Alkenes
Alkenes for IGCSE Chemistry 0620: CnH2n and the C=C bond, cracking conditions, addition with bromine, hydrogen and steam, and the bromine water test.
The IGCSE Chemistry Specialist Team · founded by Rig
Written to the Cambridge IGCSE Chemistry (0620) syllabus and mark-scheme conventions. Last updated 2026-06-11.
One colour change carries more 0620 marks than any other: aqueous bromine, orange to colourless. Alkenes supply that test, the products of cracking, and three addition reactions, and they punish loose wording harder than anywhere else in organic chemistry, because “clear” instead of “colourless” is an automatic lost mark flagged in examiner reports series after series.
What alkenes are
Alkenes are unsaturated hydrocarbons with the general formula CnH2n and a C=C double bond as their functional group. Unsaturated means the molecule contains a carbon-carbon double bond. The first three you name are ethene C2H4, propene C3H6 and butene C4H8 (positional isomers of butene are Extended material, covered under isomerism).
The double bond is the site of every reaction: it opens, and two new atoms bond to the two carbons.
Where alkenes come from: cracking
Fractional distillation produces too much of the long-chain fractions and not enough petrol, and no alkenes at all. Cracking fixes both. A large alkane molecule is broken down using a high temperature and a catalyst into a smaller alkane plus an alkene, and sometimes hydrogen:
C10H22 → C8H18 + C2H4
In any cracking equation the carbons and hydrogens must balance. Examiners give a starting alkane and one product and ask for the other. Count atoms; the missing product falls out. Cracking is also a Supplement source of hydrogen for the Haber process and fuel cells.
The bromine water test
Add aqueous bromine to the hydrocarbon and shake:
- Alkene: the mixture turns from orange to colourless. Bromine adds across the C=C.
- Alkane: the bromine water remains orange, as there is no double bond and no addition.
This is the syllabus’s distinguishing test for saturated versus unsaturated compounds, and the negative result is worth a mark too: a compare question expects both observations.
Addition reactions
In an addition reaction, the double bond opens and the reactant molecule adds across it, giving a single product. Three to learn:
| Reagent | Conditions | Product (from ethene) | Equation |
|---|---|---|---|
| Bromine | room temperature | 1,2-dibromoethane | C2H4 + Br2 → C2H4Br2 |
| Hydrogen | nickel catalyst, heat | ethane | C2H4 + H2 → C2H6 |
| Steam | 300°C, 60 atm, phosphoric acid catalyst | ethanol | C2H4 + H2O → C2H5OH |
The steam reaction is the industrial route to ethanol compared against fermentation on the alcohols page. The three conditions (temperature, pressure, catalyst) are each a marking point. Alkene molecules can also add to each other: addition polymerisation, covered under polymers.
Hold the contrast that defines the topic: alkenes add (double bond opens, one product); alkanes substitute (UV light, two products including HCl).
Worked exam question
Dodecane, C12H26, is cracked to produce octane and one other hydrocarbon, Z. (a) State the conditions for cracking. [2] (b) Deduce the formula of Z and name its homologous series. [2] (c) Describe how aqueous bromine distinguishes octane from Z. [2]
Model answer, mark by mark:
- (a) M1: high temperature. M2: catalyst. (Either alone is one mark; “heat it a lot” without “catalyst” caps at one.)
- (b) M3: C12H26 → C8H18 + Z, so Z = C4H8 by counting atoms. M4: C4H8 fits CnH2n, so it is an alkene.
- (c) M5: with Z (the alkene), aqueous bromine turns from orange to colourless. M6: with octane (the alkane), it remains orange. Both halves are needed for both marks, as the question says “distinguishes”: describe both results.
The mistakes that cost marks
- “Clear” instead of “colourless”. Clear is not a colour. A clear orange solution exists. Orange to colourless, every time.
- One observation in a distinguish question. “Distinguish” means state what happens with each substance. The alkane’s “remains orange” is a separate mark.
- Cracking conditions left vague. “Heat” alone is not “high temperature + catalyst”. Two conditions, two marks.
- Addition products that keep the double bond. After addition the product is saturated. Drawing C2H4Br2 with a C=C contradicts the reaction.
- Unbalanced cracking equations. If carbons or hydrogens don’t balance, the deduced product is wrong. Count both elements before writing the formula.
How examiners want it phrased
| Student wording | Mark-scheme wording |
|---|---|
| ”The bromine water goes clear" | "Aqueous bromine turns from orange to colourless" |
| "Alkenes have a double bond so they react" | "Alkenes are unsaturated; the C=C double bond opens in addition reactions, forming a single product" |
| "Cracking makes petrol from big molecules" | "Cracking breaks large alkane molecules into smaller alkanes and alkenes, using a high temperature and a catalyst" |
| "Ethene plus steam makes alcohol" | "Ethene reacts with steam at 300°C and 60 atm with a phosphoric acid catalyst to form ethanol” |
The bromine water test alone has appeared on Paper 6 as an observation question, Paper 2 as a multiple-choice distractor, and Paper 4 as a 2-marker. The same six words score in all three. We drill exactly this kind of high-frequency phrasing in a free trial lesson, starting from whichever paper you sit next.
Test yourself
Answer these three without scrolling back up. The answers stay hidden until you click.
Q1 (2 marks). Define the term unsaturated and state the general formula of the alkenes.
Show answer
• unsaturated: the molecule contains a carbon-carbon double bond (C=C) [1] • general formula CnH2n [1]
Q2 (3 marks). Propene reacts with hydrogen. Name the product, state the condition needed, and name the type of reaction.
Show answer
• product: propane [1] • condition: nickel catalyst (with heat) [1] • type: addition (reaction) [1]
Q3 (4 marks). Ethanol is manufactured from ethene. Write the symbol equation for the reaction and state the three conditions used.
Show answer
• C2H4 + H2O → C2H5OH [1] • 300°C [1] • 60 atm [1] • phosphoric acid catalyst [1]
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Frequently asked questions
What is the test for an alkene?
Add aqueous bromine (bromine water) and shake. With an alkene it turns from orange to colourless; with an alkane it stays orange. Both colours must be stated, and 'colourless' (never 'clear') is the word that earns the mark.
What are the conditions for cracking?
High temperature and a catalyst. Cracking breaks a large alkane molecule into a smaller alkane plus an alkene (and sometimes hydrogen). It matters industrially because demand for small molecules exceeds supply, and it is the main source of alkenes.
Which addition reactions do I need?
Bromine (room temperature, the basis of the test), hydrogen (nickel catalyst, makes an alkane), and steam (300°C, 60 atm, phosphoric acid catalyst, makes ethanol). In each case the C=C opens and a single product forms.