JC Science common mistakes
JC Science · Understand the traps and how to avoid them.
When asked to identify the independent variable, the dependent variable and the controlled variables in a described experiment, the candidate labels the thing that was deliberately changed as the dependent variable, or lists the measured outcome as a control.
The words 'independent' and 'dependent' are not transparent in everyday English, and candidates often reason that the variable the experimenter chose to set up is the one that 'depends' on the setup. Textbooks also present the three variable types together, so the definitions blur under exam pressure.
What to do instead
Use a fixed sentence frame before writing: 'We changed ___ (independent), we measured ___ (dependent), we kept ___ the same (controls).' Practise on five short scenarios and check each answer by asking 'which one did the experimenter choose, and which one responded?' The dependent variable must always be something measurable with a number and a unit.
The candidate writes a conclusion that merely restates the prediction ('the plant grew more because it grew more') instead of stating whether the hypothesis was supported and citing the actual data values, including units.
Candidates conflate the purpose of an investigation with proving themselves right, so they describe the result rather than interpreting it against the hypothesis. They also omit numbers because they are unsure which figures to quote.
What to do instead
Draft conclusions with three parts: (1) 'The hypothesis was supported/not supported because...', (2) the specific evidence with units and the comparison, e.g. 'the 20 °C sample reached 12 mm against 5 mm at 5 °C', (3) one limitation. Rehearse with past-paper style data tables until the pattern is automatic.
The candidate draws a line graph through scattered points without considering whether the data are continuous, joins points with a jagged line rather than a smooth trend line, or plots a bar chart for continuous data such as temperature over time.
Candidates learn 'graph your results' as a single procedure and do not distinguish categorical from continuous variables. They also treat joining the dots as showing the data faithfully, not realising a best-fit line or curve is expected for continuous data.
What to do instead
Apply a two-question test: is the x-axis variable continuous (time, temperature, length) or categorical (type of material, species)? Continuous means line graph with a smooth line or curve of best fit; categorical means bar chart with gaps. Label axes with quantity and unit, use a sensible scale covering more than half the grid, and never force the line through the origin unless the data justify it.
When explaining digestion, the candidate writes that food is 'digested in the stomach' and that 'nutrients are absorbed in the large intestine', and describes enzymes as being 'used up' by the reaction they catalyse.
The digestive tract is taught as an ordered list of organs, so candidates attach a vague function to each name rather than linking each organ to a specific process. The enzyme misconception arises from confusing a catalyst with a reactant.
What to do instead
Build a table with columns for organ, mechanical or chemical action, enzyme or substance secreted, pH, and substance absorbed. Fix the key points: starch begins in the mouth with amylase, protein begins in the stomach with pepsin at acid pH, fat is emulsified by bile from the liver and digested by lipase in the small intestine, and most absorption occurs in the small intestine via villi. Enzymes are unchanged and reusable.
The candidate describes breathing and respiration as the same process, writing that 'respiration is breathing in oxygen and out carbon dioxide', and cannot distinguish aerobic from anaerobic respiration in terms of reactants, products and energy released.
Everyday language uses 'respiration' for breathing, and both processes involve oxygen and carbon dioxide, so the distinction collapses. Anaerobic respiration is often memorised as a slogan ('lactic acid') without linking it to muscle cells or yeast.
What to do instead
Anchor the difference in location and purpose: breathing is ventilation of the lungs, a physical process; respiration is a chemical reaction in cells releasing energy from glucose. Write the equations from memory: aerobic glucose + oxygen gives carbon dioxide + water with a large energy release; anaerobic in muscle gives lactic acid with a small energy release; anaerobic in yeast gives ethanol and carbon dioxide. Then explain why sprinters still tire despite breathing hard.
The candidate predicts the phenotype ratio of a genetic cross without drawing a Punnett square, and confuses genotype with phenotype or treats a recessive allele as 'weaker' and therefore rarer in the population.
Ratios are memorised as 3:1 or 1:1 without the underlying segregation logic, so unusual crosses such as a test cross or a sex-linked cross break the pattern. Recessive alleles are misunderstood as being outcompeted rather than simply masked in heterozygotes.
What to do instead
Always set out gametes along the top and side of a grid, fill the four boxes, then count phenotypes and state the ratio in lowest terms. Define genotype as the allele combination and phenotype as the observable characteristic. Explain that a recessive allele can persist at high frequency because heterozygotes carry it without showing it, and practise a test cross between a dominant-phenotype individual and a homozygous recessive to reveal an unknown genotype.
When asked to separate a mixture, the candidate chooses filtration for a solution of salt in water, or evaporation for a mixture of two miscible liquids, and does not explain how the chosen technique exploits a difference in a named property.
Separation techniques are learned as a list of apparatus rather than as consequences of particle properties such as particle size, boiling point or solubility. Candidates match the technique to the appearance of the mixture instead of to the property that differs.
What to do instead
For every separation question, first name the property that differs (insoluble solid versus liquid, dissolved solid versus solvent, liquids with different boiling points, a coloured pigment's solubility), then select the technique that exploits it: filtration, crystallisation or evaporation, simple or fractional distillation, chromatography. Add one sentence justifying the choice and one safety or practical detail, such as not evaporating to dryness or pointing the condenser.
The candidate writes that neutralisation always produces a neutral solution of pH 7, that a strong acid is the same as a concentrated acid, and that pH 3 is 'twice as acidic' as pH 6.
The words 'strong' and 'concentrated' are used interchangeably in everyday speech, and the logarithmic nature of pH is not intuitive. Neutralisation is taught as acid plus base gives salt plus water, which candidates over-extend to mean the resulting solution is always neutral.
What to do instead
Separate the vocabulary: strong or weak describes degree of ionisation, concentrated or dilute describes amount per volume, so a dilute strong acid and a concentrated weak acid can both exist. State that neutralisation gives a salt plus water but the final pH depends on the amounts used, and only a strong acid with a strong base in exact proportions gives pH 7. For pH, work through one example numerically: each unit is a tenfold change, so pH 3 is one thousand times more acidic than pH 6.
The candidate calculates speed, work or power by substituting numbers into a remembered formula without checking unit consistency, writing for example 50 g in a force equation, or mixing kilowatts and seconds and reporting the answer in joules.
Formulas are recalled as strings of letters rather than as relationships between quantities, so the candidate does not notice when a mass in grams, a distance in centimetres or a time in minutes is inconsistent with the required SI units. There is also no habit of checking whether the magnitude of the answer is plausible.
What to do instead
Before substituting, convert every quantity to SI units and write the unit beside each number. After calculating, carry out a unit check on the result and a sanity check on the size (a cyclist does not travel at 300 m/s). Practise a mixed set covering speed, acceleration, weight, work and power where some values are deliberately given in non-SI units.
The candidate answers a question about why a fuse or circuit breaker is used or why a wire heats up by saying 'too much electricity flows' or 'the electricity is used up', rather than referring to current, resistance, potential difference and energy transfer.
Everyday phrases about electricity are vague and candidates reuse them in place of a model. There is also a persistent misconception that current is consumed by components, which undermines later reasoning about series and parallel circuits.
What to do instead
Rehearse the model explicitly: current is the rate of flow of charge and is the same at every point in a series loop; potential difference is energy transferred per unit charge and is shared across components in series; resistance opposes current and converts electrical energy to thermal energy. Explain a fuse as a component with a low melting point that melts when current exceeds its rating, breaking the circuit. Redraw series and parallel circuits and state what stays the same and what splits.