The 10 most common mistakes in HSC Chemistry (and how to fix them)
Every year, NESA publishes notes from the marking centre describing the patterns of error they saw across the exam. The same handful of mistakes show up in those notes every single year. They are not deep conceptual gaps. They are habits, and they each cost a mark or two.
Fix all ten and you can lift a Band 5 paper into Band 6 territory without learning any new chemistry.
1. Missing state symbols on equations
If a question asks you to write a balanced equation, the marker is looking for (s), (l), (g) and (aq) on every species. Leave them off and you usually drop a mark, even if everything else is correct.
Fix: Treat state symbols as part of the formula, not as decoration. Write them at the same time as you balance.
2. Sign errors in ΔH and electrochemistry
A negative ΔH means exothermic. A positive E°cell means a spontaneous reaction. Students get the magnitude right and the sign wrong, then conclude the opposite of the truth.
Fix: Before you write the sign, say the direction out loud: "energy released, so negative" or "spontaneous, so positive". Tie the sign to a meaning, not a memorised rule.
3. Confusing Kc and Q
Kc is the equilibrium constant (fixed at a given temperature). Q is the reaction quotient, calculated at any moment in time using current concentrations. Comparing Q to Kc tells you which direction the reaction will shift.
Fix: When you see concentrations given, ask "is the reaction at equilibrium?" If yes, you are calculating Kc. If no, or if the question says "at this instant", you are calculating Q.
4. Calling weak acids "incorrect" instead of "weak"
A weak acid is not wrong, broken or partial. It is an acid that does not fully ionise. Students lose marks describing weak acids as "not as strong" or "less able to react", which sounds like the acid is failing.
Fix: Use the textbook language. A weak acid partially ionises in water. A strong acid fully ionises in water. Same chemistry, very different mark.
5. Forgetting to include water in dissolution and acid-base equations
HCl(g) + H₂O(l) → H₃O⁺(aq) + Cl⁻(aq) is the correct full ionisation. Many students write HCl → H⁺ + Cl⁻ and lose the water and the hydronium ion that the marking guideline expects for Year 12.
Fix: For Year 12 acid-base equations, default to including water and writing H₃O⁺. For Year 11, the simpler H⁺ form is usually fine.
6. Significant figures that do not match the data
The question gives you 25.00 mL (four sig figs) and 0.10 mol L⁻¹ (two sig figs). Your answer cannot be more precise than the least precise input. Reporting 0.0023456 mol when the data only justifies two sig figs costs a mark.
Fix: When you finish a calculation, look back at the data. Round your final answer to match the lowest sig fig count in the inputs. Three is the safe default for HSC Chemistry.
7. Missing or wrong units
A bare number is not a complete answer. 5.2 is not the same as 5.2 mol L⁻¹. Concentration units, energy units (J vs kJ), and pressure units are common offenders.
Fix: Carry units through every step of your calculation, not just the final line. If your units do not cancel to what the question asks for, you have made a mistake earlier and the unit check has just saved you.
8. Vague language for equilibrium shifts
"The equilibrium increases" is meaningless. Increases what? Markers want a direction.
Fix: Use "shifts forward", "shifts to the right", or "favours the products" (and the equivalents for the reverse direction). All three are accepted. "Increases" alone is not.
9. Naming reagents instead of describing them
"An acid was added" is not a description of an experiment. The marker cannot tell whether you meant dilute acetic acid or concentrated nitric acid, and the chemistry is very different.
Fix: Name the reagent fully: concentration if relevant ("0.1 mol L⁻¹ HCl"), state, and any conditions ("in excess", "warm", "drop by drop"). When in doubt, write more.
10. Answering the wrong question
This is the largest single source of lost marks in the entire exam. The question says "evaluate" and the student describes. The question says "two factors" and the student gives one. The question is about Module 6 acids and the student answers about Module 7 organics.
Fix: Read every question twice before you start writing. Underline the directive verb (explain, evaluate, compare) and any limits ("two factors", "for one named example"). After you finish writing, re-read the question and check that your answer addresses it.
How to use this list
Pick the two errors from this list that you make most often. Just two. For your next three past papers, deliberately watch for those two errors and nothing else. Once they stop appearing, pick the next two.
This is more effective than trying to fix all ten at once. The errors are habits, and habits change one at a time.
Avocado's Practice Questions catch most of these errors automatically and explain why each one costs a mark, so you can drill the fixes against real HSC-style problems. Try it free.