IB Chemistry command terms are the instruction words that tell you what kind of answer to produce and how deeply to develop it. For example, state requires a brief answer, while explain requires reasons or causes, and evaluate requires a judgement based on strengths and limitations.
Knowing the chemistry is therefore only part of answering a question successfully. You must also present that knowledge in the form requested by the command term, while using the mark allocation and context to judge how many distinct points are needed.
Why IB Chemistry command terms matter
The current IB Chemistry guide groups command terms according to assessment objectives. Assessment objective 1 concerns knowledge, assessment objective 2 concerns understanding and application, and assessment objective 3 concerns analysis, evaluation and synthesis.
This grouping indicates general cognitive demand, but it does not assign a fixed number of marks to each term. An explain question is not automatically worth three marks, for example. Always read the mark allocation, chemical context and any additional instructions such as “using the data” or “with reference to bonding.”
Command terms are especially important in Paper 1B and Paper 2, where students construct written, mathematical and diagrammatic responses. The current course, first assessed in 2025, uses Paper 1A for multiple-choice questions, Paper 1B for data-based and experimental-work questions, and Paper 2 for short-answer and extended-response questions, as summarized in the official IB Chemistry subject brief.
The most important IB Chemistry command terms
The definitions below follow the terminology in the current Chemistry guide. The final column translates each official meaning into a practical exam response.
| Command term | What it requires | What your answer should contain |
|---|---|---|
| State | A specific name, value or brief answer without explanation or calculation | One direct fact, term or value |
| Define | The precise meaning of a term, concept or physical quantity | A complete and chemically accurate definition |
| List | A sequence of brief answers without explanation | Separate concise items |
| Identify | An answer selected from several possibilities | The relevant species, feature or value |
| Outline | A brief account or summary | The main stages or features, without detailed reasons |
| Describe | A detailed account | What happens, including relevant observations, patterns or steps |
| Explain | A detailed account including reasons or causes | What happens and why it happens |
| Calculate | A numerical answer with relevant working | Formula, substitution, working, units and sensible precision |
| Determine | The only possible answer | A result obtained logically from data or calculation |
| Show | The steps in a calculation or derivation | A transparent mathematical route to the stated result |
| Deduce | A conclusion reached from given information | Evidence from the question followed by a conclusion |
| Predict | An expected result | A chemically justified expected outcome when reasons are requested or implied |
| Suggest | A possible answer, solution or hypothesis | A plausible proposal consistent with chemical principles |
| Compare | Similarities between two or more items | Paired statements referring to both items throughout |
| Contrast | Differences between two or more items | Directly paired differences |
| Discuss | A considered, balanced review | Several relevant factors or arguments, evidence and a conclusion |
State, outline, describe and explain
These four terms are commonly confused because they can address the same chemical idea at different depths.
Consider the equilibrium:
N₂(g) + 3H₂(g) ⇌ 2NH₃(g)
State
Illustrative question: State the effect of increasing pressure on the equilibrium yield of ammonia.
Suitable answer: The equilibrium yield of ammonia increases.
No explanation is required. Writing about collision frequency or Le Châtelier’s principle adds time without improving a one-mark response unless the question separately requests a reason.
Outline
Illustrative question: Outline how increasing pressure affects this equilibrium.
Suitable answer: Increasing pressure shifts the equilibrium toward ammonia, the side with fewer moles of gas.
An outline gives the central relationship but does not need a fully developed molecular or thermodynamic account.
Describe
Illustrative question: Describe the change in equilibrium composition when pressure increases at constant temperature.
Suitable answer: The equilibrium shifts to the right. The equilibrium amounts of nitrogen and hydrogen decrease, while the equilibrium amount of ammonia increases.
A description reports what changes. It should include enough detail to account for the relevant species or trend.
Explain
Illustrative question: Explain why increasing pressure increases the equilibrium yield of ammonia.
Suitable answer: Increasing pressure favours the side with fewer moles of gas. The reactant side has four moles of gaseous particles for every two moles on the product side, so the equilibrium shifts right and the ammonia yield increases.
An explanation must make the causal chain visible. Words such as because, therefore and so can help, but they only earn credit when the chemistry connecting the statements is correct.
Numerical and data-based command terms
Calculate, determine and show
For calculate, show the relevant stages rather than writing only the final number. A strong response normally includes the equation, substituted values, answer, unit and appropriate significant figures.
For example, if asked to calculate the amount of solute in 25.0 cm³ of a 0.200 mol dm⁻³ solution:
n = cV = 0.200 mol dm⁻³ × 0.0250 dm³ = 5.00 × 10⁻³ mol
Determine is broader: use the available data or reasoning to obtain the only possible answer. Show specifically requires the steps leading to a given or required result, so a correct final value without a visible method may be insufficient.
Deduce, interpret, predict and suggest
These terms require different relationships with evidence:
- Deduce from information supplied in the question. For example, use mass-spectrum peaks to deduce isotopic composition.
- Interpret a graph, table or result by identifying trends and drawing conclusions.
- Predict an expected result using established chemical knowledge, such as a periodic trend.
- Suggest a plausible answer when more than one defensible response may exist.
If asked to suggest why an experimental yield is low, “product was lost” is often too vague. A better response identifies a credible mechanism, such as product remaining dissolved during filtration or an incomplete reaction leaving reactant unconverted.
Comparison and judgement command terms
Compare and contrast
In IB terminology, compare focuses on similarities, while contrast focuses on differences. If the instruction is compare and contrast, both are required.
Write paired statements rather than describing substance A in one paragraph and substance B in another. For example: “Both diamond and graphite contain covalently bonded carbon atoms. Diamond has four covalent bonds per carbon atom, whereas graphite has three.” This structure makes the comparison explicit.
Discuss
A discussion is a considered and balanced review. In an experimental context, this might require several factors affecting a proposed method, evidence supporting different interpretations, and a reasoned conclusion.
Do not assume that “balanced” always means writing an equal number of advantages and disadvantages. It means considering the relevant competing factors rather than presenting only one side.
Evaluate
To evaluate, weigh strengths and limitations and then judge their significance. Naming a limitation alone is not enough.
Suppose a student evaluates a calorimetry method. A developed point might be: “The polystyrene cup reduces heat transfer compared with a glass beaker, improving insulation. However, heat is still transferred to the cup and surroundings, so the measured temperature change is smaller and the calculated enthalpy change is less exothermic than the accepted value.” The strongest evaluation points connect a methodological issue to its likely effect on the data or conclusion.
Justify
A justification begins with a claim and supports it with evidence or valid reasoning. For example: “Magnesium is the stronger reducing agent because it has the more negative standard electrode potential and is therefore more readily oxidized.”
Avoid circular answers such as “The reaction occurs because it is spontaneous.” Instead, identify the evidence that establishes spontaneity, such as a positive standard cell potential under standard conditions.
Diagram and graph command terms
Draw and sketch are not interchangeable. A drawing should be accurate and labelled, while a sketch communicates the general form and essential features.
For graphs, include:
- labelled axes with quantities and units
- a sensible scale when numerical data are plotted
- accurately positioned points when required
- an appropriate straight line or smooth curve
- annotations requested by the question
If instructed to annotate, add brief explanatory notes rather than labels alone. The official Chemistry specimen papers provide useful examples of how command terms appear in the current assessment format.
How to decide how much to write
The command term gives the type and depth of response, while the marks indicate its likely scope. A practical rule is to look for enough distinct, developed ideas to match the available marks, but do not treat “one sentence per mark” as an official formula.
Before writing, use this sequence:
- Circle the command term.
- Underline the chemical subject and any constraints.
- Check the mark allocation.
- Decide what the answer must physically contain: a fact, causal chain, calculation, comparison or judgement.
- Write directly to that requirement.
For a two-mark explanation, you may need a cause and its consequence. For a four-mark evaluation, you are more likely to need multiple developed strengths or limitations and a supported judgement. Exact marking always depends on the question-specific markscheme.
Common command-term mistakes
Students frequently lose accessible marks by:
- explaining when asked to state, wasting time
- describing what happens without explaining why
- giving differences when asked to compare
- listing generic errors instead of evaluating their effects
- writing a final numerical answer without working
- ignoring qualifiers such as “using the data” or “at the molecular level”
- giving an unsupported opinion for justify, discuss or evaluate
The RevisionDojo guide to common IB Chemistry exam mistakes explains how these errors interact with calculations, data questions and timing.
How to internalise command terms efficiently
Memorising definitions helps, but the fastest route to reliable exam technique is seeing many questions worked through. You need to observe how an answer changes when the command term changes from state to explain, or from describe to evaluate.
Use the RevisionDojo IB Chemistry hub to access per-question past-paper video walkthroughs. Pause before each solution, identify the command term, predict what the response must contain, and then compare your plan with the worked answer.
Follow this with targeted practice in the IB Questionbank. Jojo AI can help you distinguish a content gap from a command-term gap, while Chemistry Predicted Papers allow you to practise selecting the correct response depth under timed conditions.
A useful error log has four columns: question, command term, what I omitted, and how I will answer next time. The broader RevisionDojo command-term guide can support this review across your other IB subjects.
Conclusion
IB Chemistry command terms define the task: state gives a brief fact, outline summarizes, describe reports what happens, explain supplies reasons, and evaluate weighs strengths and limitations to reach a judgement. The mark allocation and question context then show how broad the response should be.
The skill becomes automatic through repeated exposure rather than definition memorisation alone. RevisionDojo’s Chemistry past-paper video solutions are particularly useful because they show how each answer is built question by question; follow them with Questionbank practice and timed Predicted Papers to apply the same reasoning independently.
Sources and referenced URLs
- IB Chemistry subject brief, first assessment 2025
- Official IB Chemistry specimen papers
- IB assessment principles and practices
- IB Chemistry curriculum updates
- RevisionDojo IB Chemistry resources and video walkthroughs
- RevisionDojo IB Questionbank
- RevisionDojo Chemistry Predicted Papers
- RevisionDojo guide to using IB command terms
- RevisionDojo guide to avoiding IB Chemistry exam mistakes
