This removes the bromine (in the bromine water) and forms a halogenoalkane - so the colour is lost.
Bromine water
- As a quick note, bromine water is just water with a small amount of bromine dissolved in it.
- Bromine water is what colours the solution orange until it has reacted with the alkene.
Testing for Carboxylic Acids
We can test to see if a carboxylic acid is present in a solution.
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The test
- Firstly we must react the carboxylic acid with sodium carbonates.
- The reaction with the carbonate ions should form a salt + CO2 + H2O
- We can then test for the CO2 produced using limewater.
Limewater
- Limewater is a clear solution but it is turned cloudy if CO2 is present.
- This means we must pump the gas, produced from the reaction between the sample and the carbonate, into limewater.
- If limewater turns cloudy: carboxylic acid is present in the initial solution.
- If limewater stays clear: carboxylic acid is NOT present in the initial solution.
Test yourself on Reactions 2
A solution does NOT contain an alkene. What will happen if bromine water is added?
Which of the following is NOT needed to test for carboxylic acids?