Command terms

Every structured question part in Paper 1B and Paper 2 opens with a command term: a verb such as state, explain or deduce that sets the kind of answer the examiner wants. The IB defines each term, and markschemes are written to those definitions. Correct chemistry given in the wrong form loses marks: a bare fact where a reason was needed, a paragraph where one word would do, or a trend described when it had to be explained.

The command terms at a glance

The chemistry course has an official glossary of eighteen command terms, grouped by the assessment objective each one tests: knowing (AO1), applying (AO2), and analysing and evaluating (AO3). On past papers, state, suggest, calculate, outline, deduce, explain and determine open about two thirds of all structured parts.

Papers also use instruction words outside the glossary. Identify and justify are defined with the internal assessment criteria; the others keep their ordinary meanings. Several of them appear on almost every paper.

Naming and recalling

State

Give a specific name, formula, value, colour or condition. No explanation or working is needed, and adding one wastes time without earning anything. A 1-mark state is a word, a short phrase or a value; a 2-mark state usually asks for two separate items, such as two conditions or two observations.

The question decides whether a name or a formula is wanted. If it asks for the name of a substance, a correct formula does not score, and the reverse, unless the markscheme says otherwise. "State the name of the salt formed" needs "sodium ethanoate", not $\mathrm{C}\mathrm{H}{\phantom{X}}_3\mathrm{C}\mathrm{O}\mathrm{O}\mathrm{N}\mathrm{a}$.

Identify

Pick one answer from a set of possibilities: the oxidizing agent in an equation, the functional group in a structure, the strongest intermolecular force, the species acting as a Brønsted–Lowry base. When the question adds "giving a reason", the single mark needs both the choice and the reason.

Write and formulate

Both ask for a balanced symbol equation. A word equation, or a symbol equation that is not balanced, scores zero. State symbols are ignored unless the question asks for them, in which case they often carry a separate mark. "Formulate … using integer coefficients" rules out halves, so an equation written with $3.5\mathrm{O}{\phantom{X}}_2$ must be doubled.

Calculations

Calculate

Obtain a numerical answer, showing the relevant stages of working. Markschemes give one mark per meaningful step (amount in moles, then the ratio, then the mass or volume) and add "Award [2] for correct final answer". A bare correct answer therefore still earns full marks, but a bare wrong answer earns nothing, while a wrong answer with clear working usually earns all but one mark.

The guillemets «…» in a markscheme mark words, working or units that are not needed for the mark. The unit of a final answer is usually optional, but writing it is still the habit to keep: when a question asks for a particular unit ("in ${\text{kJ g}}^{-1}$"), stopping one step early at $\text{kJ}$ loses the mark.

Determine

Obtain the only possible answer. Determine is used for multi-step working (an enthalpy change from bond enthalpies, an order of reaction from initial-rate data, an initial rate from a tangent) and for answers read from a graph. It is marked like calculate, step by step.

Estimate

Obtain an approximate value. It is used when the data allow only a rough answer: reading a value between plotted points, extending a trend to the next member of a series, or judging the size of a quantity. Give a sensible value and, where it helps, one short phrase saying how it was obtained.

Written accounts

Outline

Give a brief account or summary. A 1-mark outline is one precise point; a 2-mark outline is two points, or one point and its consequence. It sits between state and explain: more than a single word, but no full chain of reasoning.

Describe

Give a detailed account of what is present, what is done or what is seen. A description needs no reasons. In a practical context, describe usually means the action and the observation; for bonding, it means the particles and the attraction between them.

Explain

Give a detailed account including reasons or causes. An explanation is a chain of cause and effect, and each link in the chain is usually one mark. A 2-mark explain needs two links; a 3-mark explain needs three. Stating the effect alone, however accurately, earns nothing: "first ionization energy increases across a period" describes a trend but does not explain it.

The skeleton of the answer is worth learning, because the same three links (the force in each substance, which is stronger, and the energy needed to overcome it) answer every boiling-point and volatility comparison. Periodic trends have a skeleton too: the nuclear charge, the shielding or distance of the outer electrons, and the resulting attraction.

Judging, comparing and concluding

Compare, contrast and distinguish

Compare asks for similarities, contrast for differences, and compare and contrast for both. Each point must refer to both items: "graphite conducts electricity" is not a contrast until it adds "but diamond does not".

Distinguish is a 1-mark contrast that puts both halves into one statement, usually joined by "whereas" or "but".

Deduce

Reach a conclusion from the information given: an oxidation state from a formula, a formula from data, the number of signals from a structure, the type of reaction from an equation. The data are in the question, and the answer follows from them by reasoning rather than recall. "Deduce, giving reasons" makes the reasoning part of the mark.

Predict

Give an expected result. Chemistry questions almost always say "predict, giving a reason" or "predict and explain", and a bare prediction rarely earns the mark. Where the mark scheme has two marks, one is for the prediction and one for the reason; where it has one, both are needed.

Suggest

Propose a plausible answer, often in a context the course never covered: a reason for an unexpected result, an improvement to a method, a property of an unfamiliar compound. Several answers are usually accepted, and the markscheme ends "Accept other valid answers". The answer must still be chemical and specific.

Comment

Give a judgement based on a statement or on the result of a calculation. A typical comment asks whether a difference between two results is significant, or whether a result agrees with a literature value. The judgement must be supported by a number from the question: "the difference is smaller than the uncertainty" earns the mark; "the results are close" does not.

Discuss and evaluate

Discuss asks for a balanced review: several distinct arguments or factors, ideally on both sides, and a conclusion that follows from them. Evaluate weighs strengths against limitations. Both are usually worth 2 or 3 marks, and examiners report that the commonest failing is to make one point twice in different words.

Justify and classify

Justify asks for the reason or evidence that supports a stated answer, and classify asks for a category together with the reason it applies. Both are usually worth one mark, and both lose it if the reason is missing.

Drawing, sketching and annotating

Draw

Produce an accurate, labelled diagram: a Lewis formula, a full structural formula, the repeating unit of a polymer, an arrow on an energy profile, or a best-fit line on a graph. Draw is an AO1 term, so the marks reward accuracy against fixed conventions. A Lewis formula must show every lone pair as well as every bonding pair, and an ion needs square brackets and its charge.

On graph paper, draw also covers plotting and lines of best fit: points plotted accurately, a ruler for straight lines, and a single smooth curve, not a dot-to-dot line, where the trend is curved.

Sketch

Give the general shape of a graph or diagram with its relevant features, but without accurate values. A sketch graph needs labelled axes (quantities, and units if values are marked), the correct shape, and the features that the question is testing: where the curve starts, where it peaks, which curve lies above which, where it levels off. Sketches are AO3 because the features show understanding.

Annotate, label and plot

Annotate means adding brief notes to a diagram or graph, such as marking $E_{\text{a}}$ and shading an area, or writing "equivalence point" beside the steep part of a pH curve. Label means naming the parts of a diagram (anode, cathode, salt bridge, direction of electron flow), and plot means marking points accurately on a graph. These parts are quick, but they are easy to skip when the instruction is printed under a figure rather than in the question text.

Phrases that change a command term

Many parts combine a command term with a phrase that sets an extra requirement. Each phrase usually carries part of a mark, so a bare answer that ignores it loses that part.

Command terms in one question

A single Paper 2 question moves through several command terms, and the depth of each answer changes with the term. The worked question shows how short a full-mark answer can be when the term is state or write, and how much more an explain needs.

Self-check

• To do:

  I can say what each of the eighteen command terms asks for and how many marks each typically carries.

• To do:

  I can tell from the command term whether an answer needs reasons, working or neither.

• To do:

  I can write an explanation as a chain of cause and effect with one link per mark.

• To do:

  I can give a contrast that refers to both items in every point.

• To do:

  I can write balanced symbol equations, with state symbols when asked.

• To do:

  I can show the stages of working in a calculation so that later marks are protected.

• To do:

  I can sketch a graph with labelled axes and the features the question tests.

• To do:

  I can draw a Lewis formula with every lone pair and the charge on an ion.

• To do:

  I can answer the extra requirement in phrases such as "giving a reason" and "with reference to".

• To do:

  I can give specific chemical reasons and improvements rather than "human error".