How to Improve Your HSC Chemistry Score: 2025 Examiner Report Breakdown

HSC Chemistry rewards students who combine accurate calculations with clear chemical reasoning. The 2025 exam included multi-step titration problems, equilibrium explanations, organic structure questions and a 7-mark spectroscopy question. NESA's marking feedback shows exactly which skills separated the better responses.

This guide breaks down the 2025 HSC Chemistry exam question by question. Every question is classified by module and topic using NESA's own mapping grid, so you can see exactly where the marks were. It also covers the answers, what better responses did and the areas NESA's markers said students need to improve. For the previous year's analysis, see our 2024 HSC Chemistry examiner report breakdown.

Key Takeaways

• Module 8: Applying Chemical Ideas was the biggest module, worth 28 marks. Modules 6 and 7 were worth 25 each, and Module 5 was worth 22.
• Quantitative Analysis was the single biggest topic, worth 17 marks, including the 7-mark back titration in Question 33.
• Calculations needed every step shown, with correct units and significant figures. Markers repeatedly flagged missing equations, wrong mole ratios and including outlier titres.
• Equilibrium explanations needed cause and effect. In Question 29, adding argon at constant volume doesn't disturb the equilibrium at all, so the temperature doesn't change.
• Organic structures cost many marks: missing hydrogens, wrong bonds per carbon, misreading skeletal formulae and confusing condensed and structural formulae.

HSC Chemistry Examination Structure

The exam has 5 minutes of reading time and 3 hours of working time, and is worth 100 marks. NESA-approved calculators are allowed, and a formulae sheet, data sheet and Periodic Table are provided. The HSC exam mark makes up 50% of the final HSC mark. The other 50% comes from school-based assessment.

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Marks by Module in the 2025 Exam

Every question below is classified using the mapping grid in NESA's 2025 marking guidelines, which links each question to a module and syllabus topic.

Marks by Syllabus Topic

Source: NESA 2025 HSC Chemistry marking guidelines (mapping grid).

Section I: Multiple-Choice Breakdown

Source: NESA 2025 HSC Chemistry marking guidelines (answer key and mapping grid). Descriptions of what each question tested are our own.

Multiple-Choice Questions to Watch

• Question 6: HCl is a strong acid, so [H⁺] = 0.25 mol L⁻¹ and pH = −log(0.25) = 0.60. A pH below 1 is possible for concentrated strong acids.
• Question 8: The molecular ion peak rises from 70 to 90, an increase of 20, which matches adding HF (1 + 19) across the double bond.
• Question 16: Each condensation link releases one water molecule. 1000 monomers × 90.08 − 999 × 18.02 ≈ 72 080 g mol⁻¹.
• Question 18: Here the silver ions are in the flask and NaCl is added. Red Ag₂CrO₄ forms first because silver is in excess. At the endpoint the silver runs out and the colour changes from red to yellow. Ag₂CrO₄ is the more soluble salt.
• Question 19: HCl and sodium acetate form an acetic acid/acetate buffer with equal amounts of each. Adding water dilutes both equally, so the ratio and the pH stay at 4.8.

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Section II: Question-by-Question Breakdown

Key answers are based on the sample answers in NESA's 2025 marking guidelines. Topics follow NESA's mapping grid.

What the Markers Said: Module 5 – Equilibrium and Acid Reactions (22 marks)

Question 27(c) – Sketching an equilibrium shift

• Better responses showed an immediate vertical rise at t₁, a smooth concave curve down to the new equilibrium, and a straight horizontal line at a level higher than the original.
• Markers flagged inaccurate positions, wobbly lines and curves that didn't flatten gradually. Use a ruler.

Question 29 – Adding an inert gas

• Better responses recognised that adding argon at fixed volume doesn't change the concentrations of NO₂ or N₂O₄, so Q = K and the system stays at equilibrium, and the temperature doesn't change.
• They explained this using the reaction quotient, rates or Le Chatelier's principle.
• Markers noted students should apply Le Chatelier's principle only once to a system, and that temperature is only directly proportional to pressure when volume and amount of gas are constant.

Question 32 – Predicting precipitates

• Better responses compared the ionic product with Ksp for each possible precipitate, recognised that Mg(OH)₂ is far less soluble and forms first, and then realised it uses up magnesium ions, so MgCO₃ doesn't precipitate.
• Markers flagged errors with scientific notation (10⁻⁷ is larger than 10⁻¹²), limiting reagents and the difference between molar solubility and ionic product.

Question 35 – Collision theory and Le Chatelier's principle

• Better responses used the energy profile diagram: the forward reaction is exothermic, so cooling favours it. Cooling slows both reactions, but the reverse reaction (higher activation energy) slows more, so the solution turns pink.
• Markers noted that activation energy doesn't change with temperature, and that an energy profile diagram isn't an equilibrium graph.

What the Markers Said: Module 6 – Acid/Base Reactions (25 marks)

Question 26 – Graphing and acid strength

• In 26(a), better responses recognised that the halogen is categorical data, so the right graph is a column graph, not a line graph, with an accurate scale and clear labels.
• In 26(b), better responses described a trend (acid strength decreases down the group as electronegativity decreases) instead of restating the data, and linked higher pKa to lower Ka and a weaker acid.

Question 31(b) – pH of a weak acid

• Better responses wrote an equation for N₂H₅⁺ acting as an acid in water with an equilibrium arrow, found Ka = Kw ÷ Kb using the data sheet, and calculated [H₃O⁺] and pH.
• Markers flagged missing equations, missing equilibrium arrows and unbalanced charges.

Question 33 – Back titration (7 marks)

• Better responses excluded the outlier titre, averaged the rest, used the five aliquots to find the total excess HCl, then worked back to the moles and percentage of CaCO₃.
• They wrote the calculated percentage (85.8%) and then compared it with the brands, rather than just quoting the closest brand.
• Markers recommended starting with the simple steps (equation, moles, mole ratio, average titre) and setting out working clearly.

Question 34 – Titration curve

• In 34(a), better responses annotated the graph to show the equivalence volume (0.024 L), used the half-equivalence point where pH = pKa, and showed every step.
• In 34(b), better responses explained that neutralisation and dilution by the acid aliquot both lower the hydroxide concentration, so the pH stays below 13. Markers noted this was not a buffer system.

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What the Markers Said: Module 7 – Organic Chemistry (25 marks)

Question 21 – Substitution

• Better responses gave the condition (UV light), not the reaction type, and named the product using IUPAC conventions (2-bromobutane).

Question 24 – Ethyne and chloroethene

• In 24(a), better responses drew all bonds with the right number of atoms and named the shape (linear), not just the structure.
• In 24(b), better responses wrote an addition reaction (not substitution), used formulae rather than names, and used the molar masses given. Markers flagged equilibrium arrows here, since the limiting reagent was fully used.

Question 25 – Percentage yield

• Better responses used density × volume to find the mass of ester, identified the limiting reagent (propan-1-ol) and calculated percentage yield, not percentage error or atom economy.
• Markers reminded students to use correct significant figures throughout.

Question 27(a)–(b) – Fuels and hydration of ethene

• In 27(a), better responses named a mixture (such as petrol), not a single compound, and linked its use to a specific environmental impact. Markers flagged confusion between greenhouse gases, the greenhouse effect and ozone depletion.
• In 27(b), better responses used full structural formulae, showed the C–O–H arrangement of the hydroxyl group, included every hydrogen, and wrote the acid catalyst and water above the arrow.

Question 28 – Kevlar and polystyrene

• In 28(a), better responses drew neat skeletal or full structural formulae. Markers noted students needed to recognise that the missing monomer has atoms that aren't present in the polymer, because they're lost when the polymer forms.
• In 28(b), better responses identified hydrogen bonding between Kevlar chains (and only dispersion forces in polystyrene), and explained that chains are being separated, not monomers or bonds within a chain.

What the Markers Said: Module 8 – Applying Chemical Ideas (28 marks)

Question 22 – Water quality

• Better responses explained that qualitative analysis identifies what's present while quantitative analysis measures how much, with relevant examples of ions or pollutants.

Question 23 – Improving a gravimetric procedure

• Better responses gave two valid changes that improve accuracy, such as filtering out insoluble material before adding BaCl₂ or weighing the filter paper.
• Markers flagged confusion between accuracy and reliability, unnecessary changes, and missing skills such as drying to constant mass.

Question 30 – Phosgene

• In 30(a), better responses linked a specific precaution (such as a fume hood or mask) to the risk of inhaling a toxic gas. Use correct terms: "fume hood", not "gas cabinet".
• In 30(b), better responses linked excess CO to Le Chatelier's principle and a higher yield, and the catalyst to lower activation energy and a faster rate, which saves time, energy and cost in industry. "Speeds it up" on its own wasn't enough.

Question 31(a) – Formula of hydrazine

• Better responses wrote a balanced equation from the gas volumes and showed how counting N and H atoms confirms N₂H₄. A balanced equation alone, without that reasoning, didn't address the question.

Question 36 – Spectroscopy (7 marks)

• The first step was reading the skeletal formula correctly as propanoic acid. Many students misread it as butanoic acid, ethanoic acid, a ketone or an alcohol, which affected every part of the answer.
• Better responses described all four spectra and linked each feature to the structure:
• IR: a broad O–H (acid) absorption and a strong C=O absorption around 1700 cm⁻¹.
• ¹³C NMR: three carbon environments, including the carbonyl carbon at about 160–185 ppm.
• ¹H NMR: three environments with chemical shift, integration and splitting (triplet, quartet and singlet).
• Mass spectrum: a molecular ion peak at 74 m/z.

Question 37 – Identifying an unknown

• Better responses used every piece of data: B can't be oxidised (a tertiary alcohol), and C oxidises to a ketone rather than an acid (a secondary alcohol). This points to 2-methylbut-2-ene.
• Markers recommended revising primary, secondary and tertiary alcohols and how their structure controls their reactivity.

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Common Mistakes Across the 2025 Exam

• Skipping equations: Several calculation questions required a balanced chemical equation, and leaving it out cost marks.
• Significant figures and units: Markers reminded students to apply significant figures correctly throughout.
• Including outliers: In Question 33, the first titre needed to be excluded before averaging.
• Wrong arrows: Equilibrium arrows where a reaction goes to completion (Q24(b)), and missing equilibrium arrows for weak acids (Q31(b)).
• Structural formula errors: Missing hydrogens, the wrong number of bonds per carbon and misreading skeletal formulae.
• Restating data instead of describing a trend (Q26(b)), and stating effects without explaining cause (Q30(b)).
• Line graphs for categorical data (Q26(a)).

How to Prepare for HSC Chemistry

• Practise multi-step calculations such as back titrations, Ksp predictions and weak acid pH, writing the equation and mole ratio first every time.
• Learn to read skeletal formulae and convert them to full structural formulae with every hydrogen shown.
• Practise predicting spectra for simple molecules, covering IR, ¹³C NMR, ¹H NMR and mass spectrometry together.
• Explain equilibrium shifts with cause and effect, using Q vs K, rates or Le Chatelier's principle, and practise sketching concentration–time graphs.
• Know the difference between accuracy, reliability and validity for practical questions.
• Complete full past papers under timed conditions to practise managing 3 hours of working time.

For more preparation:

• see everything you need to know for HSC Chemistry
• studying Biology too? See our 2025 HSC Biology examiner report breakdown
• studying Physics too? See our 2025 HSC Physics examiner report breakdown
• understand HSC performance bands
• see the HSC exam timetable 2026 for this year's exam date.

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Sources: NSW Education Standards Authority, 2025 HSC Chemistry examination, marking guidelines and HSC marking feedback 2025.