50 IGCSE Chemistry 0620 keywords to write exactly: words the mark scheme won't accept, and spellings that turn into another word

Knowing the answer but writing a word that means nearly the same thing can still lose the mark. Here are 42 keywords examiners look for, across the 12 topics of the 0620 syllabus, each next to the word students often write instead, plus 8 pairs where one wrong letter turns a word into another syllabus term. There is a 10-question quiz, and a PDF summary on LINE.

In short

Written IGCSE Chemistry answers are marked point by point: the examiner looks for a word or phrase that matches each point. Most of it can be said in your own words, but for some keywords the usual substitute means something different in science, such as more collisions versus more frequent collisions, or clear versus colourless. One wrong word and that point is lost.

The other group is words where one wrong letter makes another syllabus word, such as ethane and ethene. Cambridge's marking rules say these must be clearly separable or they earn nothing. Together that is 50 words.

"It means the same thing", so why no mark?

Every 0620 mark scheme opens with its Science-Specific Marking Principles. The two that matter most here, summarised from the 0620/42 May/June 2025 scheme:

Principle 1: examiners consider the context and scientific use of keywords. A keyword that is present but used incorrectly earns no mark.

Principle 3: spelling does not have to be correct, but a syllabus term must be clearly distinguishable from another syllabus term it could be confused with. Cambridge's own example is ethane / ethene.

To be fair, mark schemes accept equivalent wording in many places (marked ALLOW or owtte). The problem is not that every word must be memorised exactly; it is that the substitute a student picks often has a different scientific meaning from the one the examiner wants, and the student does not notice. Students who think in another language and translate run into this most.

42 keywords to write exactly, by topic

Each keyword shows what students often write (red), what scores (green) and why. Pick a topic, or press "Test yourself" to hide the answers and recall them first.

42 keywords

1. random
Often written (no mark)particles move around
Write this (scores)particles move randomly (in all directions)
Diffusion and gases are explained by random motion; move around does not say random
2. evaporation
Often written (no mark)boiling
Write this (scores)evaporation (from the surface, below the boiling point)
Boiling happens throughout the liquid at the boiling point; evaporation is at the surface, below it
3. isotopes
Often written (no mark)same element with a different mass
Write this (scores)same number of protons, different numbers of neutrons
Both protons and neutrons are needed; a different mass does not say why
4. ions
Often written (no mark)molecules of sodium chloride
Write this (scores)oppositely charged ions (in sodium chloride)
Ionic compounds have no molecules
5. electrostatic attraction
Often written (no mark)the ions share electrons
Write this (scores)electrostatic attraction between oppositely charged ions
Sharing is covalent; an ionic bond is the attraction between charges
6. shared pair
Often written (no mark)atoms share electrons
Write this (scores)a shared pair of electrons
The 0620 mark scheme gives a separate mark for pair
7. intermolecular forces
Often written (no mark)the covalent bonds break
Write this (scores)weak intermolecular forces are overcome (simple molecules)
Melting or boiling separates molecules; the bonds inside them stay
8. giant covalent
Often written (no mark)it has strong covalent bonds
Write this (scores)giant covalent structure with many strong covalent bonds
Simple molecules have strong covalent bonds too; giant is what explains the high melting point
9. delocalised electrons
Often written (no mark)free ions carry the charge
Write this (scores)delocalised (mobile) electrons carry the charge (metals, graphite)
In metals and graphite electrons carry the charge, not ions
10. metallic bonding
Often written (no mark)metal atoms bonded together
Write this (scores)attraction between positive ions and delocalised electrons
Name both sides of the attraction: positive ions and electrons
11. layers
Often written (no mark)the metal bends easily
Write this (scores)layers of ions (atoms) slide over each other
Bends easily just restates malleable; the examiner wants layers sliding
12. dm³
Often written (no mark)24 cm³
Write this (scores)24 dm³ (or 24 000 cm³) per mole of gas
A missing or wrong unit normally loses the final calculation mark
13. limiting reactant
Often written (no mark)the reactant there is less of
Write this (scores)the reactant that is used up first
Less mass is not fewer moles; compare moles using the equation
14. empirical formula
Often written (no mark)the smallest formula
Write this (scores)the simplest whole-number ratio of atoms
Ratio and whole-number are the marking words
15. mol/dm³
Often written (no mark)concentration in mol
Write this (scores)concentration in mol/dm³
mol is an amount; concentration is per volume
16. ions free to move
Often written (no mark)electrons are free to move
Write this (scores)ions are free to move (molten or in solution)
In a molten or dissolved ionic compound the ions carry the charge
17. bubbles
Often written (no mark)hydrogen is produced
Write this (scores)bubbles (at the electrode)
When asked what is seen: you see bubbles, not hydrogen
18. paler blue
Often written (no mark)the solution becomes clear
Write this (scores)the solution becomes paler blue / colourless
Clear means transparent, and a blue solution is already clear; give the colour
19. endothermic bond breaking
Often written (no mark)breaking bonds releases energy
Write this (scores)bond breaking is endothermic, bond making is exothermic
Breaking bonds always takes energy in; a very common swap
20. minimum energy
Often written (no mark)the energy needed to start a reaction
Write this (scores)activation energy: the minimum energy colliding particles need to react
Minimum is the heart of the definition
21. frequency of collisions
Often written (no mark)more collisions
Write this (scores)more frequent collisions (more collisions per second)
Rate is about time, so collisions must be more frequent
22. activation energy
Often written (no mark)the particles have more energy
Write this (scores)a greater proportion of collisions have energy ≥ the activation energy
What makes temperature different from concentration is the proportion of collisions reaching Ea
23. lower activation energy
Often written (no mark)a catalyst gives the particles more energy
Write this (scores)a catalyst provides a pathway with a lower activation energy
A catalyst gives no energy; it lowers the energy needed
24. rates equal
Often written (no mark)the amounts of reactants and products are equal
Write this (scores)dynamic equilibrium: forward and reverse rates are equal
At equilibrium concentrations are constant, not necessarily equal
25. reducing agent
Often written (no mark)a reducing agent is reduced
Write this (scores)a reducing agent is oxidised (it gives away electrons)
It reduces something else, so it is itself oxidised
26. completely ionised
Often written (no mark)a strong acid is concentrated
Write this (scores)a strong acid is completely ionised (dissociated) in water
Strong = fully ionised; concentrated = a lot per dm³
27. proton donor
Often written (no mark)an acid is a hydrogen donor
Write this (scores)an acid is a proton (H⁺) donor
Hydrogen could mean an atom or a molecule; what is donated is H⁺
28. effervescence
Often written (no mark)carbon dioxide is given off
Write this (scores)effervescence / fizzing (carbonate + acid)
If the question asks what is observed, a gas name is not an observation
29. white precipitate
Often written (no mark)the solution turns white
Write this (scores)a white precipitate forms
Ion tests score on precipitate and its colour
30. crystallisation
Often written (no mark)evaporate all the water
Write this (scores)heat until crystals start to form (saturated), then leave to cool
Evaporating to dryness gives a powder, not crystals, and may drive off the water of crystallisation
31. outer-shell electrons
Often written (no mark)the same number of electrons
Write this (scores)the same number of outer-shell electrons
Elements in a group have different totals; the outer shell is what matches
32. full outer shell
Often written (no mark)noble gases are stable
Write this (scores)noble gases have a full outer shell
Stable restates unreactive; it gives no reason
33. alloy
Often written (no mark)an alloy is a compound of metals
Write this (scores)an alloy is a mixture of a metal with other elements
An alloy is a mixture, not a compound
34. sacrificial protection
Often written (no mark)zinc forms a barrier
Write this (scores)zinc is more reactive, so it is oxidised instead of the iron
A barrier is how coating works; sacrificial protection needs reactivity
35. infrared radiation
Often written (no mark)greenhouse gases make a hole in the ozone layer
Write this (scores)greenhouse gases absorb and re-emit thermal (infrared) radiation
The greenhouse effect and the ozone hole are different topics
36. carbon monoxide
Often written (no mark)incomplete combustion produces carbon dioxide
Write this (scores)incomplete combustion produces carbon monoxide (toxic)
Incomplete combustion gives CO, which is toxic
37. C=C double bond
Often written (no mark)unsaturated: has a double bond
Write this (scores)unsaturated: contains a C=C (carbon–carbon) double bond
Carboxylic acids have C=O too; say carbon–carbon
38. colourless
Often written (no mark)bromine water turns clear
Write this (scores)bromine water turns from orange to colourless
Clear is not a colour; write colourless
39. general formula
Often written (no mark)compounds in the same group
Write this (scores)a homologous series has the same general formula and similar chemical properties
Group belongs to the Periodic Table
40. structural isomers
Often written (no mark)same formula, different shape
Write this (scores)same molecular formula, different structural formula
Name which formula is the same and which differs
41. boiling points
Often written (no mark)it separates by different densities
Write this (scores)fractional distillation separates by different boiling points
Fractional distillation works on boiling points, not density
42. solvent front
Often written (no mark)Rf = distance moved by spot ÷ length of the paper
Write this (scores)Rf = distance moved by spot ÷ distance moved by solvent front
Divide by how far the solvent moved, not the paper length

Scoring answers and common substitutes written by us in the style of a Cambridge 0620 mark scheme, not a published one.

Keep it for revision

All 50 keywords as a PDF to print

Add @boom-tutor on LINE and send "50 คำ"; the file comes straight back, with a box to tick each word you know.

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8 pairs where one letter changes the word

This is not about neat spelling. If the examiner cannot tell which word you meant, the point is lost. Practise each pair with its formula or symbol, chloride always with Cl⁻.

Word	Confused with	The difference
ethane	ethene	One letter between an alkane and an alkene; the example Cambridge itself gives
alkane	alkene	Saturated becomes unsaturated
chlorine	chloride	Chlorine = the element (Cl₂) · chloride = the ion (Cl⁻)
bromine	bromide	In halogen displacement a swap reverses the answer
sulfate	sulfite	SO₄²⁻ and SO₃²⁻ have different tests
ammonia	ammonium	NH₃ (a gas) and NH₄⁺ (an ion)
electrode	electrolyte	The rod versus the liquid that conducts
anode	anion	Anions move to the anode; misspell one and the sentence means nothing

Test yourself: 10 questions, which answer scores?

Each question has two answers: one scores, the other is what students often write. Pick one to see why, then draw a new set.

Which one earns the mark? Tap one answer per question

1. Energetics
2. Atoms and bonding
3. Electrochemistry
4. Acids, bases, salts
5. Environment
6. Atoms and bonding
7. Acids, bases, salts
8. Moles and formulae
9. Organic chemistry
10. Rates, equilibrium, redox

Answered 0/10

Scoring answers and common substitutes written by us in the style of a Cambridge 0620 mark scheme, not a published one.

Make the words stick, not just memorised

1. Understand why the word is right. Rate is events per unit time, so frequent is needed; once the reason is clear the word does not slip in the exam (memorising vs understanding).

2. Write full sentences, not single words. In the exam the keyword has to sit in a sentence that uses it correctly (the phrases examiners want in Explain answers).

3. Mark past papers point by point against the mark scheme and log every word that cost a mark in one notebook. It is usually the same few words (0620 past paper practice with the mark scheme).

For the other marking rules that cost students who understand chemistry, read what an IGCSE mark scheme is. The wording for ion and gas tests is in tests for ions, gases and flame colours.

Get the PDF

All 50 keywords as a PDF to print

Add @boom-tutor on LINE and send "50 คำ"; the file comes straight back, with a box to tick each word you know.

Get the PDF on LINE

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