What is an IGCSE mark scheme? Why a child who "understands chemistry" still misses an A, and the marking traps parents should know
Your child explains chemistry well and the teacher says they understand it, yet the result is a B or a C. The gap is usually not knowledge but marking rules most parents never see. This guide explains what a mark scheme is, how examiners award marks, 6 traps taken from the rules Cambridge prints in every 0620 mark scheme, and the myths that send revision the wrong way. There is a quiz for parents.
In short
A mark scheme is the document Cambridge uses to tell examiners where each mark is. Written answers are split into marking points (M1, M2, M3), one mark each, and the examiner looks for a word or phrase that matches each point. Marks are not given for a general sense that the student understands.
Students who understand chemistry but miss an A usually lose marks to a handful of rules: a keyword used in the wrong context, answers that contradict each other, giving more answers than asked so a wrong one counts, or a missing unit. These rules are not in the textbook; they are on the first pages of every mark scheme.
What a mark scheme is, and who uses it
After each series Cambridge publishes a mark scheme for every paper to its centres. It says it shows the basis on which examiners were instructed to award marks. Before the answers come two sets of rules: Generic Marking Principles for every subject, and Science-Specific Marking Principles.
What parents should take from the generic rules: marking is positive. Marks are not deducted for errors or omissions, marks are always whole, and examiners do not mark with grade thresholds in mind. Writing more is not punished as such, but some kinds of writing more do cost marks (traps 2 and 3).
| Part of the mark scheme | What is in it | What parents should notice |
|---|---|---|
| Generic Marking Principles | General rules: positive marking, no deductions, whole marks | Nothing is deducted, but marks can be withheld |
| Science-Specific Marking Principles | 7 science rules: keywords, contradictions, spelling, ecf, list rule, calculations, equations | Where the 6 traps below come from |
| Question-by-question answers | Marking points M1, M2 … and what to ALLOW / IGNORE | The keywords an answer must contain |
6 traps that cost students who understand chemistry
Each comes from a rule in the 0620 mark scheme (checked against 0620/42, May/June 2025). The example answers are ours.
| Trap | The rule examiners apply | Example |
|---|---|---|
| 1. The keyword is there, but used wrongly | A keyword that is present but used incorrectly earns no mark | Intermolecular forces written for sodium chloride, an ionic compound: right word, wrong kind of substance |
| 2. Contradicting yourself | The examiner does not choose; a correct statement contradicted in the same part earns nothing | Asked to name the gas at the cathode, writes "hydrogen or oxygen": the right answer is there, but the examiner will not pick it |
| 3. More answers than asked | List rule: wrong answers count towards the number asked, and two contradicting answers count as one wrong answer | Asked to state two, writes three; a wrong one among the first two costs that mark, and the correct third does not help |
| 4. A misspelling that makes another word | Spelling need not be perfect, but a syllabus term must be distinguishable from others | ethane / ethene · chlorine / chloride (8 pairs in the 50 keywords guide) |
| 5. A missing unit | Unless the unit has its own mark, a missing or wrong unit normally loses the final mark | Concentration worked out correctly as 0.200, but the unit written as mol instead of mol/dm³ |
| 6. No working, and a wrong answer | A correct final answer scores in full without working (unless the question says show your working); a wrong one leaves nothing for partial credit or ecf | A 3-mark question done in the head with one slip: 0 instead of 2 |
The list rule: why three answers can score less than two
Parents rarely know this rule, and hard-working students are the ones it catches, because writing extra feels safer. When a question asks for two, the examiner reads the answer as continuous prose. A wrong answer earns nothing but still counts as one of the two, and correct answers written after that may be ignored.
| Answer | Mark | |
|---|---|---|
| Question | State two properties of transition elements. [2] | |
| Student A | They form coloured compounds. They have high densities. | 2/2 |
| Student B (writes extra) | They are soft. They form coloured compounds. They act as catalysts. | 1/2 · soft is wrong and counts as one of the first two; catalysts comes too late to help |
| Student C (contradicts) | They have high melting points. They form coloured compounds but their compounds are white. | 1/2 · coloured and white contradict, so they count as one wrong answer |
Parents' quiz: true or false?
Several beliefs about marking send revision the wrong way. Guess each one before the answer; every answer cites a rule in the 0620 mark scheme.
1. A mistake in an answer makes the examiner take marks away
2. A correct final answer with no working still gets full marks
3. Asked for two, writing four just in case is safer
4. Small spelling mistakes never cost marks
5. If the keyword is in the answer, the mark is safe
6. Get the first step wrong and every later step that uses it is lost
7. Chemical equations always need state symbols or they lose marks
8. Examiners adjust marks to fit the grade thresholds
So why do students who understand still slip?
Because understanding and writing for the examiner are different skills. A student who understands knows what the particles are doing, but without practice against marking points they write it their own way, often without the keyword the scheme wants, or with a substitute that means something different: more collisions instead of more frequent collisions.
Students who think in one language and write in another have one more layer: the keyword gets lost in translation. We collected the words that cost marks most often in 50 IGCSE Chemistry keywords to write exactly.
For the numbers, how far apart a C and an A* are and where the marks leak, read how to get an A* in IGCSE Chemistry.
What parents can do: 4 questions to ask the teacher or tutor
You do not need to know chemistry to ask them. The answers show whether lessons train answering to the mark scheme or only teach the content.
1. When my child does past papers, are they marked point by point against the mark scheme, or just checked for the right answer? (0620 past paper practice with the mark scheme)
2. Is there a list of keywords my child often gets wrong? A teacher who can name them at once has really read the answers.
3. Have they practised command words? State, Describe and Explain need different answers (the phrases examiners want in Explain answers).
4. In calculations, are they made to write every step, even when they can do it in their head?
How Boom-Tutor trains this
Every unit has written questions that AI marks against marking points in the style of the mark scheme. The AI does not write the answer; it says which point is missing and which word is used wrongly, and the student fixes it. In live lessons Boom reads each student's answers and helps turn them into the student's own words, with every keyword kept. · Self-learn course 4,900 THB, paid once, valid until the exam. The self-learn course opens once the content is complete; live lessons with Boom can be booked now, starting with a free call.
Try: all 12 IGCSE Chemistry topics, interactive, on one page
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