IGCSE Chemistry rewards candidates who can read a question the way a marker reads an answer. The syllabus is wide, the ideas are sequential, and the difference between a grade 7 script and a grade 9 script is rarely raw knowledge. It is almost always the discipline of answering what was asked, in the form the mark scheme expects, with the right command verb at the start of every sentence. This article focuses on that gap, with worked examples drawn from the kinds of structured question that appear on Paper 2 and the extended-response items that dominate the back third of the paper.
Why Paper 2 is the grade-mover in IGCSE Chemistry
Paper 2 is the structured-question paper in IGCSE Chemistry, and for most centres it is where the bulk of marks are picked up. The format is familiar: short questions, each worth between one and six marks, moving from recall at the front of the paper to application and extended writing at the back. Many candidates treat it as a recall exercise and lose marks on the items that require explanation. In practice, the front half of the paper is forgiving — definitions, balanced equations, simple mole work — and the back half is where grade 8 and 9 candidates separate themselves from the rest of the cohort.
The mark allocation on the back half of the paper is not subtle. A single six-mark question can be the difference between a grade 7 and a grade 9 once the boundary sits at roughly 75–80% of the total. A candidate who scores full marks on three extended-response items, even if they drop two or three marks elsewhere, will usually out-score a candidate who plays it safe and writes in single-clause sentences. The trap is to assume the front-half questions are 'safe' marks. They are safe only if you do not rush them; candidates who race through definitions to save time for the long items are the ones I see dropping a mark on a balanced equation because they forgot to include state symbols, or losing a mark on a definition of 'electrolyte' because they wrote 'conducts electricity' rather than 'conducts electricity when molten or in aqueous solution, decomposing into elements'.
Another reason Paper 2 matters disproportionately is the command-verb load. Every question on the paper opens with a verb, and the verb is the most reliable predictor of the kind of answer the marker expects. 'State' is one mark, one clean fact. 'Describe' requires an account of what happens, often with observations. 'Explain' is where the marks live, because the candidate has to give the chemistry, not just the observation. 'Suggest' is even less forgiving, because the mark scheme accepts answers based on the principles in the syllabus but the candidate is being asked to apply those principles to a context they have not seen. Reading command verbs accurately is the single highest-leverage revision activity a candidate can do for IGCSE Chemistry.
Reading the command verb before reading the question
The single most common error I see in IGCSE Chemistry Paper 2 scripts is the candidate who reads the stem, ignores the verb, and writes the answer they would have written if the verb had been 'state'. 'Describe the colour change when anhydrous copper(II) sulfate is added to water' is a one-sentence answer — blue, with a marked temperature rise — but most candidates write two paragraphs because they treat 'describe' as 'explain'. They lose the one mark they had because the marker is reading a wall of text and cannot find the observation. The reverse error is worse: candidates who see 'explain' and write a single observation, dropping both the explanation marks.
Three command verbs carry the bulk of the marks on Paper 2. State asks for a fact. One mark, one sentence, no extra chemistry. Describe asks for an account of what you would see, smell, or measure. The mark scheme rewards observations, not reasons. Explain asks for the chemistry behind the observation — the reaction, the bonding, the energy change — and it usually carries two or three marks, split into distinct points. The candidate who writes 'the solution turns blue' for an explain question has not answered the question at all, because the question is not asking for the observation. Candidates who train themselves to underline the command verb before reading the rest of the question pick up marks they previously left on the table.
Five command verbs that decide your grade
- State: one fact, no explanation. Used in definitions, in identification of substances, in listing properties. Candidates who write three sentences lose the mark because the marker can only credit the first sentence.
- Describe: observations, measurements, named apparatus. No 'because'. A 'describe the test for hydrogen' question needs 'lighted splint, pops', not 'because hydrogen is flammable'.
- Explain: the chemistry. Use the language of the syllabus — 'because the acid donates protons', 'because the oxide ion reacts with hydrogen ions', 'because the activation energy is too high at room temperature'.
- Suggest: apply principles to a new context. Mark schemes usually accept any reasonable answer, so candidates who write 'rate of reaction decreases because concentration decreases as reactants are used up' should pick up the mark even if the stem was about a new reagent they had not met.
- Compare and contrast: two marks, one for each side. 'X is higher than Y' is one mark. 'X is higher than Y because X has a higher surface area' is still one mark unless the question asks for explanation.
The six-mark extended-response items use a more demanding verb: 'discuss', 'evaluate', 'describe and explain'. These are the items where the discipline of reading the verb pays the highest dividend, because the mark scheme usually has a points-based structure: a mark for one point, a mark for a second point, a mark for development of a point, a mark for a clear conclusion. Candidates who ramble lose the structure marks. Candidates who plan three points and a conclusion in the margin before they write are the ones who walk away with five or six.
Mole calculations: the 6 patterns that decide a candidate's score
The mole is the single concept that runs through IGCSE Chemistry more consistently than any other. A candidate who cannot move confidently between mass, moles, volume of gas, and concentration will lose marks on at least one question on every Paper 2 they sit. In my experience teaching this syllabus, six calculation patterns account for nearly every mole item that appears on the structured paper, and the candidates who learn to recognise the pattern in the first sentence of the question are the ones who finish the paper with time to spare.
The first pattern is the direct mass-to-moles conversion. The question gives a mass, often in grams, and asks for moles. The candidate divides the mass by the molar mass. The trap is forgetting the units: the mark scheme accepts '0.5 mol' but penalises '0.5' or '0.5 moles'. The second pattern is the reverse: given moles, calculate the mass. Multiply by the molar mass. The third pattern is the volume-of-gas conversion at room temperature and pressure (r.t.p.), where one mole occupies 24 dm³. Candidates who confuse r.t.p. with s.t.p. and use 22.4 lose the mark; candidates who do not write the units of volume lose the mark. The fourth pattern is the concentration calculation, where moles equal concentration (in mol/dm³) times volume (in dm³). Converting cm³ to dm³ by dividing by 1000 is the single most common arithmetic error on this item.
The fifth pattern is the limiting-reagent calculation, often embedded in a longer question about a reaction. The candidate must identify which reactant runs out first, then scale the answer to that reactant. Many candidates calculate the moles of both reactants and write both products; the marker credits the smaller. The sixth pattern is the theoretical-yield calculation, where a percentage yield or an actual mass is given and the candidate must work backwards to find the theoretical mass. These items usually carry three or four marks and combine the patterns above. Recognising which pattern is being tested, in the first ten seconds of reading the question, is the difference between finishing the calculation in two minutes and spending six minutes on it.
Common pitfalls and how to avoid them
- Unit conversion slip-ups. A 250 cm³ volume in a concentration calculation is 0.25 dm³, not 250 dm³. Convert before you calculate, write the unit at the end of every line, and you will not lose a mark to a missing factor of 1000.
- Using 22.4 instead of 24. The IGCSE syllabus uses 24 dm³ at r.t.p. unless the question specifies s.t.p. Candidates trained on older textbooks sometimes default to 22.4; the syllabus value is 24, and the mark scheme expects 24.
- Forgetting the state symbols. Balanced equations on Paper 2 should include (s), (l), (g), (aq). The mark is one mark and it is awarded independently of the rest of the equation, so a candidate who balances correctly but forgets the state symbols loses a mark they could have kept.
- Misreading the question stem. 'Calculate the mass of magnesium sulfate that can be made from 4.0 g of magnesium' is a yield question. 'Calculate the mass of magnesium sulfate formed when 4.0 g of magnesium reacts with excess sulfuric acid' is also a yield question but the 'excess' tells you the magnesium is the limiting reagent — a different calculation, but the same pattern. Underline the limiting-reagent cue every time it appears.
How to structure a six-mark extended response
The six-mark items at the back of Paper 2 are the single highest-leverage questions on the paper. They are also the items where the difference between a grade 7 script and a grade 9 script is most visible. The mark scheme on these items is almost always points-based: three distinct points, each developed, with a clear conclusion or comparison. A candidate who writes four paragraphs of unconnected material will pick up at most two marks, because the marker is crediting points, not prose.
The first move is planning. A six-mark item should be planned in the margin before any writing begins. Three points, one per planned line, with a clear conclusion or evaluation. The candidate who has three bullet points on the page before they write the first sentence of the answer is the candidate who will reach five or six. The candidate who starts writing immediately will reach three or four. The planning takes ninety seconds; the marks it recovers are the difference between a B and an A* on the paper overall.
The second move is signposting. Sentences should open with connectives that mark each point: 'first', 'second', 'finally', 'in addition'. The marker is reading thirty scripts an hour and is hunting for points. A candidate whose paragraphs are not signposted is asking the marker to find the points by reading the chemistry. Most markers will not. A candidate who opens each point with a clear marker is being read the way the mark scheme is being read, and the marks align.
A worked six-mark question
The question: 'Describe and explain the trends in reactivity of the Group 1 elements with water as you go down the group.' A grade 7 answer writes: 'Reactivity increases down the group. Lithium reacts slowly, sodium reacts more vigorously, potassium reacts very vigorously and produces enough heat to ignite the hydrogen. This is because the outer electron is further from the nucleus, so it is lost more easily.' A grade 9 answer plans three points: trend, observation, explanation. It writes: 'Reactivity increases down the group. Lithium fizzes gently, sodium fizzes vigorously and melts, potassium ignites with a lilac flame. The explanation is that the outer electron is further from the nucleus, so the electrostatic attraction from the nucleus is weaker, and the electron is lost more easily, producing a more vigorous reaction.' The grade 9 answer picks up the trend mark, three observation marks, two explanation marks, and a mark for clear development. Six out of six.
The third move is to make the conclusion explicit. Many mark schemes on 'discuss' or 'evaluate' items carry a conclusion mark. Candidates who write the body of the answer and stop are losing a mark they could have picked up with a single sentence. The conclusion should be specific: not 'in conclusion, hydrogen is produced', but 'in conclusion, hydrogen is produced most rapidly by potassium because the outer electron is lost most easily, so the reaction is most vigorous'.
Lab work and the Paper 6 question paper: what transfers from the practical
Paper 6 is the alternative-to-practical paper, and for candidates who have spent time in the lab, it is the easiest paper on the specification. The questions are not asking candidates to do the experiment; they are asking candidates to read a description of an experiment and answer as if they had done it. The skills being tested are measurement reading, observation recording, identifying control variables, and suggesting improvements — all of which transfer from real lab work, but only if the candidate has been paying attention to the method and not just the result.
The first skill is reading a thermometer, a measuring cylinder, or a burette to the precision the apparatus allows. A thermometer reading to 1 °C should be recorded as '24.0 °C', not '24 °C' and not '24.00 °C'. A burette reading should be recorded to 0.05 cm³, because the smallest scale division is 0.1 cm³ and the convention is to estimate one figure beyond. A measuring cylinder reading should be recorded to the nearest cm³, not to 0.1 cm³, because the smallest scale division on a 100 cm³ cylinder is 1 cm³. These are the marks that candidates drop because the practical was not taken seriously — and they are the marks that a centre running a structured IGCSE Chemistry lab programme can pick up by drilling apparatus-reading in the first two weeks of the course.