Cambridge IGCSE Chemistry (0620) Paper 4: The Most Common Mistakes and How to Avoid Them
IGCSE Chemistry Paper 4 — the Extended theory paper for 0620 — produces the same handful of errors in every series. They are not subtle, and they are entirely fixable. This guide lists them in order of how many marks they cost, with the fix for each.
What costs the most marks on 0620 Paper 4?
Mole calculations where the equation ratio is not applied, unbalanced or incorrectly written equations, explanation questions answered with description, and organic chemistry answers that give the product but not the conditions. Together these account for the majority of avoidable marks lost by capable candidates.
What we see on our own learning portal: Cambridge IGCSE Chemistry drew 1,220 visitors to Tutopiya’s learning portal over the 30 days to 5 November 2026, and was one of the few Cambridge subjects to grow through the autumn, up 14% month on month. (Tutopiya learning portal data, 30 days to 5 November 2026.)
Mistake 1: forgetting the equation ratio in mole calculations
This is the single biggest source of lost marks on the paper.
The pattern: a student correctly calculates the moles of the substance given, then uses that same number for the substance asked about, ignoring the stoichiometric ratio in the balanced equation.
The fix is a fixed five-step procedure, written out every time:
- Write the balanced equation.
- Calculate moles of the known substance (mass ÷ Mr, or concentration × volume).
- Apply the ratio from the equation — write it explicitly, for example “2 : 1, so moles of product = moles of reactant ÷ 2”.
- Convert to the required quantity (mass, volume, concentration).
- Check the units and significant figures.
Step 3 is the one students skip. Writing it as a separate line on the page makes skipping it almost impossible.
Mistake 2: equations that do not balance, or lack state symbols
Equation marks are among the easiest on the paper and among the most frequently lost.
Checks before you move on:
- Count atoms of every element on both sides.
- For ionic equations, check the total charge balances too.
- Include state symbols when the question asks for them — and only the correct ones. Aqueous and liquid are not interchangeable.
- For ionic equations, include only the species that change. Spectator ions should not appear.
Common specific errors: writing H₂ as H, giving water as H₂O₂, forgetting that group 2 metal ions carry a 2+ charge, and writing a full equation when an ionic one was requested.
Mistake 3: describing when the question says explain
Command words are not decoration.
| Command word | What the mark scheme wants |
|---|---|
| State / give | A short factual answer, no reasoning |
| Describe | What happens, in order |
| Explain | Why it happens, with a reason |
| Suggest | An application of principles to an unfamiliar situation |
| Predict | An outcome with a brief justification |
| Compare | Both items, with explicit linking language |
Example. Explain why the rate of reaction increases when temperature is raised.
Describes (partial marks): “The reaction goes faster at higher temperature.”
Explains (full marks): “At higher temperature the particles have greater kinetic energy, so they collide more frequently and a greater proportion of collisions have energy equal to or greater than the activation energy, so the rate increases.”
Three creditable ideas: frequency of collisions, proportion with sufficient energy, activation energy.
Mistake 4: organic chemistry without conditions
Paper 4 organic questions ask for conditions as often as products.
Know, for each reaction: the reagent, the catalyst, the temperature, and the pressure where relevant.
- Cracking — high temperature, catalyst
- Hydration of alkenes to alcohols — steam, catalyst, high temperature and pressure
- Fermentation — yeast, around 25 to 35 degrees Celsius, anaerobic
- Oxidation of ethanol to ethanoic acid — acidified potassium manganate(VII), warm
- Esterification — carboxylic acid plus alcohol, acid catalyst
- Addition polymerisation — alkene monomers, high pressure, catalyst
Also distinguish clearly between addition reactions of alkenes and substitution reactions of alkanes, including the requirement for ultraviolet light in the latter.
Mistake 5: electrolysis answers without the reasoning
Electrolysis appears in most series. Candidates name the products but cannot explain the selection.
The reasoning chain you need:
- Identify the ions present, including those from water in aqueous solutions.
- At the cathode: the less reactive cation is discharged. For aqueous solutions, this means hydrogen is produced unless the metal is below hydrogen in reactivity.
- At the anode: halides are discharged in preference to oxygen from water; otherwise oxygen is produced.
- Write half equations with electrons on the correct side.
- Explain the observation — gas bubbles, colour change, metal deposited.
Mistake 6: losing calculation marks for presentation
Cambridge awards marks for method. A bare answer that is wrong scores zero; the same wrong answer with visible correct working frequently scores most of the marks.
- Show the formula, then the substitution, then the answer.
- Carry units through the calculation.
- Give significant figures as instructed, usually three unless stated.
- If you realise an earlier value was wrong, note it and carry on — own-figure credit applies.
A revision plan by question type
| Weeks | Focus |
|---|---|
| 1–2 | Mole calculations: forty questions across all types, five-step method enforced |
| 3 | Equations: thirty balancing and ionic equation questions |
| 4 | Explanation questions by command word: twenty past questions |
| 5 | Organic chemistry: reactions, conditions and products, tested as a table from memory |
| 6 | Electrolysis and redox, including half equations |
| 7–8 | Full past Paper 4s under time, marked against official schemes |
| 9 | Error log and examiner reports |
For related technique work, see Cambridge IGCSE Chemistry 0620 common mistakes and command words and keywords for 0620.
Sources and a caveat
Syllabus content, paper structure and assessment objectives are from the Cambridge IGCSE Chemistry (0620) syllabus published by Cambridge International. Paper numbering and component options vary between syllabus versions and between Core and Extended routes — confirm which components your centre has entered you for. Mark schemes and examiner reports for recent series, available through your school, are the definitive guide to how marks are awarded.
Primary sources
- Cambridge IGCSE Chemistry (0620) syllabus
- Cambridge International — Cambridge IGCSE
- Cambridge International — programmes and qualifications
Portal figures are Tutopiya learning portal data read from the Learning Portal growth and health dashboard on 6 November 2026, comparing the 30 days to 5 November 2026 with the preceding 30 days.
Next step: take twenty past mole calculation questions and work every one using the five-step method, writing the ratio line explicitly each time. If that single change recovers marks — and for most candidates it does — apply the same discipline to equations and explanations. An IGCSE Chemistry tutor can mark a set and show you exactly where the scheme splits the marks.
Frequently asked questions
What is Paper 4 in Cambridge IGCSE Chemistry 0620? +
Paper 4 is the Extended theory paper, covering the full extended syllabus with structured questions. It carries the largest share of the marks for Extended candidates and includes calculations, equations, explanations and application to unfamiliar contexts.
What is the most common mistake in IGCSE Chemistry Paper 4? +
Mole calculations where the student works out the moles of one substance and then forgets to apply the ratio from the balanced equation. It appears in nearly every series and costs multiple marks each time.
How do I get full marks on equation questions? +
Balance the equation, include state symbols where asked, and write ionic equations with the correct charges and only the species that change. Check that both sides balance for atoms and for charge before moving on.
How much organic chemistry is on Paper 4? +
A substantial section, covering homologous series, functional groups, isomerism, addition and substitution reactions, polymers and the conditions for each reaction. The conditions are examined as often as the products.
How should I revise for Paper 4? +
By question type rather than by topic. Calculations, equations, explanations and practical application are the four recurring demands, and each needs its own drill. Mark everything against the official scheme.
How many students use Tutopiya's Cambridge IGCSE Chemistry resources? +
Cambridge IGCSE Chemistry drew 1,220 unique visitors to Tutopiya's learning portal in the 30 days to 5 November 2026, and was one of the few Cambridge subjects to grow through the autumn, up 14% month on month. Figures come from Tutopiya's own learning portal analytics, read from the Learning Portal growth and health dashboard on 6 November 2026.
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Mahira Kitchil
Project Head of AI Buddy
Mahira Kitchil is Project Head of AI Buddy at Tutopiya, where she works with schools and families across the UK, Malaysia, India and the Gulf. She follows Cambridge, Pearson Edexcel, AQA, OCR and IB announcements as they land in each market, and writes these guides from published syllabuses, board timetables and regulator guidance rather than second-hand summaries.
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