IB Chemistry Command Terms and Mark Schemes: What Do They Mean and How Are Marks Awarded?

Command terms are the exact verbs — state, explain, calculate, deduce — that tell you how much depth an examiner expects. Marks are awarded against a published mark scheme using method (M), accuracy (A) and sometimes reasoning (R) marks, each tied to what that specific command term demands.

"State" wants a bare fact, no reasoning attached. "Describe" wants detail or a process laid out, still without justification. "Explain" needs a reasoned why, linked to a scientific principle. "Discuss" wants a balanced look at multiple factors — and it almost always carries more marks than the other three.

"Define" wants a precise, learnt statement of meaning — the kind found in the syllabus glossary word-for-word. "Identify" wants you to name or pick out a substance, structure or trend from data given in the question, with no explanation required. Mixing the two up costs marks constantly on Paper 1 and Paper 2 short-answer sections.

Yes — SL and HL draw on the same command term list from the IB Chemistry guide; there's no separate HL glossary. What changes is depth: HL pushes terms like "explain" and "evaluate" into multi-step reasoning, mechanisms and quantitative work that SL versions of the same question rarely demand.

M marks reward the correct method — the equation, approach or working step you chose. A marks reward the accurate value that follows from it. Miss an M mark through a wrong formula, and you can often still scoop the A marks if your numbers stay internally consistent from that point on.

Usually because the mark scheme rewards the reasoning route, not just the final value. Command terms like "deduce" or "explain" need the linking steps written down on the page — examiners can't award marks for a correct number or conclusion with no visible working or justification behind it.

ECF, or error carried forward, lets you keep marks on later steps of a multi-part calculation even after an earlier mistake — provided your method from that point stays correct and internally consistent. It's the reason examiners insist you always show full working; one slip doesn't have to sink the whole question.

Paper 1's multiple-choice questions are marked strictly right or wrong, no partial credit available. Paper 2 and Paper 3 use point-based mark schemes with M, A and R marks tied to short-answer and extended-response command terms, and Paper 3 layers in practical-based and option-topic questions marked on the same logic.

Underline the command term in every question before you write a word — it tells you exactly how much detail earns marks. Match your answer's length and structure to that verb, so you're not wasting time over-explaining a "state" question or losing marks by under-explaining a "discuss" one.

Show every step: write the formula, substitute values with correct units, then state the final answer to the right number of significant figures — each of those typically earns its own mark. Jumping straight to a final number, even a correct one, risks losing every method mark on offer.

The one I see constantly: writing a "describe" answer to an "explain" question — students list what happens without ever saying why it happens, so reasoning marks go unclaimed. A close second is treating "deduce" like "calculate," skipping the logical link between given data and the stated conclusion.

Yes, substantially. Examiner reports repeatedly flag misread command terms — not missing content knowledge — as a leading cause of lost marks in Papers 2 and 3. A student who knows the syllabus cold but writes "describe" answers to "explain" questions can lose the equivalent of a full grade boundary across a paper.

Start with the IB's own Chemistry guide, which includes an official command term glossary worth learning by heart early in the course. Beyond that, practise with real past-paper mark schemes, not just questions — that's how you actually see how M, A and R marks map onto each verb in context.