Row Reagent Alkane result Alkene result A bromine water turns colourless stays orange B limewater turns milky no change C damp litmus turns red turns blue D...
Question 1 Report
| Row | Reagent | Alkane result | Alkene result |
|---|---|---|---|
| A | bromine water | turns colourless | stays orange |
| B | limewater | turns milky | no change |
| C | damp litmus | turns red | turns blue |
| D | bromine water | stays orange | turns colourless |
A student wants to be sure which row of the table correctly describes a laboratory test that distinguishes an alkene from an alkane. Only one row gives both the correct reagent and the correct results. Which row is correct?
Answer Details
The correct reagent is bromine water, and the two results must run the right way round. An alkane has only single bonds, so it leaves the bromine water orange, while an alkene opens its carbon-carbon double bond and adds bromine across it, so the orange colour disappears:
\[ \mathrm{C_2H_4 + Br_2 \rightarrow C_2H_4Br_2} \]
The row giving stays orange for the alkane and turns colourless for the alkene is therefore the correct one.
The other bromine water row simply reverses the results. Limewater detects carbon dioxide, so it would respond to the products of burning either gas, and damp litmus tests acidic or alkaline gases, and neither hydrocarbon is either.
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