How the on-screen exam works: three tasks, the tools, the clock and the command terms
By the end of this lesson you will be able to
- Describe the structure of the 120-minute, 100-mark Chemistry on-screen examination and how its questions fall into three tasks worth 25, 50 and 25 marks.
- Use each on-screen tool — calculator, periodic table, graph plotter, table tool, drawing canvas, measuring tool, drop-downs — the way the markscheme rewards.
- Plan your 120 minutes so that no marks are lost to the clock.
- Distinguish the demands of the command terms, from state to discuss and evaluate, and match the length of an answer to its marks.
- Apply the rules that decide calculation marks: working seen, units, significant figures, standard form and error carried forward.
You met this before
- Every MYP science task is judged against four criteria: A Knowing and understanding, B Inquiring and designing, C Processing and evaluating, D Reflecting on the impacts of science. In class each is marked out of 8; on the exam paper each is worth about 25 marks.
- A fair test changes one independent variable, measures one dependent variable and keeps the others controlled at stated values (Year 1 Being a scientist).
- A table puts the independent variable in the first column with units in the headings only; a graph puts it on the x-axis with an even scale (Year 1).
- Command terms tell you how much to write: state is a word or value, explain needs a reason, evaluate needs strengths, limitations and a judgement (every unit of this course).
- Calculations in this course are written one step per line: formula, substitution with units, answer with unit and sensible significant figures (Units 2, 3 and 8).
A question says “Calculate the amount of carbon dioxide, in moles, in 11.0 g of the gas. Give your answer to 3 significant figures. [2]”. Write the full-mark answer and say what each of the two marks is for.
Reveal
M(CO₂) = 12.01 + 2 × 16.00 = 44.01 g mol⁻¹
n = m ÷ M
n = 11.0 g ÷ 44.01 g mol⁻¹
n = 0.250 mol
One mark for a correct method (dividing mass by molar mass, seen), one for the answer rounded as asked: 0.250, not 0.25 and not 0.24994. The two-mark pattern — method, then answer with precision — is the one the Chemistry markschemes use again and again.
Revise it in full: Year 1 · Being a scientist · Lesson 3 — Variables and the fair test · Year 1 · Being a scientist · Lesson 4 — Tables and graphs: making the pattern visible · Year 1 · Being a scientist · Lesson 5 — Conclusions and evaluation: was I right, and how would I know?
The puzzle
Two friends revise the same chemistry for the same months. On the day, one scores 66 and the other 82. On the May 2026 Chemistry boundaries that is the difference between a grade 5 (61–71) and a grade 7 (82–100).
When their teacher compares the scripts line by line, the second student did not know more chemistry.
The 16 marks came from somewhere else: a unit left off an answer, a “?” left in a drop-down, an “explain” answered with a “state”, a graph with an uneven scale, a discussion with no conclusion, and eleven blank marks at the end when the clock ran out. Where exactly do marks like that hide — and how do you collect them?
Before you read on, predict: in a 120-minute, 100-mark paper, how many minutes can you spend on one mark? And which of these do you think costs the most marks across the six Chemistry reports from 2023 to 2026: missing units, “amount” instead of mass or volume, or controls without values?
Compare your answer
120 minutes ÷ 100 marks is 1.2 minutes per mark — a 2-mark calculation deserves about two and a half minutes, a 12-mark discussion about fifteen. Reading the sources takes some of that time, so the real budget is tighter.
All three are reported repeatedly. Units and significant figures appear in almost every Chemistry report (M23, M24, N24, M25, M26), “amount” in M23, M24, M25, N25 and M26, and unqualified controls in about twenty sessions across all four sciences. None of them needs new chemistry — only habits, which is what this lesson builds.
The science of the exam — structured notes
1.1 The paper: 120 minutes, 100 marks, one story per question
The MYP Chemistry eAssessment is a single on-screen examination of 2 hours, marked out of 100. It assesses all four criteria, each worth about 25 marks. You answer every question; there is no choice of questions.
Each numbered question is built around one context — candles, cola, fire extinguishers, frozen desserts — and its parts move from short knowledge items to longer reasoning. Stimulus arrives as text, photographs, data tables, graphs, short videos (with transcripts), animations and interactive simulations.
In this course we group the paper into three tasks, because that is how the marks fall:
- Task 1 — Criterion A, about 25 marks. Short items: select, state, identify, calculate, explain. Usually the first two or three questions.
- Task 2 — Criteria B and C, about 50 marks. Investigations: a research question, a hypothesis, variables, a full “Design” item of 15–17 marks, then data to tabulate, plot, process and evaluate.
- Task 3 — Criterion D, about 25 marks. One or two real-world issues with sources, ending in a long “Discuss and evaluate” answer of 9–15 marks.
The boundaries are not strict — a data question may carry an A mark, a D question a calculation — but if you prepare for these three tasks you have prepared for the whole paper.
| Criterion | What the marks reward | Typical items on screen | Where marks leak |
|---|---|---|---|
| A Knowing and understanding (≈25) | Recall and use of chemistry; explaining with particles, electrons and energy; applying it to unfamiliar contexts | Select, state, identify, calculate, explain, draw a Lewis structure, balance with drop-downs | Missing units and significant figures; a statement where an explanation was asked; no particle-level reasoning |
| B Inquiring and designing (≈25) | Research questions, hypotheses with a “because”, variables, a replicable method, sufficient data, specific safety | Formulate, identify, design (15–17 marks) | “Amount” instead of mass or volume; controls without values; the dependent variable a calculated quantity; five values not listed |
| C Processing and evaluating (≈25) | Tables, graphs, means, trends, conclusions with data, validity, errors, improvements, extensions | Present, plot, calculate, determine, outline, evaluate | Uneven scales; means with the wrong precision; conclusions with no values; errors named as mistakes |
| D Reflecting on the impacts of science (≈25) | Using science to weigh consequences across economic, environmental, social, ethical and political factors, with a justified appraisal | Suggest, outline, discuss, evaluate (9–15 marks) | Copying the sources; one-sided answers; cost as the only economic point; no conclusion |
The Chemistry grade-7 boundary was 81 in May 2023, 82 in May 2024, 73 in November 2024, 83 in June 2025, 73 in November 2025 and 82 in May 2026. The grade-4 boundary in May 2026 was 51. What do these numbers tell you about how to use practice papers?
Reveal answer
Boundaries move by up to 10 marks between sessions because papers differ in difficulty; they are set after marking so that a grade means the same standard. A score on one practice paper therefore translates only roughly into a grade — use the boundary for that paper’s own session.
The more useful number is your marks per criterion: 25 each. A student on 51 with 20 in A and 8 in D has a different job to do from a student on 51 with 11 in A and 17 in D.
1.2 The on-screen tools, and how each one earns or loses marks
Every tool below has appeared in a real Chemistry or Integrated Sciences paper between 2021 and 2023. The IB’s familiarisation package lets you practise them before the day; every Chemistry report from 2023 to 2026 asks teachers to use it, and the May 2023 report says why — so that you “use their time wisely in situations such as producing graphs”.
| Tool | What you do | Real item | The mark-earning habit |
|---|---|---|---|
| Calculator | A scientific calculator opens on screen | Every mole calculation, e.g. November 2021 Q8b, 12 kg of lithium to moles | Type the working into the answer box as well — the method mark needs to be seen |
| Periodic table and data | Relative atomic masses, groups and periods; some papers give a formula or metric-multiplier sheet | May 2021 Q1b, group and period of silicon | Use the table’s values, not remembered ones (Cl is 35.5 on many tables, not 35) |
| Drop-downs and drag-and-drop | Choose coefficients, labels, states or words | May 2023 Q1b, balancing propane with drop-downs | Never leave a “?” — the May 2024 report saw students who “did not interact with the dropdowns offered” |
| Graph plotter | Drag points onto a grid, type the scale values and axis labels | May 2023 Q4c, temperature from −15 to −2.5 °C against percentage frozen | Label with quantity and unit; type an even scale; check negative values sit left of zero |
| Table tool | Type headings and values into cells | November 2023 Q4a, sugar solubility data | Units in the headings only; independent variable first and in order; same decimal places |
| Drawing canvas | Drag atoms, bonds, electrons or particles into a box | November 2023 Q1c, the Lewis structure of CO₂; May 2023 Q4a, water molecules before and after freezing | Count electrons; particles in a solid touch in a regular pattern, in a liquid touch irregularly |
| Measuring tool | An on-screen ruler over an image with a scale bar | May 2023 Q4b, the width of an ice crystal | Measure against the scale bar, then convert units, then standard form |
| Symbols palette | Subscripts, superscripts, °, arrows, ⇌ | November 2023 Q1d, CO₂(aq) ⇌ CO₂(g) | Find ° and the subscripts before the exam — a biology report warned students to learn where the degree symbol is |
| Video, animation, simulation | Watch (a transcript is given) or change a variable and read an output | November 2022 Q4 fire-extinguisher video; the May 2026 report’s “interactive model” for voltage | Read values off the simulation with their units; note the settings you used |
A micrograph of a frozen dessert has a 20 µm scale bar. With the on-screen ruler, the ice crystal at A measures about 55 µm across. Give the size in metres in standard form.
Measure against the scale bar. The crystal is 2.75 scale bars long.
size = 2.75 × 20 µm
size = 55 µm (first mark: 55 ± 5 µm, the width, not the perimeter)
Convert. 1 µm = 10⁻⁶ m, so 55 µm = 55 × 10⁻⁶ m
Standard form. 55 × 10⁻⁶ m = 5.5 × 10⁻⁵ m (second mark)
The markscheme said “55 × 10⁻⁶ is NOT accepted as standard form”: the number in front must be between 1 and 10. The May 2023 report asked teachers to practise “conversion between units in a stepwise fashion” — exactly these three lines.
A drop-down question asks you to balance the complete combustion of butane, C₄H₁₀: ? C₄H₁₀ + ? O₂ → ? CO₂ + ? H₂O. [2]
Carbon. 4 C atoms per butane, so 4 CO₂ per butane.
Hydrogen. 10 H atoms per butane, so 5 H₂O per butane.
Oxygen. 4 × 2 + 5 × 1 = 13 O atoms needed, which is 6½ O₂ per butane.
Whole numbers. Double everything: 2 C₄H₁₀ + 13 O₂ → 8 CO₂ + 10 H₂O
Check: carbon 8 and 8; hydrogen 20 and 20; oxygen 26 on the left and 16 + 10 on the right. Where a coefficient is 1, choose “1” if the drop-down offers it — May 2021 Q3c reminded markers that pentane’s coefficient is 1, not “?”. Any box left on “?” scores nothing, even if the others are right.
A student presents the data table in the graph tool with the x-axis labelled “Temperature” and a scale typed as −15, −7, −4.5, −3, −2.5; the y-axis is labelled “% frozen”. Find three errors the markscheme would penalise.
Reveal answer
1. The x-axis label has no unit: it must be “Temperature / °C” (the May 2023 markscheme gave a mark for “Temperature AND °C”).
2. The scale uses the data values as scale values, so the gaps are uneven — the classic error the Biology M23 report describes; type an even scale such as −16, −14 … 0.
3. “% frozen” should be the full quantity, “Percentage of ice cream frozen / %”. A quantity name, then the unit.
1.3 The clock: a plan for 120 minutes
Students in the May 2023, May 2024 and June 2025 Chemistry sessions told the IB that time was limited. The examiners’ answer, in several reports, is that time is lost by copying the question back and by not knowing the tools. A plan protects you:
- Minutes 0–30: Task 1. About 25 marks at 1.2 minutes each. Short items first; if a calculation stalls, flag it and move on.
- Minutes 30–85: Task 2. About 50 marks. Read the whole investigation before writing — later parts often explain the set-up. Give the big “Design” item about 18–20 minutes.
- Minutes 85–115: Task 3. About 25 marks. Read the sources with the question in mind; plan the long answer for 3 minutes under the bullet headings before writing.
- Minutes 115–120: check. Units, “?” left in drop-downs, blank boxes, the conclusion of the discussion.
A student reaches minute 70 with the Design item (16 marks) and a 6-mark processing question left in Task 2, plus the whole of Task 3 (25 marks). Make a new plan for the remaining 50 minutes.
Reveal answer
Marks left: 16 + 6 + 25 = 47 in 50 minutes, about 1.06 minutes per mark — tight but possible.
A sound plan: Design 15 minutes (use the bullet points as headings; a list of five IV values, three trials and a mean is worth marks even if the method is short); processing 6 minutes; Task 3 26 minutes; 3 minutes to check units and drop-downs. What must not happen is spending 30 minutes on the design and leaving 25 marks of Task 3 half-written.
1.4 Command terms: the verb decides the answer
Every item begins with a command term, and the markscheme is written to it. The Chemistry reports return to this every session: “The command term outline requires more than a statement” (M23); the term determine “was not fully understood” (M24); students “stated a range of values” where determine asked for one (M24).
| Command term | What it demands | Typical marks | A chemistry answer that earns them |
|---|---|---|---|
| Select / State | Choose from options / give a name, value or brief answer, no reason | 1 | “Exothermic.” |
| Identify | Pick out and state briefly a feature or answer from possibilities | 1–2 | “The alcohol group, –OH.” |
| Outline | A brief account: the point plus one supporting point | 2 | “A catalyst speeds up a reaction by providing a pathway of lower activation energy.” |
| Describe | A detailed account of what happens or what the data show, with values | 2–3 | “As temperature rises from 20 to 60 °C, time falls from 120 to 16 s.” |
| Explain | Reasons or causes: the mechanism, usually with particles, electrons or energy | 2–4 | “More particles per cm³, so more frequent collisions, so a faster rate.” |
| Calculate / Determine | A numerical answer with working shown / the only possible answer | 1–4 | Formula, substitution, answer with unit, rounded as asked |
| Estimate / Predict | An approximate value / an expected result, from a graph or trend | 1–2 | “−6 °C” read from the line, with the minus sign and unit |
| Suggest | Propose a reasonable answer to an unfamiliar situation | 1–2 | “Ammonia is less flammable than hydrogen, so a leak is less likely to ignite.” |
| Formulate | Express precisely: a research question or a hypothesis with “because” | 1–4 | If … then … because (science) |
| Design | Produce a full plan against the bullet points | 15–17 | Variables, equipment, method, sufficient data, safety |
| Compare (and contrast) | Similarities (and differences), referring to both throughout | 2–6 | “Aluminium is less dense than tin-plated steel (2.70 against 7.30 g cm⁻³), so …” |
| Justify | Give valid reasons or evidence for an answer | 1–2 | “… because it melts at 1066 °C, above 1050 °C.” |
| Evaluate | Weigh strengths and limitations, then judge | 2–4 | A strength, a limitation, “so the method is only partly valid” |
| Discuss (and evaluate) | A balanced review of factors and arguments, supported by evidence, ending in a conclusion | 9–15 | Lesson 4 of this unit |
“For a 3-mark Explain question, writing three correct facts about the substance will always earn three marks.”
Reveal answer
False. The marks go to three linked points that answer the question — usually a cause, a particle-level mechanism and the consequence. Three true facts that do not connect (“sodium is in group 1; it is soft; it floats”) explain nothing.
November 2021 Q1d is the model: sodium’s outer electron is easier to remove, because it is further from the nucleus, because sodium has more shells — three marks, one chain.
1.5 How calculation marks are awarded
Calculation items carry most of the “easy” marks of Task 1, and they are lost for reasons that have nothing to do with chemistry. The markschemes follow five rules:
- Working earns marks. A method mark is given for the right equation with the right numbers substituted, even if the arithmetic slips.
- Error carried forward (ECF). If a later part uses a wrong earlier answer correctly, the later part still scores. So always attempt the later part.
- Units. Some items give a separate mark for the unit (November 2023 Q2a: “98 | g mol⁻¹”).
- Precision. If asked for 3 significant figures, 0.114 scores and 0.11393 does not (November 2023 Q2b).
- Convert first. Kilograms to grams before n = m ÷ M; cm³ to dm³ before c = n ÷ V.
Calculate the amount of hydrogen produced when 8.00 kg of methane react with excess steam: CH₄(g) + H₂O(g) → CO(g) + 3H₂(g).
Molar mass. M(CH₄) = 12 + 4 × 1 = 16 g mol⁻¹ [1]
Convert first. 8.00 kg = 8 000 g
n(CH₄) = m ÷ M
n(CH₄) = 8 000 g ÷ 16 g mol⁻¹
n(CH₄) = 500 mol [1]
Mole ratio. 1 mol CH₄ gives 3 mol H₂ [1]
n(H₂) = 3 × 500 mol
n(H₂) = 1 500 mol [1]
The May 2023 report: “many candidates did not convert to grams first and so were a factor of 1000 out”. A student who wrote 0.5 mol and then 1.5 mol still earned the molar-mass and ratio marks by ECF — which is why you never leave a later step blank.
November 2021 Q8b asked for the amount of lithium in a 12 kg car-battery supply, “to one decimal place”, with M(Li) = 7. Write the answer, then say what a student who wrote “1.7 mol” would score and why.
Reveal answer
12 kg = 12 000 g
n = 12 000 g ÷ 7 g mol⁻¹
n = 1 714.285… mol
n = 1 714.3 mol to one decimal place
The markscheme gave 1 mark for 1.7 — the right method with kilograms left unconverted — and the second only for 1714.3. One decimal place means one digit after the point, not one significant figure.
What if the first part of a question asks for the molar mass of calcium carbonate and you write 90 g mol⁻¹ (a slip: you used 30 for Ca)? The next part asks for the amount in 25.0 g. What should you do in the next part, and what can you still score?
Reveal answer
Use your own 90 g mol⁻¹ correctly in the next part:
n = 25.0 g ÷ 90 g mol⁻¹
n = 0.278 mol
With ECF, the second part scores full marks even though the right answer (M = 100.09 g mol⁻¹, n = 0.250 mol) is different. You lose one mark, in the first part, not three. Leaving the second part blank because “the first might be wrong” throws marks away.
1.6 Reading the stem: the question contains the chemistry you need
Every report since 2023 says the same thing in different words: students do not use the information in the question. “Students did not use the prompts and information provided in the question which is designed to support students to develop an answer” (Chemistry N25). “This may be because the students did not fully read the information provided in the stem” (Chemistry M24, on the “because” of a hypothesis).
A reliable routine takes 20 seconds per item:
- Circle the command term and the number of marks.
- Underline the data you are given — values, units, a named substance.
- Ask which unit of the course this really is: an unfamiliar context (fingerprints, banknotes, candy floss) always rests on familiar chemistry (sublimation, alloys, states of matter).
- Add what the stem does not say. Repeating the stem scores nothing.
November 2024 Chemistry asked why a bottle of fizzy drink produces bubbles when the cork is removed; the report said many students could not answer “even though many students will have experienced this phenomenon”. Which unit of this course is the question really about, and what is the two-point answer?
Reveal answer
Gas laws and equilibrium (Units 3 and 9): the solubility of a gas rises with its pressure above the liquid.
Point 1: in the sealed bottle the carbon dioxide above the drink is at high pressure, so a large mass of CO₂ is dissolved (CO₂(g) ⇌ CO₂(aq) lies to the right). Point 2: opening the bottle lowers the pressure, the solubility falls, and the excess gas comes out of solution as bubbles.
A question shows a photograph of candy floss and says: “The sugar is heated to 160 °C, spun out through small holes and cools rapidly into threads.”
It then asks you to select the state of the sugar inside the spinning container and in the collection bowl, and to explain, using particles, why the threads keep their shape in a new container. Write the full-mark answer and name the course unit each idea comes from.
Reveal answer
Inside the container: liquid (the sugar has melted at 160 °C). In the bowl: solid (it cooled quickly below its melting point) — this was November 2023 Q4e.
Explanation: in the solid the sucrose molecules are held in fixed positions by intermolecular forces and only vibrate, so the threads keep their shape in any container; in the liquid the molecules can move past one another, so it flows and takes the shape of its container.
Units: states of matter and the kinetic theory (Unit 3 Gas laws, Lesson 1) and intermolecular forces (Unit 7 Bonding and structure). The unfamiliar part — spinning sugar — is decoration around a Year 1 idea.
Try it — read a simulation like an examiner
Real papers now include interactive models: you change a setting, read an output, and answer from your readings. This StadiraLab behaves the same way. Marble chips react with hydrochloric acid and the carbon dioxide is collected in a 100 cm³ gas syringe for 60 s; you set the acid temperature and read the volume.
Treat it as an exam item: record exactly what the screen shows, with units, and notice what the instrument cannot tell you.
Using the pattern you found, a student writes: “The volume is directly proportional to the temperature.” Evaluate this statement with numbers from your results.
Reveal answer
Not supported. From 20 to 40 °C the Celsius temperature doubles but the volume quadruples (about 12 to 48 cm³); from 10 to 20 °C it doubles and the volume doubles only by coincidence of the numbers.
Direct proportion needs a straight line through the origin, and a graph of volume against temperature is a curve that gets steeper — each 10 °C multiplies the volume by about 2. The relationship is a positive, non-linear one; and °C is not an absolute scale, so “proportional to temperature in °C” has no physical meaning anyway.
Common mistakes to avoid
- Leaving a “?” in a drop-down — every box must be chosen, including coefficients of 1 (May 2024 report).
- Answering the stem, not the command term — “outline” needs more than a statement; “determine” needs one value, not a range.
- No working in the answer box — the on-screen calculator does not show the marker your method; type it.
- Not converting kg to g, or µm to m — “a factor of 1000 out” (May 2023 report).
- Standard form that is not standard — 55 × 10⁻⁶ m is not accepted; 5.5 × 10⁻⁵ m is.
- Axis labels without units — “Temperature / °C”, never “Temperature” alone.
- Skipping parts because an earlier part may be wrong — error carried forward rewards correct use of a wrong value.
- Running out of time for Task 3 — a quarter of the paper; plan your minutes.
Study smart: how this is examined
Study routine. Today: copy the command-term table onto one card and write one chemistry example of your own for each term. In two days: open the familiarisation package with your teacher (or the practice course) and use every tool once — graph, table, drawing, measuring, symbols.
In a week: do one full past-paper question of 12–17 marks against the clock at 1.2 minutes per mark, then mark it with the markscheme and list where each lost mark went.
How we found out — the first chemistry exam with unfamiliar questions
In 1869 Dmitri Mendeleev, teaching chemistry in St Petersburg, arranged the 63 known elements by atomic mass and by their properties. Where the pattern demanded it, he left gaps — and in 1871 he did something few scientists dare: he described elements nobody had seen.
He called them eka-aluminium, eka-boron and eka-silicon, and predicted their atomic masses, densities, oxides and chlorides. His table was being set a test he could not revise for.
In 1875 the French chemist Paul-Émile Lecoq de Boisbaudran found a new metal in zinc ore by its spectrum and named it gallium. He measured its density as 4.7 g cm⁻³. Mendeleev wrote to say that eka-aluminium should be about 5.9 and that the sample was probably impure. Lecoq purified it, measured again, and found 5.9.
Scandium followed in 1879 (Lars Fredrik Nilson, Sweden) and germanium in 1886 (Clemens Winkler, Germany), whose atomic mass of about 72.6 and density of about 5.3 g cm⁻³ sat close to Mendeleev’s 72 and 5.5.
What persuaded chemists was not that the table organised what they already knew — several earlier tables did that — but that it answered questions about the unknown. That is exactly what an examination paper does when it puts silane beside methane, or gallium in a solar panel, and asks you to predict.
Where it is used now
- Laboratories that report numbers. A water-quality or mine-assay laboratory reports every result with its unit, its precision and its method — the same three habits as a Criterion A calculation mark.
- Pharmacy and medicine. A dose calculation in mg per kg, converted to cm³ of a solution of known concentration, is Unit 2 stoichiometry with lives attached; a factor-of-1000 slip is a real danger.
- Engineering standards. Safety data sheets use the same hazard pictograms the exam asks you to select (flammable, toxic, corrosive, oxidising).
- Computer-based testing everywhere. Driving theory tests, university entrance tests and professional licensing exams are now taken on screen; the skill of reading a question carefully and using on-screen tools under time pressure transfers directly.
- Science in the news. Discussing whether a country should build a desalination plant or recycle clothing is exactly what Criterion D rehearses — citizens and ministers weigh the same evidence.
Science now — and the scientist you need to be
What is happening in 2026
The MYP on-screen examinations continue under the current framework, with published subject reports after each session; the 2026 Chemistry report again asks for “the need to include state symbols in equations, and units in calculations” and for students to be “familiarized with the on-screen examination package”. The IB has announced a new framework whose first new-style eAssessment is planned for May 2030; the chemistry and the exam habits in this unit carry forward.
Around the exam, AI assistants have become common study tools. Used well, they generate practice questions and explain steps; used badly, they give confident answers with wrong units or invented data. The skill that protects you is the one this lesson trains: check every step, every unit and every command term yourself.
The mindshift
What you need to be able to do
Say what each criterion rewards; use every on-screen tool without hesitation; keep to about 1.2 minutes per mark with a plan for the three tasks; read the command term and write to it; and write every calculation as method, substitution and answer with unit and precision.
See an example
A typical weakness: asked to “evaluate the use of a pH probe rather than indicator paper”, an assistant may list advantages only.
The November 2022 Q3b markscheme needed a strength of the probe (numerical data, e.g. pH 3.53), a limitation of the paper (a colour judged against a range), and then the judgement that the probe gives more valid data — the judgement mark only after the first two. If the assistant’s answer has no judgement, it would not score full marks: you have found its gap, and practised the skill.
Key terms
| On-screen examination | The 2-hour, 100-mark MYP eAssessment taken on a computer, assessing all four criteria. |
| Criterion | One of the four assessed strands (A knowing and understanding, B inquiring and designing, C processing and evaluating, D reflecting on the impacts of science), each worth about 25 marks. |
| Command term | The verb that begins an item and defines the kind and depth of answer required. |
| Markscheme | The list of creditworthy points, with accept and reject notes, that answers are marked against. |
| Error carried forward (ECF) | Credit for using an earlier wrong answer correctly in a later step. |
| Standard form | A number written as a × 10ⁿ with 1 ≤ a < 10 and n a whole number. |
| Significant figures | The digits of a number that carry meaning about its precision, counted from the first non-zero digit. |
| Familiarisation package | The IB practice environment that shows the item types and tools of the on-screen examination. |
| Grade boundary | The mark range for each grade in one session, set after marking so that grades represent the same standard. |
| Stimulus | The text, data, image, video or simulation that a question is built on. |
Think like a scientist
Criterion A Kettles in areas with hard water become coated with limescale, mainly calcium carbonate. Descalers contain acids. One brand uses citric acid; a laboratory tests a 25.0 g sample of limescale with excess hydrochloric acid and collects the carbon dioxide at room conditions (molar volume 24.0 dm³ mol⁻¹). Each part below uses a different command term — write to the term.
(a) State the formula of calcium carbonate. [1]
(b) Select the correct balanced equation: A CaCO₃ + HCl → CaCl₂ + CO₂ + H₂O · B CaCO₃ + 2HCl → CaCl₂ + CO₂ + H₂O · C CaCO₃ + 2HCl → CaCl + CO₂ + H₂O · D 2CaCO₃ + HCl → CaCl₂ + CO₂ + H₂O. [1]
(c) Calculate the volume of carbon dioxide produced, assuming the sample is pure calcium carbonate (M = 100.09 g mol⁻¹). [3]
(d) Outline why the fizzing slows down and stops. [2]
(e) Explain, using collision theory, why warm descaler works faster than cold descaler of the same concentration. [3]
(f) Suggest why the manufacturer chose citric acid rather than hydrochloric acid for a product sold for home use. [2]
Model answer
(a) CaCO₃ (the 3 as a subscript).
(b) B. (A is not balanced for Cl and H; C gives the wrong formula for calcium chloride; D is not balanced for Ca.)
(c) n(CaCO₃) = m ÷ M
n(CaCO₃) = 25.0 g ÷ 100.09 g mol⁻¹
n(CaCO₃) = 0.250 mol [1]
1 mol CaCO₃ gives 1 mol CO₂, so n(CO₂) = 0.250 mol [1]
V = n × 24.0 dm³ mol⁻¹
V = 0.250 mol × 24.0 dm³ mol⁻¹
V = 6.00 dm³ [1]
(d) The acid (or the calcium carbonate) is used up as it reacts, so the concentration of acid particles falls and there are fewer collisions per second [1]; when the limiting reactant has run out the reaction stops and no more CO₂ is made [1].
(e) In warm acid the particles have more kinetic energy and move faster [1], so they collide with the limescale surface more often [1], and a greater proportion of collisions have energy equal to or greater than the activation energy, so there are more successful collisions per second [1].
(f) Citric acid is a weak acid and a solid, so it is less corrosive and safer to store and handle at home [1]; hydrochloric acid is a strong, corrosive acid that gives off an irritating vapour and could damage the kettle’s metal or harm the user [1]. (Also accepted: citric acid is a food substance, so traces left in the kettle are not toxic.)
Probe deeper — exam-style questions
Attempt every question before opening the answer. Marks in brackets show what a full answer is worth. Write to the command term.
State the number of minutes you should aim to spend on a 12-mark question in the 120-minute, 100-mark paper.
Reveal answer
12 × 1.2 = about 14–15 minutes [1].
Convert 0.000 043 m to standard form, and then to micrometres.
Reveal answer
0.000 043 m = 4.3 × 10⁻⁵ m [1]
4.3 × 10⁻⁵ m ÷ 10⁻⁶ m µm⁻¹ = 43 µm [1]
In a can of cola there is 39 g of sucrose (M = 342.3 g mol⁻¹). Calculate the amount of sucrose to 3 significant figures.
Reveal answer
n = 39 g ÷ 342.3 g mol⁻¹ [1]
n = 0.113 94 mol, which is 0.114 mol to 3 s.f. [1]
(November 2023 Q2b: 2 marks for 0.114 alone; 0.1139 would lose the precision mark.)
Outline the role of a catalyst in a chemical reaction.
Reveal answer
A catalyst increases the rate of a reaction [1] by providing an alternative pathway with a lower activation energy, without being used up [1]. (May 2021 Q2f: the second mark for any one of these further points; “speeds it up” alone is a statement, worth 1.)
Explain, using the Bohr model, why sodium is more reactive than lithium.
Reveal answer
Sodium’s outer electron is easier to remove than lithium’s (it needs less energy) [1], because it is further from the nucleus and so is attracted less strongly [1], because sodium has more occupied electron shells (three against two), which also shield the outer electron [1].
A student’s answer to “Determine the melting point of the alloy from the graph” is “between 995 and 1005 °C”. Explain why it scores at most 1 of 2 marks, and give the kind of answer that scores both.
Reveal answer
Determine means obtain the only possible answer: one value read from the line [1]. A full-mark answer is a single value with its unit, for example “1000 °C” [1] — the November 2021 Q4f markscheme gave one mark for a value in the accepted range and one for the unit.
7. The May 2023 paper’s hydrogen question used CH₄ + H₂O → CO + 3H₂. A plant converts 1.00 tonne of methane per hour. Calculate the volume of hydrogen made per hour at room conditions (24.0 dm³ mol⁻¹), in m³, in standard form to 3 significant figures, and state one conversion that a hurried student would be likely to miss.
Reveal answer
1.00 t = 1.00 × 10⁶ g
n(CH₄) = 1.00 × 10⁶ g ÷ 16.04 g mol⁻¹
n(CH₄) = 6.23 × 10⁴ mol [1]
n(H₂) = 3 × 6.23 × 10⁴ mol = 1.87 × 10⁵ mol [1]
V = 1.87 × 10⁵ mol × 24.0 dm³ mol⁻¹ = 4.49 × 10⁶ dm³
V = 4.49 × 10³ m³ (since 1 m³ = 1 000 dm³) [1]
The likely misses: tonnes to grams (10⁶, not 10³) or dm³ to m³ (÷ 1 000) [1]. (With M = 16 exactly, 4.50 × 10³ m³ is equally accepted.)
8. A 4-mark item reads: “Evaluate the claim that a pH probe gives more valid data than universal indicator paper for measuring the pH of rain water.” Write a full-mark answer and label which part of your answer earns each mark.
Reveal answer
Strength of the probe [1]: it gives a numerical, quantitative reading to 0.01 pH units (e.g. 5.62), so small changes can be detected and averaged.
Limitation of the paper [1]: the colour is matched by eye to a chart, so the reading is qualitative, only to about 1 pH unit, and depends on judgement and lighting.
Limitation of the probe [1]: it must be calibrated with buffer solutions and rinsed between samples; an uncalibrated probe gives a systematic error in every reading.
Judgement [1]: so, provided it is calibrated, the probe gives the more valid data — the claim is justified with that condition.
Summary
- The Chemistry eAssessment is 120 minutes and 100 marks, with about 25 marks for each criterion; in this course the paper is three tasks: A (25), B and C (50), D (25).
- Every tool — calculator, periodic table, graph plotter, table tool, drawing canvas, measuring tool, symbols, drop-downs — has cost marks when students did not know it; practise them before the day.
- Budget about 1.2 minutes per mark: Task 1 by minute 30, Task 2 by 85, Task 3 by 115, then check.
- The command term decides the answer: state is brief, outline adds support, explain gives the mechanism, determine gives one value, evaluate and discuss weigh both sides and judge.
- Calculation marks come from working, error carried forward, units, the requested precision and converting before calculating.
Next time → Criterion A across the whole course: a topic-by-topic checklist of every eAssessment topic in Chemistry, from periodic trends to the atmosphere, with a retrieval question for each and the unit to go back to if you miss it.