WACE Chemistry: the ATAR Chemistry exam, units 3 and 4

The WACE Chemistry ATAR exam

SCSA publishes an examination design brief inside the syllabus, so the shape of the paper is fixed even though the questions change every year. The brief for the Year 12 Chemistry ATAR course says the exam has three sections and asks for a representative sample of Unit 3 and Unit 4.

Calculations make up 15 to 20 percent of the whole paper and sit in Sections Two and Three. The instructions tell you to show working and to give answers to a sensible number of significant figures, with units. The supervisor hands you a Chemistry data booklet, and you may bring up to three calculators that cannot store programs or text. Section Three questions are built on a scenario, such as a passage, a history of a discovery or a table of results, and they can ask for equations, multi-step calculations or a flowchart in the same question.

What Unit 3 and Unit 4 cover

• Unit 3, equilibrium, acids and bases, and redox: reversible reactions and dynamic equilibrium, Le Chatelier's principle, the equilibrium constant K_(c), proton transfer between conjugate acid-base pairs, strong and weak acids, pH and the ionic product of water, then oxidation and reduction and electrochemical cells.
• Unit 4, organic chemistry and chemical synthesis: how structure, properties and reactions link across the functional groups, then the design of syntheses, including yield, cost, safety and the properties of plastics such as polyethene, nylon and PET.

The notional teaching time for the Year 12 pair is 110 class contact hours. Our Organic Chemistry course and Reaction Kinetics course give you the groundwork for Unit 4 and for rate ideas that feed into equilibrium.

Worked examples in SCSA style

Equilibrium constant and a disturbance

A sealed flask holds dinitrogen tetroxide and nitrogen dioxide at equilibrium. The concentrations are 0.040 mol L⁻¹ of N₂O₄ and 0.12 mol L⁻¹ of NO₂. For N₂O₄ ⇌ 2NO₂:

Adding more N₂O₄ at constant temperature makes the reaction quotient fall below K_(c), so the system shifts right until the ratio is back to 0.36. The value of K_(c) itself changes only if the temperature changes. Marks in Section Three go to this reasoning, written in terms of rates of the forward and reverse reactions, not just to the arithmetic.

pH of a strong base

For 0.0050 mol L⁻¹ barium hydroxide at 25 °C, each formula unit releases two hydroxide ions, so the hydroxide concentration is 0.010 mol L⁻¹.

The syllabus takes K_(w) as 1.0 × 10⁻¹⁴ at 25 °C, so that is the value to use. A common slip is to forget the second hydroxide and answer 11.70.

Rate and equilibrium ideas run together in Unit 3. Try a rate-equation question from the Reaction Kinetics course, then press Mark to check it there:

School assessment, grades and the ATAR

Your school sets the tasks, but SCSA sets the weightings for Year 12 Chemistry. School-based assessment is science inquiry (practical work and at least one investigation) 20%, extended response 10%, tests 20% and examinations 50%. Schools report A to E for the course and give each student a school mark out of 100.

SCSA then statistically moderates the school marks against the ATAR exam marks at that school, so a mark from one school is comparable with another. The combined mark is the average of the moderated school mark and the exam mark. TISC takes the combined results across your ATAR courses and calculates the ATAR. Results go online through the SCSA student portal in December; the date for 2026 had not been confirmed when the 2026 Information Handbook was published.

Dates

For 2026 SCSA published the written timetable on 10 June. Expect the 2027 timetable to appear around the same point in the year, and check it on the SCSA site before you plan study leave.

Use the daily paper as a trial exam

Schools in Western Australia run their own trial and semester exams, so a trial here means whatever your teacher sets. Uni-Sine's free daily paper on the Year 12 Chemistry track works as a short, timed trial: 8 questions worth 25 to 35 marks, marked as you go. It is not a full three-hour WACE paper, and it was not written by SCSA.

• Where it helps: its rotation includes acids, bases and buffers, electrode potentials, organic synthesis and equilibrium constants, which sit inside Unit 3 and Unit 4.
• Where it does not: it also visits topics that are not in the WACE Year 12 syllabus, such as transition metals, NMR and Born-Haber cycles, and it has no data booklet. Skip those days, or use them as a stretch.
• The Year 11 track: the Year 11 Chemistry paper is a good source of quick revision on bonding, moles and rates before you tackle equilibrium.

Exam answers

Worked answers and the mark scheme for each daily paper come with the paper once it has been marked. They are Uni-Sine's own, and they are not official SCSA answers, past paper answers or marking keys.

Common WACE Chemistry mistakes

• Writing a calculation answer with no units, or with ten significant figures when the data has two.
• Explaining a shift in equilibrium by saying the system "wants" to balance. Use the rates of forward and reverse reactions.
• Mixing up the data booklet's values with ones you remember. Take constants from the booklet that the supervisor gives you.
• Leaving Section Three until the last ten minutes. It carries 40 percent, so give it its full 70.

Read next

Australia · WACEWACE Chemistry data booklet: what SCSA gives youThe SCSA Chemistry ATAR data booklet for the Year 12 exam: six formulae, solubility rules, colours and 42 reduction potentials, and what is missing.