Cambridge IGCSE Chemistry 0620: Command words and keywords

Cambridge IGCSE Chemistry (0620) uses the standard Cambridge command words. Mark schemes include science-specific marking principles: examiners consider the context and scientific use of keywords—correct terms used incorrectly do not receive credit. Spellings of syllabus terms must allow clear separation from other terms (e.g. ethane vs ethene).

Command words in Chemistry 0620

Command word	What to do in Chemistry	Common mistake
State	Short answer; one point per mark	Long explanation
Define	Precise meaning	Including examples when not asked
Describe	What happens; no “why”	Adding explanation
Explain	Why or how; use “because”	Only describing
Suggest	Apply to new context	Textbook recall
Calculate	Show working; units; correct significant figures	Wrong units or formula
Write	Balanced equation; state symbols if required	Unbalanced or wrong products
Draw	Displayed formula; correct bonds	Wrong structure (e.g. ethane vs ethene)
Complete	Fill table or equation	Missing state symbols
Compare	Similarities and differences	No comparative language
Predict	What will happen based on knowledge	Vague or unsupported
Evaluate	Judge; conclude with evidence	No conclusion

Keywords examiners look for

Chemistry mark schemes reward syllabus terminology. Incorrect use of a keyword (e.g. wrong context) does not earn credit. Spelling must allow unambiguous separation from other terms—e.g. ethane/ethene, acid/alkali.

States and structure

Term	Use correctly	Avoid
Solid / liquid / gas	State of matter	Vague “matter”
Ion	Charged particle	Atom when ion is meant
Molecule	Covalent bond; shared electrons	Ionic compound
Ionic bond	Electrostatic attraction between ions	”Shared electrons”
Covalent bond	Shared pair of electrons	”Transfer” for covalent
Metallic bond	Sea of delocalised electrons	Covalent or ionic
Electrolyte	Conducts when molten or dissolved	Solid salt (doesn’t conduct when solid)

Organic chemistry

Term	Use correctly	Avoid
Alkane	C–C single bonds only; saturated	Alkene
Alkene	C=C double bond; unsaturated	Alkane
Ethane	C₂H₆; single bond	Ethene (C₂H₄)
Ethene	C₂H₄; double bond	Ethane (C₂H₆)
Homologous series	Same functional group; same general formula	Random family
Functional group	Part that determines reactions	Whole molecule
Combustion	Reaction with oxygen	Incomplete definition
Cracking	Breaking long chains into shorter ones	Wrong process
Polymerisation	Joining monomers	Depolymerisation
Addition	Single product (e.g. alkene + Br₂)	Substitution
Substitution	One atom replaced	Addition

Acids, bases and salts

Term	Use correctly	Avoid
Acid	H⁺ donor	”Sour” (observation not definition)
Base	H⁺ acceptor / oxide or hydroxide	”Alkali” for insoluble base
Alkali	Soluble base; OH⁻ in solution	Base (broader)
Neutralisation	Acid + base → salt + water	Incomplete equation
Salt	Compound from acid (H⁺ replaced)	Any ionic compound
Titration	Finding concentration by reaction	Vague “mixing”
End point	When neutralisation complete	”When it changes colour” (indicator dependent)

Electrochemistry and energy

Term	Use correctly	Avoid
Oxidation	Loss of electrons	Gain of oxygen only
Reduction	Gain of electrons	Loss of oxygen only
Oxidising agent	Accepts electrons	Reduces
Reducing agent	Donates electrons	Oxidises
Exothermic	Releases energy (ΔH negative)	“Hot”
Endothermic	Takes in energy	”Cold”
Activation energy	Minimum energy for reaction	”Energy to start” (vague)
Catalyst	Speeds up reaction; unchanged	”Used up” or “consumed”

Quantitative chemistry

Term	Use correctly	Avoid
Relative atomic mass	Weighted mean mass of isotopes	Mass of one atom
Mole	6.02 × 10²³ particles	”Amount” without definition
Concentration	mol/dm³ or g/dm³	”Strength”
Percentage yield	(actual/theoretical) × 100	Wrong formula
Limiting reactant	Runs out first	”Used up” (vague)

Mark scheme principles (Chemistry 0620)

Cambridge Chemistry mark schemes state:

• Context matters—keywords must be used correctly in context.
• Spelling—must allow clear separation from other syllabus terms (e.g. ethane vs ethene).
• Contradictory statements—no credit if one part contradicts another.
• Credit given positively—marks awarded for what is correct; not deducted for errors.
• Alternative answers—mark schemes list acceptable alternatives.

Common mistakes from examiner reports

• Ethane vs ethene—confusion in displayed formulae; double bond for ethene.
• Oxidation vs reduction—wrong direction (oxidation = loss of electrons).
• Acid/base definitions—using observations instead of H⁺/OH⁻.
• Units—missing or wrong (e.g. kJ not kJ/mol).
• State symbols—(s), (l), (g), (aq) omitted in equations.
• Balanced equations—wrong stoichiometry or products.
• Describe vs Explain—describing when explanation is required.

Tips for Chemistry 0620

1. Learn the pairs: ethane/ethene, oxidation/reduction, acid/base, alkane/alkene.
2. Use keywords in context—don’t just drop them in.
3. Check spelling for terms that could be confused (ethane, ethene).
4. Show working in calculations; include units.
5. State symbols in equations when required.
6. Past papers + mark schemes show acceptable wording.

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Based on Cambridge IGCSE Chemistry 0620 syllabus and mark schemes. Check the latest syllabus for your exam series.