Cambridge IGCSE / A-Level
IGCSE Chemistry 0620 Paper 4: Structure, Question Styles and Technique
Paper 4 is the Extended theory paper for IGCSE Chemistry 0620: 1 hour 15 minutes, 80 marks, worth 50 percent of your grade. It mixes short answers, calculations and longer structured questions across both Core and Supplement content. Moles, organic chemistry and electrochemistry carry the most marks in most sessions.
I sit Cambridge IGCSE, and Paper 4 is the paper that decides whether the sciences go well for me. It covers everything in the Extended syllabus, it moves fast, and it punishes vague answers. When I started past-paper practice I kept losing marks on questions I understood, purely because my answers were not specific enough for the mark scheme. This page is the set of notes I wish I had at the start: exactly how the paper is built, which topics come up heaviest, three original exam-style questions worked through properly, and the technique habits that stopped me bleeding marks.
How Paper 4 is structured
Paper 4 is 1 hour 15 minutes for 80 marks, which means you are working at nearly a mark a minute once you allow reading time. It is 50 percent of the whole qualification, and it examines the full Extended syllabus, so Core content appears alongside Supplement content in the same question. Expect around 6 to 8 long structured questions, each broken into parts that climb in difficulty: a definition or recall part, then application, then a calculation or an extended explanation worth 3 to 5 marks. The other papers in the Extended route are Paper 2, the 45 minute multiple choice paper worth 40 marks and 30 percent, and Paper 5 or Paper 6 for practical skills at 20 percent. There is no choice of questions on Paper 4, and you answer directly in the booklet. I check the mark total in brackets before writing anything, because a 1 mark part wants one clear point, not a paragraph, and a 4 mark part is asking for four separate scoring statements.
Topic weighting: what actually comes up
Cambridge does not publish a fixed percentage per topic, but after working through several years of past papers a pattern is obvious. Stoichiometry is the most reliable heavy scorer: mole calculations, limiting reagents, percentage yield and empirical formulas appear every session, often hiding inside other topics. Organic chemistry is usually a full question, covering homologous series, isomers, addition and substitution reactions, and polymers. Electrochemistry and chemical energetics both lean Supplement, so they turn up on Paper 4 in a way they barely do on Core papers, with electrode half-equations a regular 2 to 3 marks. Acids, bases and salt preparation reappear constantly, usually with a practical flavour. The Periodic Table, metals and reactivity, and rates of reaction fill out the middle of the paper, and there is nearly always a data or graph interpretation part. My rule from practice: if I could not do moles, organic reactions and half-equations cold, I was giving up 20 or more marks before the paper started.
Question styles you need to recognise
Every part of every question starts with a command word, and the command word tells you what the mark scheme wants. State and give want a short factual answer with no explanation. Describe wants what happens; explain wants why it happens, usually in terms of particles, electrons or energy. Deduce and predict mean the answer is reachable from the data in front of you. Suggest is the one that used to scare me: it means the situation is unfamiliar on purpose, and you are meant to apply known chemistry to it, so a sensible application scores even if you have never seen the example before. Calculations must show working, because method marks are real: a wrong final answer sitting on correct working can still take most of the marks. The longer 4 to 6 mark explanation parts are marked as separate scoring points, so I write them as short, separate sentences rather than one flowing paragraph, which makes it easier for the examiner to find each point.
Exam technique I actually use
Three habits changed my Paper 4 scores. First, I answer in the language of the syllabus. If the question is about rate, the scheme wants collision frequency and energy of collisions, not it reacts faster. If it is about bonding, it wants precise phrases like electrostatic attraction, delocalised electrons and intermolecular forces. Second, I never leave a calculation blank. Writing the relevant equation, converting to moles, or even just converting units can score method marks. Third, I write charges and state symbols carefully, because half-equations and ionic equations lose the mark for a single missing charge. On timing, I aim to be at 40 marks by 35 minutes, which leaves slack for the long organic or electrolysis question near the end. And I read the whole question stem before part (a), because later parts often reuse the same data, and the stem usually contains the exact substance names the mark scheme expects.
Worked questions, step by step
A student heats 6.4 g of copper(II) oxide with excess carbon. The equation for the reaction is 2CuO + C -> 2Cu + CO2. Calculate the maximum mass of copper that could be produced. (Ar: Cu = 64, O = 16, C = 12) [3]
- Find the moles of CuO: Mr of CuO = 64 + 16 = 80, so moles = 6.4 / 80 = 0.08 mol.
- Use the ratio from the equation: 2CuO -> 2Cu, so the ratio is 1 to 1 and moles of Cu = 0.08 mol.
- Convert to mass: mass = moles x Ar = 0.08 x 64 = 5.12 g.
Answer: 5.12 g of copper.
Where marks slip: One mark for moles of CuO, one for using the 1 to 1 ratio, one for the final mass. Show every step: if the arithmetic slips, correct working still earns the first two marks.
Try one yourself: Try the same question with 12.0 g of iron(III) oxide reduced by carbon monoxide: Fe2O3 + 3CO -> 2Fe + 3CO2 (Ar: Fe = 56, O = 16). The ratio is no longer 1 to 1, which is exactly where most people slip.
Concentrated aqueous sodium chloride is electrolysed using inert electrodes. (a) Name the product at the negative electrode (cathode). (b) Write the ionic half-equation for the reaction at the positive electrode (anode). (c) Explain why sodium metal is not produced in this electrolysis. [4]
- (a) At the cathode, hydrogen gas is produced, not sodium. In aqueous electrolysis the less reactive species is discharged.
- (b) At the anode, chloride ions are oxidised: 2Cl- -> Cl2 + 2e-. Check the equation balances for both atoms and charge.
- (c) Sodium is more reactive than hydrogen, so hydrogen ions from the water are discharged in preference to sodium ions.
- The sodium ions stay in solution, which is why the liquid left behind is sodium hydroxide solution.
Answer: (a) Hydrogen. (b) 2Cl- -> Cl2 + 2e-. (c) Sodium is more reactive than hydrogen, so H+ ions from the water are discharged instead of Na+ ions.
Where marks slip: The half-equation mark needs correct species, balancing and electrons on the correct side. Writing 2Cl- - 2e- -> Cl2 is also accepted, but a missing charge loses the mark instantly.
Try one yourself: Repeat for dilute sulfuric acid with inert electrodes: name both products and write both half-equations. Then try concentrated hydrochloric acid and note what changes at the anode.
Ethene reacts with steam to form ethanol. (a) Name this type of reaction and give the essential conditions used in industry. (b) State the general formula of the homologous series that ethanol belongs to. (c) Ethanol is also made by fermentation. Give one advantage and one disadvantage of fermentation compared with the reaction of ethene with steam. [6]
- (a) It is an addition reaction, specifically hydration, because water adds across the C=C double bond. Conditions: about 300 degrees C, 60 atm, phosphoric acid catalyst.
- (b) Ethanol is an alcohol, and the general formula of the alcohols is CnH2n+1OH.
- (c) Advantage of fermentation: it uses renewable plant material such as sugars, and it runs at low temperature, so energy costs are lower.
- Disadvantage: it is slow, it is a batch process, and it produces impure ethanol that needs fractional distillation.
- In comparison answers, make both sides explicit: renewable compared with a finite crude oil feedstock scores; the single word renewable may not.
Answer: (a) Addition (hydration); around 300 degrees C, 60 atm, phosphoric acid catalyst. (b) CnH2n+1OH. (c) For example: fermentation uses renewable sugars, but it is slow and gives impure ethanol that needs distilling.
Where marks slip: Condition marks are all or nothing per condition, so learn the catalyst by name. One advantage plus one disadvantage means exactly two clearly separated points, each a genuine comparison of the two methods.
Try one yourself: Try the parallel question for cracking: describe how alkanes are cracked to produce alkenes, give the conditions, and explain why cracking matters to the petrochemical industry.
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Solve my question →Quiz me on this topicQuestions students ask
How long is IGCSE Chemistry 0620 Paper 4 and how many marks is it worth?
It is 1 hour 15 minutes, 80 marks, and counts for 50 percent of the whole IGCSE. Extended candidates sit it alongside Paper 2, the multiple choice paper worth 30 percent, and Paper 5 or 6 for practical skills at 20 percent.
Is Paper 4 only Supplement content?
No. It examines the whole Extended syllabus, which is Core plus Supplement. Plenty of marks come from Core ideas asked with more depth, so do not skip Core topics when revising for it.
Do I lose marks for wrong significant figures in calculations?
Usually not, provided your value rounds correctly and you have not truncated too early. Give at least 3 significant figures unless told otherwise, and keep the full calculator value between steps of a multi-part calculation.
Can I sit Paper 4 if I have only studied Core content?
No. Paper 4 is part of the Extended route. Core-only candidates sit Papers 1 and 3 instead, which cap the available grade at C. Your school decides the entry, usually based on mock results.