Cambridge IGCSE Chemistry Paper 3 and Paper 4: structured answers

Paper 3 and Paper 4: what does the Theory paper assess?

This is the largest-weighted component of the qualification. Alongside content knowledge, it assesses whether a student can handle data, transfer a principle to an unfamiliar setting, build a checkable calculation and answer in the form required by the question’s verb.

Command words: the question’s verb determines the work

Command word	Answer core	What should be visible?
state	Concise fact	A clear statement without unnecessary explanation
describe	Features / change	What do the data, structure or observations show?
explain	Reason + link	Why does it happen and how does it cause the outcome?
compare	Shared criterion	Address both sides using the same feature
calculate	Visible working	Relationship, substitution, unit and result
suggest	Defensible proposal	A possible answer based on the given information

Under Cambridge’s official definitions, state asks for a clear expression, describe for characteristics or main features, explain for reasons and clear relationships, compare for similarities and/or differences, calculate for working from given facts, and suggest for a knowledge-based proposal where more than one answer may be possible.

Do not infer the number of sentences from the mark value alone. The command word and task content together show the necessary evidence. Repeating a property in an “explain” response is not a reason; a lengthy explanation in a “state” response may waste time.

The anatomy of a strong explanation

1Claim

The reaction is faster at a higher temperature.

2Chemical reason

The particles have greater average kinetic energy.

3Causal link

A larger proportion reaches the activation energy, so more successful collisions occur per unit time.

The claim → chemical reason → link to the stated outcome structure works for many, though not all, explanations. For bonding and structure: graphite conducts electricity → each carbon atom contributes a delocalised electron → these charge carriers can move along the layers. For reaction rate: higher temperature → greater average kinetic energy → a larger proportion of particles reaches the activation energy.

Avoid empty connectors. “Therefore it is faster” has value only when the relevant particle-level cause appears before “therefore”.

Four weak → missing → improved answer pairs

1. Why does molten sodium chloride conduct electricity?

Weak: “Because it is ionic.”
Missing: the charge carrier and its ability to move.
Improved: “When molten, the Na⁺ and Cl⁻ ions are free to move and can therefore carry electric charge.”

2. Why is a reaction faster at a higher concentration?

Weak: “There are more collisions.”
Missing: why, and which collisions increase the rate.
Improved: “There are more reactant particles per unit volume, so successful collisions occur more frequently per unit time and the rate increases.”

3. Describe the graph

Weak: “It increases.”
Missing: range, nature of the change and relevant data.
Improved: “From 0–40 s the gas volume increases rapidly, then the increase slows and reaches a constant value of about 48 cm³ from approximately 70 s.”

4. Suggest an improvement to measuring a temperature change

Weak: “Be more accurate.”
Missing: a specific limitation, apparatus or method, and expected effect.
Improved: “Use a covered, insulated polystyrene cup so that less heat transfers to the surroundings; the measured temperature change will then be less underestimated.”

An improved answer is not necessarily long. Every word performs work required by the command word.

Calculations: make the working traceable

1. 1Data and units
2. 2Relationship used
3. 3Rearrangement / ratio
4. 4Substitution
5. 5Result + unit
6. 6Order-of-magnitude check

A reliable order is:

1. list the data used with units;
2. name or write the relationship;
3. rearrange before inserting numbers where needed;
4. substitute and keep sufficient intermediate digits;
5. give the final value with a unit and justified precision;
6. check the order of magnitude.

In stoichiometry, the balanced equation and mole ratio are part of the calculation. Visible working exposes where an incorrect result began; a single calculator line hides it. The moles and stoichiometry guide gives a complete worked example.

Data and graph questions: separate reading from explanation

First state what the data show, then explain it. If temperature increases while reaction time decreases, that is an observed trend. A larger proportion of particles possessing the activation energy is the explanation.

A useful four-step routine:

• axis/data: what changes and in which unit;
• trend: increases, decreases, reaches a plateau, or contains an anomaly;
• evidence: one or two relevant pairs of values;
• Chemistry: which model or relationship accounts for it.

For a compare question, write about both systems using the same criterion. Do not create two unrelated descriptions and leave the reader to infer the comparison.

Unfamiliar context: the given information is a bridge, not a distraction

An AO2 question may present a substance or process not seen before. It does not demand a new syllabus topic; it asks for transfer of a known principle. Mark separately:

1. what the question states with certainty;
2. which known chemical model connects to it;
3. what conclusion follows from the two;
4. which claim is supported by data or an equation.

For example, if two energy diagrams for an unfamiliar catalyst are provided, its name is unnecessary. Infer a greater reaction rate from the pathway with lower activation energy while recognising that the difference between starting and finishing energy levels is unchanged.

75 minutes, 80 marks: let the mark value guide time

The average is under one minute per mark, though reading and checking make blocks more useful than a rigid rule. A practice starting point is:

• first 5 minutes: quick scan and begin with a stable question;
• approximately 60–63 minutes: continuous answering with length matched to marks;
• final 7–10 minutes: check omissions, units, equations, command words and answer positions.

If a two-mark explain item consumes five minutes, mark it and move on. Leave a long calculation in a checkable state—data, relationship and partial steps—so returning does not mean starting from zero.

A mark scheme is a diagnostic tool, not model prose

1. 1Independent answer
2. 2Mark with evidence
3. 3Name the missing idea
4. 4Rewrite the answer
5. 5New question after 48–72 hours

For every mark awarded, identify the evidence in the student’s answer. For every lost mark, write the missing idea, not the entire mark scheme. Then close it, rewrite the answer concisely and complete a new similar question 48–72 hours later.

Remember that older past-paper content may not match the current syllabus. It can develop skills, but a full simulation must fit the 2026–2028 content, Core/Extended route and current format.

A weekly 35-minute answer-building routine

• 10 minutes: two short command-word questions and one calculation independently.
• 8 minutes: self-marking with evidence identified.
• 7 minutes: rewrite two weak sentences as claim–reason–link.
• 5 minutes: one targeted task for a calculation or unit error.
• 5 minutes: update the error log and set the reattempt date.

Include Paper 1/2 multiple choice and Paper 5/6 practical reasoning once each week because the same knowledge becomes markable in different forms.

Six-line answer check

• I performed the work required by the command word.
• I did not merely repeat the statement in the question.
• In an explanation, the link between cause and outcome is visible.
• In a comparison, I addressed both sides using the same criterion.
• In a calculation, the relationship, ratio, unit and final result are visible.
• A specific reading or trend supports my data statement.

The aim is not to memorise “mark scheme language” but to make your own chemical knowledge visible in a precise, checkable form suited to the question.