Paper 3 Theory (Core): Complete 0620 Answer Strategy
A complete Cambridge IGCSE Chemistry 0620 Paper 3 guide: 80 marks in 75 minutes of Core structured theory, command-word discipline, answer technique and Core calculations.
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 2026-08-24.
Paper 3 is the Core structured-theory paper: 80 marks in 1 hour 15 minutes, worth 50% of the qualification. It assesses Core content only, through definitions, equations, calculations, descriptions, explanations and data handling. Paper 3 is the largest single component on the Core route (Paper 3 + Paper 1 + Paper 5 or 6), where grades run C to G. A grade above C requires the Extended route; the routes are compared in the Core vs Extended guide.
The paper does not reward the longest answer. It rewards the required Chemistry, expressed clearly enough for each mark point to be identified.
The first reading rule
Before writing, identify three things:
- Command word: state, describe, explain, calculate, deduce, compare, suggest or another listed command.
- Mark allocation: the minimum number of creditable points or stages expected.
- Chemistry target: the principle being assessed, not merely the story around it.
For example:
Describe what you would see when magnesium reacts with dilute hydrochloric acid. [2]
A complete answer names two observations: bubbles/effervescence of gas, and the magnesium getting smaller / dissolving. “It reacts” scores nothing because it names no observation.
No Supplement content
Everything on Paper 3 is drawn from the Core column of the syllabus. This has two practical effects. First, Core candidates should not spend revision hours on Supplement-only calculations and explanations they will not be assessed on. Second, if a practice question requires a Supplement rule, it is a Paper 4 question and is not representative of what Paper 3 will ask. Confirm your route and entered papers using which paper am I taking.
A timing framework
The paper offers an average of about 56 seconds per mark. Use that as a warning against over-writing, not as a rigid per-item timer.
A practical approach:
- answer short, secure items efficiently
- show working immediately for calculations
- mark a difficult subpart and continue rather than losing several minutes
- reserve time to check units, equations, unanswered lines and graph readings
When practising, record where time was lost. A student who finishes late because of one calculation needs a different correction from one who over-writes every two-mark description.
Build answers from mark points
State, give and identify
Answer directly. Do not bury the fact in a paragraph.
State the colour of aqueous copper(II) sulfate. [1]
Blue.
Describe
Report what happens, what is observed or what pattern is shown.
The temperature rises, then stays constant, then falls slowly.
Explain
Give the cause and connect it to the result.
Metals conduct electricity because they contain a sea of delocalised electrons that are free to move and carry charge.
Compare
Make both items visible in the same point.
Diamond is hard because every carbon atom forms four strong covalent bonds, whereas graphite is soft because its layers can slide over one another.
Deduce
Link the evidence to the conclusion.
The gas relights a glowing splint, so it is oxygen.
The command-word guide covers the subject-specific command words in full.
Calculations at Core level
Core calculations are real but bounded. Expect relative formula mass, moles from mass, mass from moles, percentage composition and reacting-mass steps, rather than the more demanding Supplement routes such as titration, percentage yield or gas-volume ratios.
Use a vertical method so method marks survive an arithmetic slip:
- write the relationship or equation
- calculate Ar or Mr where required
- apply the balanced-equation ratio
- convert to the requested quantity
- state the unit
Original worked example: moles from mass
Calculate the number of moles in 12 g of carbon. [Ar: C = 12]
Step 1: relationship — moles = mass ÷ molar mass
Step 2: substitute — moles = 12 ÷ 12
Step 3: answer — 1 mol
The written relationship earns the method mark even if the final figure is mis-keyed.
Equations and formulae
Before moving on:
- count every atom on both sides
- use the correct formula for each substance
- include state symbols when requested or useful
- give a symbol equation when one is asked for; a word equation does not replace it
Structured-answer technique
Paper 3 questions build several small parts around one context. Two habits protect marks:
- Answer the part that is asked. Part (b)(ii) is marked on its own line. Do not answer it inside part (b)(i).
- Match points to marks. A two-mark description needs two distinct observations. A three-mark explanation needs three linked reasons. Count the marks, then count your points.
Practical and experimental questions inside Paper 3
Paper 3 can include experimental contexts even though Paper 5 or 6 is the dedicated practical component. Distinguish:
- observation: what is seen, measured or heard
- inference: what substance or process the observation supports
- variable: what is changed, measured or controlled
- improvement: a specific change addressing a named problem
“Human error” is not specific. Name the mechanism, such as heat loss or gas escaping.
The Paper 3 error log
Record more than the topic:
| Field | Example |
|---|---|
| Topic | Acids and bases |
| Question type | Describe [2] |
| Lost mark cause | Named only one observation |
| Correct answer | Effervescence and the solid dissolving |
| Follow-up | Write three fresh observation answers |
Useful categories: missing knowledge, wrong equation, calculation setup, unit, incomplete answer, command-word mismatch, data not quoted, imprecise observation, time management.
How Paper 3 differs from Paper 4
Paper 3 and Paper 4 share format, length, marks and weighting: 80 marks, 75 minutes, 50%. The differences are content and route:
- Paper 3 assesses Core content only; Paper 4 assesses Core plus Supplement.
- Paper 3 sits on the Core route (grades C to G); Paper 4 sits on the Extended route (grades A* to G).
- Paper 4 demands deeper explanation chains, harder calculations and more unfamiliar-context reasoning.
If a route change is under discussion, the Paper 4 theory guide shows the additional demands directly.
Final readiness checklist
A Paper 3-ready Core candidate can:
- finish an 80-mark paper within 75 minutes
- select the correct command-word response type
- give as many distinct points as there are marks
- show calculation stages and units
- balance equations
- write precise observations
- identify recurring error categories and repair them with fresh questions
Use the free diagnostic to locate weak Core topics, the topical questions and practice hub for targeted repair, and the Paper 1 multiple-choice (Core) guide to prepare the same knowledge for the other Core paper. The full topic notes cover every Core topic.