Command Words and What Examiners Expect

Command words are the instruction verbs at the start of a question — state, describe, explain, calculate, compare, evaluate and so on. They tell you exactly what kind of answer the examiner wants and how much to write. Misreading or ignoring the command word is one of the most common reasons students lose marks across all three sciences: two candidates can know identical science, but the one who responds correctly to the command word scores higher. Because Combined Science A spans biology, chemistry and physics, the same command word can feel very different depending on the paper — yet what it demands never changes.

By the end of this lesson you should know the command words OCR uses, be able to recognise what each demands, apply them across all three sciences, and avoid the two classic traps — describe versus explain and compare versus contrast.

Reading the command word tells you which AO a question targets — state/describe usually signal AO1, explain/calculate/suggest signal AO2, and evaluate/justify signal AO3 — so this technique underpins all three objectives.

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The OCR Command Words at a Glance

Command word	What it asks for	What to write
State / Name / Give / Identify	A short fact, no explanation	A word or short phrase
Define	The precise scientific meaning of a term	A concise, exact definition
Describe	What happens or what the data shows	An ordered account of features, steps or trends — no reasons
Explain	What happens and why	A linked chain of reasoning with "because"/"so"
Compare	Similarities and differences	Explicit, linked point-by-point comparisons
Suggest	Apply knowledge to an unfamiliar context	A reasonable, scientific response using the given information
Calculate	Work out a numerical value	Formula, substitution, working and answer with units
Determine	Use given data or a calculation to find a value	Working that leads to the value the data imply
Estimate	A rough but reasoned value	A sensible figure with brief justification
Predict	Say what will happen	An expected outcome, often with a short reason
Evaluate	Strengths and weaknesses, then a conclusion	A balanced judgement supported by evidence
Justify	Reasons supporting a decision or conclusion	The decision plus the evidence/reasoning for it
Plot / Draw / Sketch / Label	Produce or annotate a graph or diagram	Accurate points/lines/labels; a ruler where straight lines are needed

The sections below give a precise meaning and examples drawn from all three sciences for the most important command words.

Exam Tip: Before writing anything, underline the command word. It forces you to check what is actually being asked and sets the depth and style of your answer.

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State / Name / Give / Identify

Meaning: write a short, factual answer. No explanation is needed. Usually 1 mark.

• Biology: "State the organelle where aerobic respiration takes place." → Mitochondria
• Chemistry: "Name the gas produced when a metal reacts with an acid." → Hydrogen
• Physics: "Give the unit of electrical resistance." → Ohm (Ω)

Exam Tip: If the word is state or name, keep it to a word or short phrase. A long explanation wastes time and can introduce an error that contradicts your correct answer.

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Define

Meaning: give the exact scientific meaning of a term. Usually 1–2 marks.

• Biology: "Define osmosis." → "The net movement of water molecules from a dilute solution (higher water concentration) to a concentrated solution (lower water concentration) through a partially permeable membrane."
• Chemistry: "Define an isotope." → "Atoms of the same element with the same number of protons but a different number of neutrons."
• Physics: "Define acceleration." → "The rate of change of velocity."

Exam Tip: OCR definitions are marked on specific key words. Forgetting "partially permeable membrane" for osmosis, or "same number of protons" for isotopes, can cost the mark even though the rest is right.

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Describe

Meaning: say what happens, or what the data shows — an ordered account of the main features, steps or patterns. Do not give reasons. Usually 2–4 marks.

• Biology: "Describe how to test a leaf for starch." → boil in water, then in ethanol to remove chlorophyll, rinse, add iodine; blue-black shows starch.
• Chemistry: "Describe what you see when sodium reacts with water." → it floats and moves on the surface, fizzes, melts into a ball, and eventually disappears.
• Physics: "Describe the motion shown by a horizontal line on a distance–time graph." → the object is stationary (not moving).

Exam Tip: When describing data, quote figures. "The rate increases" is weak; "the rate increases from 5 cm³/min at 20 °C to 18 cm³/min at 40 °C" is a describe answer that scores.

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Explain

Meaning: say what happens and why — give reasons or mechanisms, not just an account. Usually 2–6 marks.

• Biology: "Explain why enzyme activity falls above the optimum temperature." → the active site changes shape because the bonds holding the enzyme's shape break, so the substrate no longer fits — the enzyme is denatured.
• Chemistry: "Explain why increasing concentration increases the rate of reaction." → there are more reactant particles in the same volume, so there are more frequent collisions, so more successful collisions per second.
• Physics: "Explain why a parachute reduces a skydiver's speed." → it increases air resistance, so the upward force now exceeds weight, so the skydiver decelerates to a lower terminal velocity.

Exam Tip: The single most common command-word error is describing when asked to explain. Make sure every explain answer contains a reason — signalled by "because", "this means that" or "so".

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Compare

Meaning: give similarities AND differences between two or more things, using explicit linking language. Usually 2–4 marks.

• Biology: "Compare mitosis and meiosis." → "Both are preceded by DNA replication. However, mitosis produces two identical cells, whereas meiosis produces four genetically different cells."
• Chemistry: "Compare ionic and covalent bonding." → "Both involve the outer electrons of atoms. However, ionic bonding transfers electrons whereas covalent bonding shares them."
• Physics: "Compare series and parallel circuits." → "In both, components form part of a circuit. However, in series the current is the same everywhere, whereas in parallel it splits between branches."

A genuine comparison links the two things in each point ("A does X, whereas B does Y"). Listing features of one, then separately listing features of the other, is weaker and often loses marks because the examiner has to do the comparing for you.

Exam Tip: Compare is not the same as contrast. Contrast asks for differences only; compare asks for both similarities and differences. Use both, whereas, however, similarly, unlike to make each comparison explicit.

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Suggest

Meaning: apply your knowledge to a new or unfamiliar situation. There may be more than one acceptable answer. Usually 1–3 marks.