What mistakes do students make with Reactions of alkenes?

Describing addition as substitution, in which an atom is replaced. Addition adds atoms across the double bond with no atoms removed. Saying the double bond remains after reaction. The C=C becomes a single C–C bond. Writing that bromine water turns colourless with alkanes. Alkanes do not decolourise bromine water under these conditions, so the test distinguishes alkenes from alkanes.

What are the exam tips for Reactions of alkenes in Chemistry AQA GCSE?

Show the structural change by drawing the C=C as a single bond in the product and adding the new atoms to the two carbons. Name the catalyst and condition for each addition: nickel for hydrogenation, phosphoric acid and steam for hydration. Use the bromine water test result as evidence that a compound is unsaturated.

AQA · GCSE · Chemistry

Course version: 8462 · Version from Sept 2018

Alkenes react with hydrogen, water and the halogens, by the addition of…

Full specification wording

Reactions of alkenes: Alkenes react with hydrogen, water and the halogens, by the addition of atoms across the carbon-carbon double bond so that the double bond becomes a single carbon-carbon bond.

The C=C functional group allows alkenes to undergo addition reactions.

Checked against the AQA 8462 specification (opens in a new tab) on 21 September 2026.

Point 4 of 5
2 key terms

Study notes

From the checked teaching notes for this specification point.

The idea

What earns the marks

Addition reactions of alkenes involve atoms adding across the C=C, which becomes a single C–C bond.
Hydrogen adds to an alkene in the presence of a nickel catalyst to produce the corresponding alkane.
Water adds as steam in the presence of a phosphoric acid catalyst to produce an alcohol, for example ethene to ethanol.
Halogens add across the double bond, for example bromine reacting with ethene to form 1,2-dibromoethane.
Bromine water changing from orange-brown to colourless is the standard test for an alkene.
The product of an addition reaction is saturated because it contains only single carbon-carbon bonds.

Common mistakes and how to put them right

Describing addition as substitution, in which an atom is replaced.
Put it right: Addition adds atoms across the double bond with no atoms removed.
Saying the double bond remains after reaction.
Put it right: The C=C becomes a single C–C bond.
Writing that bromine water turns colourless with alkanes.
Put it right: Alkanes do not decolourise bromine water under these conditions, so the test distinguishes alkenes from alkanes.

Exam tips

Show the structural change by drawing the C=C as a single bond in the product and adding the new atoms to the two carbons.
Name the catalyst and condition for each addition: nickel for hydrogenation, phosphoric acid and steam for hydration.
Use the bromine water test result as evidence that a compound is unsaturated.

Try the lesson

The start of the Reactions of alkenes: Alkenes react with hydrogen, water and the halogens, by the addition of atoms across the carbon-carbon double bond so that the double bond becomes a single carbon-carbon bond. lesson, made from checked teaching notes for this specification point.

Have a go first

What structural change occurs to the carbon-carbon double bond when an alkene reacts with a halogen?

Tip: press 1 to 4 to choose

Listen

The C=C functional group allows alkenes to undergo addition reactions. Atoms add across the double bond, which opens out to become a single carbon-carbon bond, so the product is saturated. Hydrogen adds in the presence of a nickel catalyst to form an alkane: ethene + hydrogen → ethane. Water adds as steam with a phosphoric acid catalyst to form an alcohol: ethene + steam → ethanol. Halogens add directly: bromine water is decolourised from orange-brown to colourless, which is the test for an alkene, and ethene + bromine → 1,2-dibromoethane. In each case two new single bonds form to the carbons and the C=C is lost. A strong answer earns marks for these points. Addition reactions of alkenes involve atoms adding across the C=C, which becomes a single C–C bond. Hydrogen adds to an alkene in the presence of a nickel catalyst to produce the corresponding alkane. Water adds as steam in the presence of a phosphoric acid catalyst to produce an alcohol, for example ethene to ethanol. Halogens add across the double bond, for example bromine reacting with ethene to form 1,2-dibromoethane. A common slip: Describing addition as substitution, in which an atom is replaced. Instead, addition adds atoms across the double bond with no atoms removed. Exam tip: Show the structural change by drawing the C=C as a single bond in the product and adding the new atoms to the two carbons.

See it

A top-down flowchart. The top node reads 'Alkene: C=C double bond' and points to 'Atoms add across the C=C'. That node points to 'C=C becomes a single C–C bond', which points to 'Product is saturated'. The 'Atoms add across the C=C' node also branches to three reaction routes: 'Hydrogen + nickel catalyst', leading to 'Alkane'; 'Water as steam + phosphoric acid catalyst', leading to 'Alcohol'; and 'Halogen added directly', which leads to two nodes, 'Bromine water: orange-brown to colourless' and '1,2-dibromoethane from ethene + bromine'. — Shows how atoms add across the alkene C=C, which becomes a single C–C bond, giving saturated products with hydrogen, steam and halogens.

Key words

Say what each word means before you turn the card.

Addition reactions

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Unsaturation

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See every definition

Addition reactions: Reactions in which atoms add across the carbon-carbon double bond so that the double bond becomes a single carbon-carbon bond.
Unsaturation: The presence of a carbon-carbon double bond, shown by bromine water being decolourised from orange-brown to colourless.

Label the diagram

Some boxes have lost their names. Pick the right name for each number.

The diagram again, with 3 boxes numbered instead of named. A top-down flowchart. The top node reads 'Alkene: C=C double bond' and points to 'Atoms add across the C=C'. That node points to 'C=C becomes a single C–C bond', which points to 'Product is saturated'. The 'Atoms add across the C=C' node also branches to three reaction routes: 'Hydrogen + nickel catalyst', leading to 'Alkane'; 'Water as steam + phosphoric acid catalyst', leading to 'Alcohol'; and 'Halogen added directly', which leads to two nodes, 'Bromine water: orange-brown to colourless' and '1,2-dibromoethane from ethene + bromine'.

1Box 1
2Box 2
3Box 3

0 of 3 labelled

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