IGCSE Chemistry: Organic Chemistry Practice Questions
Organic chemistry is the chemistry of carbon compounds. IGCSE focuses on four homologous series: alkanes, alkenes, alcohols and carboxylic acids, each with a general formula, a functional group and a set of characteristic reactions.
Topic 11 of Cambridge IGCSE Chemistry 0620 rewards pattern recognition more than memorisation. Once you know the general formulae and what makes a molecule unsaturated, most questions reduce to applying the same three or four reactions. The questions below cover the tests, the equations and the polymer work that appear most often.
What you need to know for Organic Chemistry
- Homologous seriesA family of compounds with the same general formula and functional group, differing by CH2 each step, showing a gradual change in physical properties and similar chemical properties.
- AlkanesGeneral formula CnH2n+2. Saturated, containing only single carbon to carbon bonds. They are generally unreactive apart from combustion and substitution with chlorine in ultraviolet light.
- AlkenesGeneral formula CnH2n. Unsaturated, containing a carbon to carbon double bond. They undergo addition reactions, which is why they are far more reactive than alkanes.
- The bromine water testAqueous bromine is orange or brown. An alkene decolourises it rapidly. An alkane leaves it unchanged. This is the standard test for unsaturation.
- AlcoholsGeneral formula CnH2n+1OH. Ethanol is made by fermentation of glucose using yeast, or by addition of steam to ethene with a catalyst.
- Addition polymerisationMany alkene monomers join by opening their double bonds to form one long chain, with no other product. Ethene forms poly(ethene).
IGCSE Chemistry Organic Chemistry questions and answers
4 exam-style questions written to the 0620 syllabus. Try each one on paper first, then open the worked answer to check your method against the marks.
Describe a chemical test that distinguishes ethene from ethane. State the reagent and the result with each gas.
Show the worked answer
- Add the gas to aqueous bromine, often called bromine water, which is orange or brown.
- Ethene contains a carbon to carbon double bond and is unsaturated, so it reacts by addition and decolourises the bromine water quickly.
- Ethane is saturated with only single bonds, so no reaction occurs at room temperature and the bromine water stays orange or brown.
- The product with ethene is 1,2-dibromoethane.
Ethanol can be made by fermentation. Write the equation for the reaction and state two conditions required.
Show the worked answer
- Glucose is broken down by enzymes in yeast.
- The equation is C6H12O6 giving 2C2H5OH + 2CO2.
- Condition 1: yeast must be present to supply the enzymes.
- Condition 2: a temperature of roughly 25 to 35 degrees Celsius, warm enough for a fast reaction but not hot enough to denature the enzymes. Oxygen must also be excluded.
Ethene molecules join together to form poly(ethene). Name this type of polymerisation and explain, in terms of bonding, why ethene can undergo this reaction but ethane cannot.
Show the worked answer
- The reaction is addition polymerisation, in which many monomers join with no other product formed.
- Ethene contains a carbon to carbon double bond, so it is unsaturated.
- One bond of the double bond breaks, allowing each molecule to form new bonds to its neighbours and build a long chain.
- Ethane is saturated with only single bonds, so there is no bond available to open and it cannot polymerise.
State the general formula of the alkenes and give the molecular formula of the alkene containing four carbon atoms.
Show the worked answer
- The alkene general formula is CnH2n.
- Substituting n equal to 4 gives 8 hydrogen atoms.
- The molecular formula is therefore C4H8, which is butene.
Common mistakes in this topic
- Confusing the alkane and alkene general formulae. Alkanes are CnH2n+2, alkenes are CnH2n.
- Saying bromine water turns clear rather than colourless.
- Forgetting to balance combustion equations. Complete combustion of a hydrocarbon always gives carbon dioxide and water.
- Writing that fermentation needs oxygen. It is an anaerobic process.
- Naming products without checking the number of carbon atoms. Ethanol has two carbons, propanol has three.
Exam tips
- Learn the first four members of each series by name and formula. Meth, eth, prop, but covers almost every question.
- Any question mentioning bromine water is testing for unsaturation. Say decolourises, not turns clear.
- For polymer questions, draw the repeat unit with the two bonds extending outwards through the brackets and n outside.
- When asked to compare fermentation with the catalytic route to ethanol, think about raw material, rate, purity and whether the process is batch or continuous.
- Combustion equations are easier if you balance carbon first, then hydrogen, then oxygen last.
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Organic Chemistry FAQs
What is the test for an alkene in IGCSE Chemistry?
Shake the substance with aqueous bromine, also called bromine water. An alkene decolourises the orange bromine water rapidly because the double bond opens and bromine adds across it. An alkane leaves the bromine water orange or brown at room temperature. Always describe the result as decolourised rather than clear.
What is the difference between saturated and unsaturated?
A saturated compound contains only single carbon to carbon bonds, so no more atoms can be added to it. An unsaturated compound contains at least one carbon to carbon double bond, which can open to allow atoms to add across it. Alkanes are saturated and alkenes are unsaturated.
How is ethanol manufactured, and which method is better?
Ethanol is made by fermentation of glucose using yeast, or by the catalytic addition of steam to ethene. Fermentation uses a renewable raw material and low temperatures but is slow, batch based and gives impure ethanol. The catalytic route is fast, continuous and produces pure ethanol but uses ethene from a finite crude oil supply.
What is a homologous series?
A homologous series is a family of organic compounds that share the same general formula and functional group, with each member differing from the next by CH2. Members show a gradual trend in physical properties such as boiling point, and very similar chemical properties because they share a functional group.
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Written to the published Cambridge IGCSE Chemistry (0620) syllabus. Check your school entry code and syllabus year, because Core and Extended candidates are assessed on different content. Last reviewed 2026-08-12.