IGCSE Chemistry Command Words Decoded

Command words are not decorative verbs. They define the job the answer must do. A student can know the Chemistry and still lose the mark by describing when the question asks for an explanation, giving a conclusion without evidence under deduce, or listing advantages without reaching a judgement under evaluate.

The current reference point for this page is the Cambridge IGCSE Chemistry 0620 syllabus overview and the command-word table in the 2026-2028 syllabus. The subject-specific list contains 22 command words.

The complete 22-word list

Command word	What your answer must do	Chemistry example
Analyse	Break information into parts and show the important relationships	Analyse a graph to connect concentration, gradient and reaction rate
Calculate	Work out a numerical answer from the information given	Calculate moles, mass, concentration, energy change or percentage yield
Compare	State relevant similarities and/or differences	Compare fermentation with hydration of ethene
Consider	Review the supplied information before responding	Consider the data and decide which metal is most reactive
Contrast	State relevant differences	Contrast ionic and covalent structures
Deduce	Reach a conclusion from evidence and show the link	Deduce an ion from test results
Define	Give the precise meaning of a term	Define isotope, oxidation or relative atomic mass
Demonstrate	Show clearly through reasoning, working or evidence	Demonstrate that a formula is consistent with percentage composition
Describe	State observations, features, changes or trends	Describe the colour change or the shape of a graph
Determine	Obtain an answer from data, measurement or calculation	Determine concentration from titration results
Discuss	Develop the relevant arguments or factors in a structured way	Discuss advantages and disadvantages of two industrial routes
Evaluate	Judge quality, suitability or importance using evidence	Evaluate an experimental method and decide whether the result is reliable
Examine	Inspect information closely and draw out relevant features	Examine results for anomalies and patterns
Explain	Give reasons and make cause-and-effect relationships clear	Explain why a higher temperature increases reaction rate
Give	Supply an answer from recall or a supplied source	Give one use of limestone
Identify	Name, select or recognise the required item	Identify the gas, apparatus, ion or anomalous result
Justify	Support a choice or conclusion with evidence or reasoning	Justify the best apparatus for measuring a gas volume
Predict	State what is expected from a pattern or chemical principle	Predict the effect of increasing pressure on equilibrium yield
Show (that)	Use working or reasoning to establish the stated result	Show that the empirical formula is CH2O
Sketch	Draw the essential shape or features without exact plotting	Sketch an energy-level diagram or rate curve
State	Express the answer clearly and directly	State the colour of chlorine gas
Suggest	Apply Chemistry to a situation where several sound answers may exist	Suggest one source of error or one reason actual yield is below theoretical yield

Identify includes naming. The current subject table does not list Name as a separate command word. A question can still tell you to name a substance as ordinary wording, but the syllabus definition of identify already includes naming, selecting and recognising.

The high-risk pairs

Describe versus explain

Describe answers what happened.

The solution changed from blue to colourless.

Explain answers why it happened.

Copper(II) ions were removed from the solution because they gained electrons at the cathode and formed copper atoms.

A colour change repeated under an explain question is still only a description. The explanation needs the chemical cause.

Compare versus contrast

Compare may require similarities, differences or both, depending on the wording and marks. Mention both items explicitly.

Fermentation uses a renewable raw material and is slow, whereas hydration uses ethene from petroleum and is fast.

Contrast focuses on differences.

Ionic structures contain oppositely charged ions in a giant lattice, whereas simple molecular substances contain discrete molecules with weak intermolecular forces between them.

A paragraph about only one item is not a comparison or contrast.

Deduce versus predict

Deduce starts from evidence already supplied.

The gas relights a glowing splint, so it is oxygen.

Predict extends a known pattern or principle to a new case.

Astatine should be less reactive than iodine because Group VII reactivity decreases down the group.

For deduce, show the evidence-to-conclusion link. For predict, state the pattern or principle supporting the prediction.

Calculate, determine and show (that)

All three can involve working, but the purpose differs.

• Calculate: obtain a numerical answer.
• Determine: use data, measurement or calculation to find the required result.
• Show (that): establish a result already stated in the question.

For all three, write the method, substitution, answer and unit. A bare answer removes the evidence needed for method marks.

Discuss versus evaluate

Discuss develops the relevant factors or viewpoints. Evaluate goes further and reaches a supported judgement.

A weak evaluation:

Method A is better.

A creditable evaluation:

Method A gives more precise volume readings because the gas syringe has smaller scale divisions and avoids gas dissolving in water. However, it may leak if the apparatus is not airtight. Overall, it is the better method when the joints are checked first.

The judgement is supported and acknowledges the limitation.

Suggest versus justify

Suggest asks for a chemically reasonable possibility. Justify asks you to defend a choice or conclusion.

Suggest why the yield is low: some product was lost during transfer.

Justify using a volumetric pipette: it delivers a fixed volume more precisely than a measuring cylinder because it has a smaller uncertainty for that volume.

One topic, different command words

Consider a rate experiment using calcium carbonate and hydrochloric acid.

State one factor that increases the rate.

Increase the acid concentration.

Describe the graph when the concentration is increased.

The curve has a steeper initial gradient and reaches the same final gas volume sooner.

Explain the increased rate.

There are more acid particles per unit volume, so collisions with the calcium carbonate surface occur more frequently and there are more successful collisions per second.

Analyse the graph.

The initial gradient is greater for the higher concentration, showing a faster initial rate, while both curves level off at the same volume because the same amount of limiting reagent was used.

Evaluate the method.

Measuring gas volume with a gas syringe is suitable because it records the product directly, but any leak lowers the measured volume. Checking airtight connections and repeating the experiment would improve reliability.

The Chemistry is related, but each answer performs a different job.

Worked Paper 4 example

A student adds aqueous bromine to hexane and hexene.

(a) Describe the result in each test tube. [2]
Hexane: the aqueous bromine remains orange. [1]
Hexene: the aqueous bromine changes from orange to colourless. [1]

(b) Explain the result with hexene. [2]
Hexene contains a carbon-carbon double bond / is unsaturated. [1]
Bromine adds across the double bond in an addition reaction. [1]

(c) Deduce which unlabelled sample is hexene. [1]
The sample that decolourises aqueous bromine is hexene. [1]

Part (a) rewards observations. Part (b) rewards the chemical reason. Part (c) rewards a conclusion linked to the test evidence.

Worked Paper 6 example

A student measures temperature using a thermometer marked every 1°C.

Identify the measuring instrument.

Thermometer.

State the smallest scale division.

1°C.

Suggest one source of error.

Heat is lost to the surroundings before the maximum temperature is recorded.

Justify one improvement.

Use an insulated cup with a lid because this reduces heat transfer to the surroundings and makes the recorded temperature change closer to the true value.

Evaluate the result.

The trend may be valid if repeats agree, but one reading alone is insufficient to judge reliability; repeated measurements and a mean are needed.

Paper 6 often changes command words within the same experiment. Reading the verb first prevents a correct observation from being placed where a reason or judgement was required.

The mistakes that cost marks

1. Learning an outdated or generic list. Use the 22 words in the current Chemistry syllabus, not a shortened revision-site list.
2. Treating “name” as a separate official table entry. Naming is contained within identify in the current list.
3. Explaining a describe question. Extra reasoning does not replace a missing observation.
4. Describing an explain question. Repeating the trend does not give its cause.
5. One-sided comparisons. Name both substances, methods or conditions.
6. Unsupported conclusions. Deduce and justify need the evidence or reasoning link.
7. No judgement in an evaluation. Advantages and disadvantages alone are discussion, not a completed evaluation.
8. Hidden working. Calculate, determine and show (that) should expose the method.
9. Skipping suggest questions. A scientifically reasonable proposal can earn the mark even when the situation is unfamiliar.
10. Writing by length instead of marks. Use one distinct creditable point per mark as the starting rule.

A practical answer routine

Before writing:

1. Circle or underline the command word.
2. Read the mark allocation.
3. Decide the answer type: fact, observation, cause, calculation, evidence-based conclusion or judgement.
4. Write one distinct point per available mark.
5. Check that the final sentence actually completes the command.

For a three-mark explanation, ask whether the answer contains a complete chain. For an evaluation, ask whether it contains a supported judgement. For a comparison, ask whether both items are visible.

Pair this guide with the dedicated Paper 2 guide, Paper 4 guide and Paper 6 guide, because the same command word behaves differently when the task is a calculation, data response or practical evaluation.