One phrase short of full marks: what examiners look for in 6 common IGCSE Chemistry Explain questions
Explain questions are marked phrase by phrase, not on understanding. A student who has the right idea but leaves out one phrase gets 2 out of 3, every time. This guide splits six common questions into the phrases you must write, lets you hide them to test yourself, and lists the words to swap so you stop losing marks for free.
The short answer
The number in brackets at the end of a question, such as [3], is the number of marking points. The examiner looks for a phrase matching each one: all there, full marks; one missing, one mark lost, however long and well understood the answer is.
Explain asks for the reasons why or how, not just what happens. What each command word asks for is in the Paper 4 guide.
Why understanding still gets 2/3
Most students stop writing when the explanation feels complete. Examiners do not mark feelings; they mark phrases. A student explains that higher temperature makes particles collide more often, then stops, and the activation energy point, another mark, is never written.
Students who think in another language and translate have one more trap: the phrase the mark scheme wants gets shortened on the way. The fix is to learn the English phrases for the common questions whole.
6 common Explain questions, phrase by phrase
Pick a topic and see which phrase earns each mark. Below it is what students often write, and the phrase that is missing. To test yourself, hide the phrases and think before you tap.
Explain, in terms of particles, why the rate of reaction increases when the temperature is increased. [3]
Phrases the examiner looks for (1 phrase = 1 mark)
1. particles have more kinetic energy / move faster
2. more frequent collisions (more collisions per second)
3. a greater proportion of collisions have energy ≥ the activation energy
What students often write
The particles move faster and collide more often, so the rate increases.
- Phrase 1
- Phrase 2
- Phrase 3
- 2/3
Missing the activation energy point, the one that makes temperature different from concentration. Most students stop at collisions.
Words to swap: one wrong word, one lost mark
Some words sound alike but score differently. This table collects the ones that come up most in answers from students who know the chemistry and still lose marks.
Tap a row to highlight it
more collisions
Scoresmore frequent collisions / more collisions per second
WhyRate is about time, so the collisions must be more frequent
covalent bonds break (when a simple molecular substance melts or boils)
Scoresweak intermolecular forces are overcome
WhyThe bonds inside each molecule stay intact; the molecules separate
molecules (for sodium chloride)
Scoresions / oppositely charged ions
WhyIonic compounds have no molecules
electrons move (in a molten or dissolved ionic compound)
Scoresions are free to move
WhyFree electrons belong to metals and graphite
a strong acid is concentrated
Scoresa strong acid is completely ionised (dissociated) in water
WhyStrong = fully ionised; concentrated = a lot per dm³. Different ideas
the equilibrium moves right
Scoresthe position of equilibrium shifts to the right, because there are fewer moles of gas on the right
WhyDirection and a reason you can count
the metal can change shape (why is it malleable?)
Scoreslayers of positive ions slide over each other
WhyThe examiner wants the layers sliding, not a restatement of malleable
hydrogen is produced (when asked what is seen)
Scoresbubbles / effervescence
WhyYou see bubbles, not hydrogen
A 4-step method for Explain answers
1. Read the number in brackets and count how many phrases you need.
2. Start from what happens to the particles, ions or bonds, not from what you see.
3. Link cause and effect with so or because, so the examiner sees what each phrase leads to.
4. Before moving on, check your answer against the table above for any word from the red column.
Where Boom-Tutor helps
Every topic in the course ends with exam-style written questions. The student writes the answer, and AI marks it against the marking points and says which phrase is missing, so this can be practised every day without waiting for a teacher.
The self-learn IGCSE Chemistry course is ฿4,900, paid once, with access until the exam session. The platform is expected to open around December 2026. To see where marks are being lost now, start with the free check below.
Try: all 12 IGCSE Chemistry topics, interactive, on one page
Stuck on something the AI explanation did not fix? Book a lesson with Boom.