BJC General Science common mistakes
BJC General Science · Understand the traps and how to avoid them.
When asked to identify the independent and dependent variables in a described experiment, the candidate labels the variable the experimenter deliberately changes as the dependent variable, or names the measured outcome as the independent variable, then builds conclusions around that reversed pairing.
The words 'independent' and 'dependent' are memorised as labels without a working definition, and in scenarios where the manipulated factor is a treatment (fertiliser amount, temperature) the candidate reasons that the outcome 'depends on' the setup rather than on the manipulation, so the labels swap.
What to do instead
Before answering, write the sentence 'The experimenter changes ___ and measures ___' using the scenario's own nouns, then assign independent to the first blank and dependent to the second. Practise on ten short scenarios where the manipulated factor is a quantity, a category and a presence/absence, and check each answer by asking whether the outcome could exist without the manipulation.
The candidate treats a single trial as sufficient evidence and writes a conclusion such as 'the fertiliser made plants grow faster' after one measurement per condition, ignoring the absence of repeats, a control group, or stated constants.
School practicals often produce one dramatic result, and candidates conflate 'the result I saw' with 'a reliable finding'. They also do not distinguish a control group from controlled variables, so they believe constants have been handled when they have not.
What to do instead
Adopt a fixed three-part evaluation: state whether repeats were done, whether a control or comparison condition existed, and whether other variables were held constant. If any is missing, phrase the conclusion as tentative and name the specific repeat or control needed. Drill by critiquing short experiment summaries and rewriting each conclusion with a limitation clause.
On density and measurement questions the candidate substitutes numbers into a formula without converting to consistent units, for example dividing a mass in grams by a volume in cubic centimetres mixed with millilitres of a different substance, or reporting a density in g/cm³ when the mass was in kilograms.
Formulae are recalled as letter patterns (d = m/V) rather than as relationships between quantities, and unit prefixes are treated as decoration. Candidates also read '1 mL' and '1 cm³' as unrelated units, so they do not see when a conversion is free and when it is not.
What to do instead
Force a unit line before any calculation: write mass with its unit, volume with its unit, and convert both to grams and cubic centimetres (or kilograms and cubic metres) explicitly on paper. Learn the anchor equalities 1 mL = 1 cm³, 1 kg = 1000 g, 1 L = 1000 cm³, and finish every density answer by checking that the magnitude is plausible for the material named.
The candidate classifies a substance or change using surface appearance alone, calling a clear liquid a pure substance or a compound, naming a mixture as an element because it looks uniform, or labelling dissolving as a chemical change because the solid disappears.
Everyday language uses 'pure' to mean clean or unadulterated, and 'chemical' to mean anything to do with chemicals, so the technical definitions of element, compound and mixture never get applied. The disappearance of a solid in water looks like destruction rather than dispersal of particles.
What to do instead
Apply a decision sequence to every example: can it be separated by physical means (mixture) or not (pure substance); can it be broken down chemically into simpler substances (compound) or not (element); are new substances formed with different properties (chemical change) or is only the arrangement or state altered (physical change). Practise on deliberately ambiguous cases such as salt water, air, brass, dissolving sugar and burning magnesium.
When choosing a separation technique the candidate picks the method by name association rather than by property difference, for example using filtration to separate dissolved salt from water, evaporation to separate two miscible liquids, or a separating funnel for a solid-liquid mixture.
Methods are learned as a list with one stock example each, so the underlying principle (particle size, boiling point, solubility, magnetism) is never the deciding factor. Candidates also forget that filtration only works when one component is insoluble.
What to do instead
For each mixture, first name the property that differs between the components (particle size, boiling point, solubility, magnetic attraction), then select the technique that exploits that property. Build a two-column table of property difference against method and test yourself by inventing mixtures, including ones where two methods could work, and justifying the better choice.
The candidate describes osmosis as water moving from a high-concentration solution to a low-concentration solution, or states that water moves 'into the cell' without reference to the solute concentrations on each side, and then mispredicts whether a plant cell becomes turgid or plasmolysed.
The phrase 'high concentration' is ambiguous between solute concentration and water concentration, and candidates silently pick the wrong one. Diffusion and osmosis are also merged into a single idea, so the requirement for a partially permeable membrane is dropped.
What to do instead
Standardise the wording: water moves from the region of higher water potential (lower solute concentration) to the region of lower water potential (higher solute concentration) through a partially permeable membrane. For each scenario, explicitly label the solute concentration on both sides, state the direction of water movement, then state the effect on the cell (turgid, flaccid, plasmolysed, lysed, crenated).
In questions on enzyme activity or photosynthesis, the candidate claims the rate keeps rising indefinitely with temperature or light intensity, or states that an enzyme is 'killed' or 'destroyed' by heat, rather than denatured above an optimum.
The graph's initial rising section is generalised to the whole range, and the plateau caused by a limiting factor is not recognised. Everyday phrasing about heat killing things is carried into molecular explanations.
What to do instead
Learn each relationship as a shape with a reason: enzyme rate rises to an optimum then falls sharply because the active site changes shape (denaturation, not death); photosynthesis rises then plateaus when light is no longer the limiting factor, and the plateau can be lifted by raising carbon dioxide or temperature. Sketch both curves from memory and annotate the reason for every section, including the plateau.
The candidate confuses the direction of blood flow and the oxygen status of vessels, stating that arteries always carry oxygenated blood and veins always carry deoxygenated blood, or that the left side of the heart receives oxygenated blood from the body.
The rule 'artery = oxygenated' is taught as a shortcut and works for most of the systemic circuit, so the pulmonary and umbilical exceptions are never integrated. Heart diagrams are memorised as pictures rather than traced as circuits.
What to do instead
Define arteries and veins by direction relative to the heart (away versus towards), then treat oxygenation as a separate fact that must be checked per vessel. Trace blood as a continuous loop from vena cava through the right side, lungs, left side and aorta, saying 'oxygenated' or 'deoxygenated' aloud at each stage, and add the pulmonary artery, pulmonary vein and umbilical vessels as named exceptions.
In genetics problems the candidate writes a cross without distinguishing genotype from phenotype, assigns gametes incorrectly, or reports a 3:1 ratio for a test cross or a sex-linked cross where the expected ratio differs.
Punnett squares are completed mechanically as a grid-filling exercise, and the meanings of homozygous, heterozygous, dominant and recessive are not connected to the symbols. Candidates also forget that a son inherits his X chromosome from his mother.
What to do instead
Before drawing the grid, write the parental genotypes with a one-line justification, list the gametes each parent can produce, and only then fill the square. After filling, count genotypes and phenotypes separately and state both ratios. For sex-linked traits, label the sex chromosomes on every gamete and check inheritance of the X explicitly.
In energy and forces questions the candidate treats energy as used up or destroyed, adds forces that act on different objects, or calculates speed, acceleration or work without tracking the direction and sign of the quantities involved.
Everyday talk of 'using up' energy and 'losing' force encourages a substance model of energy. Free-body reasoning is skipped in favour of formula hunting, and the vector nature of force and velocity is not applied.
What to do instead
State the energy transfer chain for every problem (store to store, with the dissipated pathway named) and use conservation language: transferred, not destroyed. For forces, draw and label only the forces acting on the single object named, then sum them with directions. For motion, write the known quantities with signs and units before choosing an equation, and check that the answer's direction matches the physical situation.
On circuit questions the candidate applies series rules to parallel branches, believes current is 'used up' by each component so that it decreases around a series loop, or claims that adding a resistor in parallel increases total resistance.
Current is imagined as a fluid consumed by lamps, and the rules for series and parallel are memorised as lists without the underlying conservation of charge and energy. Candidates also do not redraw complex circuits in a simplified form.
What to do instead
Anchor on two principles: charge is conserved, so current entering a junction equals current leaving, and in a series loop the current is the same at every point; voltage is shared in series and equal across parallel branches. Redraw every circuit as a clean schematic before calculating, mark the current at each point, and verify with the rule that adding a parallel path always lowers total resistance.
When explaining day and night, seasons or Moon phases, the candidate says the Earth is closer to the Sun in summer, that night occurs because the Earth is in the Sun's shadow in a way that varies, or that phases are caused by the Earth's shadow falling on the Moon.
Two-dimensional diagrams and everyday phrases such as 'the dark side' encourage shadow-based explanations, and the axial tilt is known as a fact but not used as the mechanism. The scale of the Earth-Sun distance variation is also not appreciated.
What to do instead
For each phenomenon, name the mechanism explicitly and rule out the popular alternative: day and night from axial rotation; seasons from the constant 23.5-degree tilt changing the angle and duration of sunlight, not distance; phases from the changing angle between the Sun, Moon and observer, with Earth's shadow relevant only during a lunar eclipse. Draw a top-down Sun-Earth-Moon diagram for each case and label the illuminated half.
In acid-base and reaction questions the candidate uses 'acid' and 'alkali' as synonyms for 'dangerous' and 'safe', predicts the pH of a dilute strong acid from its concentration alone, or writes a neutralisation word equation without identifying the salt formed.
Safety labelling language and household product advertising dominate everyday exposure, and pH is treated as a property of a named substance rather than of a solution at a given concentration. Salt naming requires combining the metal from the base with the acid's anion, a step that is often skipped.
What to do instead
Separate the three ideas: strength (degree of ionisation), concentration (amount per volume) and pH (a measure of hydrogen ion concentration in that solution). For neutralisation, always derive the salt by pairing the base's metal or ammonium ion with the acid's anion, and name it, before writing the equation. Use the pH scale with benchmark values (stomach acid, pure water, soap) to sanity-check predictions.
On science-and-society items the candidate answers from personal opinion or media framing, accepting a claim because it is repeated or because a graph looks steep, without checking sample size, control, source or whether the graph's axes are scaled consistently.
These questions look like general knowledge rather than data interpretation, so evaluation steps are not applied. Candidates also confuse correlation with causation when a headline implies one.
What to do instead
Use a fixed evaluation checklist on every data or claim item: who produced the data, how many subjects, was there a comparison group, what exactly was measured, and does the axis start at zero. State explicitly whether the evidence supports causation or only association, and phrase the answer around what the data show rather than what seems likely.