HKDSE Chemistry common mistakes
HKDSE Chemistry · Understand the traps and how to avoid them.
Writing ionic equations that keep spectator ions (e.g. full molecular equation for precipitation), or omitting state symbols and charges on ions in the final equation.
Candidates memorise 'word equation to formula equation' steps and stop there; they also treat state symbols as decoration rather than part of the required answer format. Under time pressure the last line written is the least checked.
What to do instead
Practise a fixed three-line drill for every reaction: formula equation, then split all soluble strong electrolytes into ions, then cancel identical ions on both sides. Always write charges above the arrow-balanced ions and add (s), (l), (g), (aq) as you write, not afterwards. Mark every ionic equation out of 2 in your own practice and lose a mark deliberately when a state symbol is missing, so the habit sticks.
Calculating moles with the wrong molar mass or mixing up the limiting reagent in a two-reactant problem, then reporting a product mass based on the excess reactant.
Candidates grab the first mass given and divide by a molar mass recalled from memory (often forgetting waters of crystallisation or using Ar of the element instead of Mr of the compound). They also skip the step of dividing moles by stoichiometric coefficients, so the limiting reagent is never identified explicitly.
What to do instead
Force a written table: substance, mass or volume, Mr or concentration, moles, divide by coefficient, then circle the smallest quotient as limiting. Compute Mr from the formula written out with every atom counted, including water of crystallisation. Do five mixed limiting-reagent problems and, for each, state in one sentence which reactant runs out and why.
Confusing the direction of electron flow, ion migration, and the site of oxidation in electrochemical cells, or writing the wrong half-equation at an electrode.
The mnemonic 'oxidation is loss' is known but not applied to the actual species present; candidates also assume the anode is always negative, which is true for galvanic cells but reversed in electrolysis. They then write half-equations for species that are not in the cell.
What to do instead
For every cell question, first label the cell type (galvanic or electrolytic) and then write 'anode = oxidation' at the top of your answer. List the actual ions or molecules present at each electrode before choosing the half-reaction, and check that the two half-equations balance in atoms and charge and add to the overall equation. Practise one example of each cell type side by side and annotate the electron-flow arrow on the diagram you draw yourself.
Describing organic reaction mechanisms or products without distinguishing substitution, addition, and elimination, or ignoring Markovnikov orientation and the role of the reagent (e.g. NaOH in aqueous versus alcoholic solvent).
Candidates learn reactions as isolated equations and never build a decision rule based on substrate type (alkane, halogenoalkane, alkene) and conditions. The solvent and temperature, which decide the pathway, are treated as incidental detail.
What to do instead
Build a one-page decision tree: identify functional group, then class of halogenoalkane (primary, secondary, tertiary), then reagent and conditions, then mechanism (SN1, SN2, elimination) and product. For each reaction you revise, write the conditions in the same line as the reagent, e.g. 'NaOH(aq), warm' versus 'NaOH in ethanol, heat'. Test yourself by predicting the major product for three similar substrates and justifying the choice in one sentence.
Treating equilibrium as 'the reaction stops' or claiming that adding a catalyst shifts the position of equilibrium, and failing to state that rates become equal rather than concentrations becoming equal.
Everyday language suggests equilibrium means 'balanced amounts'. The dynamic nature of equilibrium and the distinction between rate and position are abstract, and textbooks' shorthand ('equilibrium shifts right') is misread as a change in the equilibrium constant.
What to do instead
Write the definition in full each time: forward and reverse rates are equal, concentrations remain constant, and the reaction is dynamic. For every disturbance, answer two separate questions: what happens to the rate, and what happens to the position. Explicitly note that a catalyst changes the rate of both directions equally and leaves K unchanged. Practise Le Chatelier predictions on the Haber process with a table of change, effect on position, effect on K, effect on yield.
Giving vague qualitative explanations for trends such as boiling point, solubility, or acidity, without naming the specific intermolecular force or the structural feature responsible.
Candidates know the vocabulary (hydrogen bonding, van der Waals forces) but do not connect it to molecular size, branching, or the presence of an O–H bond. They also omit the comparison, writing 'has hydrogen bonds' rather than 'forms more hydrogen bonds per molecule than the other substance'.
What to do instead
Use a fixed comparison sentence pattern: 'X has [specific force] because [structural feature]; Y has only [weaker force]; therefore more energy is needed to separate X particles.' For each trend, practise three pairs (e.g. ethanol versus ethoxyethane, propan-1-ol versus butan-1-ol, ammonia versus phosphine) and name the force and the structural reason every time.
Misreading titration or enthalpy curves and reporting a value from the wrong point, or calculating enthalpy change without dividing by moles and without the correct sign.
Candidates read the graph at the start of the steep section rather than the equivalence point, or take the temperature change from the wrong pair of readings. The sign convention for exothermic reactions is often dropped because the calculation is treated as pure arithmetic.
What to do instead
For titration curves, mark the equivalence point as the steepest vertical section and read the volume at its midpoint. For enthalpy, write the full chain: heat released = mcΔT, moles = concentration × volume (or mass ÷ Mr), ΔH = −heat ÷ moles, and state 'exothermic, so negative' in words. Do a full worked example with units at every line and check that the magnitude is plausible for the reaction type.
Answering structured questions with single words or bullet fragments when the question asks for an explanation, and failing to use the data or information given in the question stem.
Candidates under time pressure treat marks as proportional to the number of facts rather than the number of linked reasoning steps. They also ignore the stem's data because they jump straight to recalling a memorised answer.
What to do instead
Before writing, count the marks and plan one sentence per mark. Use the stem explicitly ('the graph shows...', 'the table gives...') and link each sentence with 'because' or 'therefore'. Practise rewriting your own one-word answers into full explanations and check against the mark allocation; if a two-mark question has one clause, add the consequence.
Confusing tests and observations for gases and ions, e.g. reporting a 'pop' for carbon dioxide, or using limewater to test hydrogen, and omitting the preliminary test that distinguishes similar ions.
The tests are learned as a list without the observation attached, and similar-sounding tests (flame tests, precipitation tests, gas tests) blur together. Candidates also skip the confirmatory step, such as adding excess reagent to dissolve a precipitate.
What to do instead
Make a two-column table of test and exact observation, written in the exam's expected wording (e.g. 'white precipitate soluble in excess aqueous ammonia'). For each ion, include the confirmatory step and the interfering ion it rules out. Cover the observation column and recall it from the test, then reverse the direction, and rehearse the gas tests with the correct gas each time.
Losing marks on practical and investigative questions by not identifying the independent, dependent, and controlled variables, or by proposing a method that cannot actually measure what is claimed.
Candidates treat these questions as general knowledge and write a procedure from memory without checking that each step produces the required measurement. Control variables are omitted because they feel obvious.
What to do instead
Use a four-line scaffold for every experimental design answer: variables (independent, dependent, controlled), apparatus, procedure with quantities, and how the result is processed. Check the method by asking whether the dependent variable is actually measured and whether the controls are held constant in practice. Practise on two familiar experiments, such as rate of reaction with marble chips and acid, and enzyme or solubility investigations.