Okayama Public High School Entrance Examination - Science common mistakes
Okayama Public High School Entrance Examination - Science · Understand the traps and how to avoid them.
When calculating average speed or acceleration, students add the distances and divide by the number of intervals, or they use the total distance without checking whether the motion was uniform; they also treat a velocity-time graph's slope as distance.
Students memorise 'speed = distance ÷ time' as a single formula and apply it to multi-stage motion without identifying which time interval corresponds to which distance. Graph reading is taught separately from calculation, so the meaning of slope (acceleration) and area (distance) is not linked to numerical problems.
What to do instead
For every motion problem, first draw or annotate a table of stages (time, distance, velocity), then compute total distance ÷ total time only for the whole trip. For graphs, practise labelling slope and area explicitly: write 'area = distance' on the graph before substituting numbers, and check units (m/s vs m/s²) to catch slope/area confusion.
In work and energy problems, students compute work as force × distance even when the force is perpendicular to the motion (for example, the normal force or gravity on a horizontally moving object), and they equate 'work done' with 'energy used' without checking for friction or changes in potential energy.
The formula W = F × d is over-applied because textbooks present it first with a horizontal push. Students do not internalise that only the component of force along the displacement does work, and they lack a habit of drawing a force diagram before calculating.
What to do instead
Require a free-body sketch with the displacement arrow drawn first; then cross out any force perpendicular to it before multiplying. Practise a mixed set where some forces do zero work, and always write an energy balance line (input work = change in KE + change in PE + work against friction) before substituting numbers.
For light and sound, students state that a virtual image in a plane mirror or convex lens can be projected onto a screen, or they claim the image in a plane mirror is the same size but reversed left-to-right as a physical flip rather than a front-back reversal.
Everyday language ('the mirror flips things') conflicts with the ray model. Students memorise image properties as a list without connecting them to ray diagrams, so they cannot distinguish real from virtual when the question changes the setup.
What to do instead
Draw ray diagrams for every image question and mark where rays actually converge (real) versus where extensions meet (virtual). Use a simple test: 'Can light actually reach this point?' For mirrors, demonstrate the front-back reversal with a labelled object and image and avoid the word 'flip' in answers.
In current circuits, students apply the same current to every branch of a parallel circuit, or they add resistances in parallel as R1 + R2; they also ignore the internal resistance of a battery or the effect of adding a component on the total current.
Series rules are learned first and rehearsed more, so they dominate under time pressure. Students do not connect the parallel rule to the idea that each branch provides an additional path, and they rarely redraw the circuit in simplified form.
What to do instead
Adopt a fixed procedure: identify nodes, mark where current splits and rejoins, then apply 'same current in series, same voltage across parallel branches'. For parallel resistance, verify with the reciprocal formula and a sanity check that total resistance is smaller than the smallest branch. Redraw complex circuits as equivalent single-loop diagrams before calculating.
When writing chemical reaction equations, students balance by changing subscripts (for example, writing H2O2 as H2O to make the count work) or omit state symbols and conditions, and they fail to simplify ionic equations by cancelling spectator ions.
Balancing is treated as a puzzle to make numbers match rather than a consequence of conservation of atoms. Students lack a systematic counting method and do not distinguish coefficients from subscripts in their own handwriting.
What to do instead
Enforce a table method: list each element, count atoms on both sides, and adjust only coefficients. Never alter a subscript; if the formula seems wrong, recheck the compound's name. For ionic equations, write the full equation first, then delete ions appearing unchanged on both sides, and always include (aq), (s), (l), (g) and any heat or catalyst condition.
In oxidation-reduction questions, students equate 'oxidation' with 'gaining oxygen' only and cannot identify oxidation when electrons are transferred without oxygen, or they mislabel the oxidising agent as the substance being oxidised.
The oxygen-based definition is introduced first and is more intuitive, while the electron-transfer definition is abstract. Students memorise 'reduction is loss' without linking it to the mnemonic and to a concrete example, so the agent and the substance are confused.
What to do instead
Use one consistent definition (loss of electrons = oxidation) and apply it to a worked example with oxidation numbers. Label each species with its change (for example, 'Fe: 0 → +2, oxidised; it is the reducing agent'). Practise identifying the agent by asking 'which substance caused the other to be oxidised?' rather than by formula matching.
In acid-base and neutralisation problems, students treat pH as a linear scale (assuming pH 4 is twice as acidic as pH 5) or assume that mixing equal volumes of acid and base always gives a neutral solution regardless of concentration.
pH is taught as a number to memorise rather than as a logarithmic measure of hydrogen ion concentration. Students also apply 'equal amounts neutralise' without checking the stoichiometry of the reaction or the concentrations involved.
What to do instead
State explicitly that a change of one pH unit means a tenfold change in hydrogen ion concentration, and use simple examples (pH 3 vs pH 5: 100 times). For neutralisation, write the balanced equation, convert volumes and concentrations to moles, and compare mole ratios before deciding whether the result is acidic, basic or neutral. Use an indicator's colour change as a check.
In genetics problems, students confuse genotype with phenotype, treat a dominant allele as always the more common one in a population, and mis-predict offspring ratios by drawing one Punnett square for a cross that actually requires tracking two genes or sex chromosomes.
The word 'dominant' in everyday Japanese and English suggests 'stronger' or 'more frequent'. Students also memorise 3:1 without understanding that it arises only from a heterozygous × heterozygous cross, and they rush to draw a square before writing parental genotypes.
What to do instead
Before any cross, write the parental genotypes with defined symbols and state which phenotype each genotype gives. Draw the Punnett square only after confirming the number of genes involved. For sex-linked traits, label X and Y chromosomes explicitly. Add a check question: 'Does the ratio follow from the genotypes I wrote?'
In earth science, students read a geological cross-section by assuming the lowest layer is always the oldest, even when the sequence has been overturned or intruded by a magma body, and they misidentify an igneous intrusion as a layer that formed at the same time as the surrounding sediment.
The principle of superposition is taught as a rule of thumb without the exceptions. Students do not look for cross-cutting relationships or baked margins, and they treat diagrams as pictures to describe rather than evidence to interpret.
What to do instead
Apply a checklist to every cross-section: superposition, cross-cutting relationships (an intrusion is younger than what it cuts), inclusions, and baked contact zones. Practise by writing a relative-age order as a numbered list with a one-line justification for each step, then check that no intrusion is listed as contemporaneous with the sediments it cuts.
In weather and astronomy questions, students explain seasonal temperature differences by Earth's changing distance from the Sun, or they confuse the cause of tides, phases of the Moon and eclipses, describing a lunar eclipse as Earth passing between the Sun and the Moon at any phase rather than at full moon.
Diagrams in textbooks are static and not to scale, so orbital geometry is learned as a picture rather than a model. The distance explanation for seasons is intuitive and widely repeated outside school, which makes it resistant to correction.
What to do instead
Use a physical or hand-held model to demonstrate axial tilt and revolution, and state the key fact in one sentence: seasons are caused by the tilt of Earth's axis changing the angle and duration of sunlight, not by distance. For Moon events, build a phase sequence and mark where eclipses can occur (new moon for solar, full moon for lunar), then answer with both the geometry and the phase.