# Alkene

A hydrocarbon with a carbon-carbon double bond. The general formula is CnH2n. Alkenes are unsaturated and more reactive than alkanes.

Chemistry (school) · Part 08: Carbon chemistry and the Earth · Card 5 of 10 · Last updated 2026-10-19

## Analogy

An alkene has a spare hand, the double bond, ready to grab something new.

## Example

**Alkene names**

Ethene C2H4
Propene C3H6
Butene C4H8
Test: bromine water decolourises

## In practice

Ethene is used to make poly(ethene). Bromine water turns from orange to colourless with an alkene.

## Practice

### Flashcards

- **What is the test for an alkene?** Shake with bromine water. It turns from orange to colourless.

## Questions people ask

### Why are alkenes more reactive than alkanes?

The carbon-carbon double bond can open up, which lets other atoms add on. Alkanes have only single bonds and nothing to open. That is why alkenes react with bromine water, hydrogen, and steam, and why they can join together to make polymers.

### What does unsaturated mean?

An unsaturated compound has at least one carbon-carbon double bond, so it holds fewer hydrogen atoms than the maximum. Alkenes are unsaturated. Ethene, C2H4, has two fewer hydrogens than ethane, C2H6, which is the saturated version with two carbons. Bromine water detects this feature.

## Related concepts

- [Hydrocarbon](https://studyvisually.com/learn/science/chemistry-school/chemistry-school-part-8/chemistry-school-part-8-concept-2)
- [Alkane](https://studyvisually.com/learn/science/chemistry-school/chemistry-school-part-8/chemistry-school-part-8-concept-4)

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