Which homologous series do I need to know for 0620?

Alkanes, alkenes, alcohols and carboxylic acids. For each you need the general formula, the first four members, functional group, typical reactions and a test to distinguish them. Extended candidates also need esters.

How do I name organic compounds in the exam?

Count the carbon chain length for the prefix (meth-, eth-, prop-, but-), then add the suffix for the homologous series (-ane for alkanes, -ene for alkenes, -ol for alcohols, -anoic acid for carboxylic acids, -yl ...-anoate for esters). For branched chains, name the longest chain and add the branch as a prefix.

What is the test to distinguish alkanes from alkenes?

Add bromine water. An alkene decolourises bromine water (from orange/brown to colourless) because it undergoes addition across the double bond. An alkane does not react with bromine water, so it stays orange/brown. The exam awards marks for the reagent, the observation and the explanation.

Do I need to draw structural formulae or are molecular formulae enough?

The exam often asks for displayed or structural formulae, not molecular formulae. A displayed formula shows every bond; a structural formula shows the grouping of atoms (e.g. CH3CH2OH). If the question says 'draw the structural formula', a molecular formula like C2H5OH alone will not score.

Organic Chemistry: IGCSE Chemistry Exam Guide

How to answer every organic chemistry question in IGCSE Chemistry 0620. Homologous series, naming, reactions and isomerism exam technique.

Published by IGCSEChemistry.com.my

Chemistry teaching team: K. S. Tan (15+ years teaching IGCSE Chemistry) and Ms Yash (10+ years teaching IGCSE Chemistry) and Ms Kartini (15+ years teaching IGCSE Chemistry).

Mapped to Cambridge IGCSE Chemistry 0620 (2026–2028). Last updated 2026-08-19.

Organic chemistry questions appear in substantial blocks on every Paper 4 and in clusters on Paper 2. The topic carries high marks but has a reputation for being difficult to revise because it feels like a separate subject bolted onto the rest of chemistry. The key insight for the exam is that organic questions are highly patterned: the same reaction types, the same naming rules, the same distinguishing tests. Learn the patterns and the marks become predictable. The underlying content is covered under organic chemistry; the extended exam technique guide is at organic chemistry exam technique.

What the exam actually tests

Organic chemistry questions on 0620 test five things, in roughly this order of frequency:

Naming and drawing. Given a structure, name it. Given a name, draw the structure. Given a molecular formula, draw possible isomers.
Reactions of each homologous series. Combustion, addition, substitution, oxidation, esterification. The question tells you the reactants and conditions; you give the product and the equation.
Distinguishing tests. How to tell an alkane from an alkene, or an alcohol from a carboxylic acid.
Polymers. Draw the repeating unit from a monomer, or identify the monomer from a polymer structure.
Isomerism. Draw or identify structural isomers of a given molecular formula.

The homologous series table you must be able to reproduce

You do not need to memorise long lists. You need four rows and six columns:

Series	General formula	Functional group	First four members	Key reaction	Test
Alkanes	CnH2n+2	None (C-C single bonds)	CH4, C2H6, C3H8, C4H10	Combustion, substitution with halogens (UV light)	Does NOT decolourise bromine water
Alkenes	CnH2n	C=C double bond	C2H4, C3H6, C4H8	Addition (Br2, H2, H2O, HBr)	Decolourises bromine water
Alcohols	CnH2n+1OH	-OH	CH3OH, C2H5OH, C3H7OH, C4H9OH	Combustion, oxidation, esterification	Burns with clean blue flame; oxidised by acidified K2Cr2O7 (orange to green)
Carboxylic acids	CnH2n+1COOH	-COOH	HCOOH, CH3COOH, C2H5COOH, C3H7COOH	Neutralisation, esterification	Turns litmus red; reacts with carbonates (fizzing, CO2)

If a Paper 4 question asks you to compare two homologous series, it is asking for differences you can read from this table.

Naming: the two-part system

Every organic name has a prefix (chain length) and a suffix (functional group):

Prefix: meth- (1C), eth- (2C), prop- (3C), but- (4C)
Suffix: -ane (alkane), -ene (alkene), -ol (alcohol), -anoic acid (carboxylic acid), -yl …-anoate (ester)

The exam error that costs the most marks: writing “ethanol” as “ethenol” (which would be a different compound with a double bond and an OH group). Spell carefully.

For esters, the naming flips: the alcohol part comes first as a prefix (-yl), then the acid part (-anoate). Ethanol + ethanoic acid gives ethyl ethanoate.

Reactions: what the examiner wants

Each reaction question follows the same structure: reactants + conditions -> products. The marks are split between naming the product, writing the equation, and stating the conditions.

Conditions that must be stated for full marks:

Alkene + bromine: no special conditions (room temperature, aqueous or pure).
Alkene + hydrogen: nickel catalyst, heat (hydrogenation).
Alkene + steam: acid catalyst (phosphoric acid), high temperature and pressure (hydration).
Alkane + halogen: UV light (substitution).
Alcohol + carboxylic acid: acid catalyst, heat (esterification).
Alcohol oxidation: acidified potassium dichromate, heat.

Missing the catalyst or the condition typically loses one mark per question.

Drawing structures that earn marks

When the question says “draw the structural formula” or “draw the displayed formula”:

Displayed formula: show every single bond as a line. Every C-H bond, every C-C bond, every C=C bond. Nothing is implicit.
Structural formula: group the atoms around each carbon. CH3CH2CH2OH is acceptable; C3H7OH is a molecular formula and may not score.

When drawing, make bonds clear and straight. A wobbly line that could be one bond or two will not be credited. Double bonds must be visibly double.

Isomerism: the systematic approach

When asked to draw isomers of a molecular formula:

Draw the longest possible straight chain first.
Shorten the chain by one carbon and attach it as a branch. Move the branch along the chain to find distinct positions.
If a functional group is present (OH, C=C), move it along the chain for more isomers.
Check each structure has the correct molecular formula.

For C4H10, there are two isomers: butane (straight chain) and methylpropane (three-carbon chain with a methyl branch). For C4H8, you get but-1-ene, but-2-ene and methylpropene.

Polymers: the repeating unit

Addition polymers form from alkene monomers. The double bond opens, and the monomers join in a chain.

To draw the repeating unit from a monomer:

Open the C=C double bond to single bonds.
Extend bonds from each end to show continuation.
Put square brackets around the unit with an “n” subscript.

To identify the monomer from a polymer:

Find the repeating unit inside the brackets.
Rejoin the two extension bonds as a C=C double bond.

The mark scheme is strict: the brackets, the extension bonds and the “n” must all be present.

Worked exam question

Q (Paper 4): But-1-ene reacts with hydrogen bromide. (a) Name the type of reaction. (1) (b) Draw the structural formula of the product. (2) (c) Explain why butane does not react with hydrogen bromide under the same conditions. (1)

Model answer: (a) Addition (1). (b) CH3CH2CHBrCH3 — the H adds to the terminal carbon of the double bond and the Br to the other carbon (2: 1 for correct formula, 1 for correct position of Br). (c) Butane has only single C-C bonds / no C=C double bond, so there is no site for addition to occur (1).

The reasoning mark in (c) requires linking the absence of a double bond to the inability to undergo addition. “Butane is saturated” alone may score, but “butane is less reactive” will not.

Organic chemistry rewards systematic study more than any other 0620 topic. If drawing structures or predicting products is a persistent difficulty, a trial lesson can identify whether the gap is in naming conventions, reaction conditions, or structural drawing and target it directly.