GCE O/L Science common mistakes
GCE O/L Science · Understand the traps and how to avoid them.
When calculating magnification or actual size from a scale drawing, candidates divide the wrong way round, reporting an answer such as 'actual size = image size × magnification' or giving magnification as an area ratio rather than a length ratio.
The formula triangle is memorised as a jumble of symbols without a mental model of what magnification means. Candidates also forget to convert both measurements to the same unit before dividing, so a 50 mm image of a 0.05 mm cell comes out as 1000 instead of 1000× being checked against a sensible cell size.
What to do instead
Fix the meaning first: magnification tells you how many times bigger the image is than the real object, so magnification = image length ÷ actual length. Always write the equation, substitute with units converted to the same unit (usually mm), then sanity-check: a light-microscope cell should give a magnification of a few hundred to a few thousand, not 0.002. Practise the reverse direction by covering a different term in the equation triangle.
Candidates describe diffusion, osmosis and active transport interchangeably, for example saying 'water moves by active transport into a root hair cell' or 'minerals are absorbed by osmosis', and omit the partially permeable membrane and the concentration gradient when defining osmosis.
The three processes share the vocabulary of movement and concentration, and candidates learn definitions as isolated sentences rather than as a comparison table. The word 'transport' in active transport misleads them into thinking it is a type of osmosis.
What to do instead
Build a three-column comparison of substance moved, direction relative to the gradient, energy requirement and membrane involvement, and memorise osmosis as a full definition: net movement of water molecules from a dilute (high water potential) solution to a concentrated solution through a partially permeable membrane. Then classify every example you meet: root hair cells absorbing water (osmosis), absorbing nitrate ions (active transport), oxygen entering an alveolus (diffusion).
When asked to test a leaf for starch, candidates list iodine solution and 'it goes black' but omit the decolourising step in hot ethanol and the washing in warm water, or they boil the leaf in ethanol directly over a Bunsen flame.
The procedure is learned as a list of steps without the purpose of each, so steps get dropped under exam pressure. The safety reason for using a water bath rather than a naked flame is rarely taught explicitly.
What to do instead
Learn the sequence with a reason attached to each step: boil in water (kills tissue, breaks membranes), boil in ethanol in a water bath (removes chlorophyll so the colour change is visible; ethanol is flammable so no naked flame), wash in warm water (softens leaf and removes ethanol), add iodine (starch turns blue-black). Rehearse the order aloud and state the hazard when writing the method.
Candidates write the word equation for aerobic respiration and photosynthesis as if they were the same reaction reversed, then claim 'plants respire only at night' or that respiration produces oxygen.
The equations look like mirror images, so candidates assume one replaces the other in time. The idea that respiration is continuous in all living cells, including in daylight, is often missed because textbooks emphasise gas exchange in leaves.
What to do instead
Write both equations side by side and label the energy term: photosynthesis is endothermic and stores energy in glucose using light; respiration is exothermic and releases energy from glucose continuously, day and night, in plants and animals. Use the classic exam scenario of a plant in a sealed bell jar in the dark to show net oxygen uptake, and explain that in daylight photosynthesis usually outpaces respiration rather than replacing it.
In genetics questions candidates confuse genotype with phenotype, write gametes as 'Bb' instead of 'B' and 'b', and cannot complete a Punnett square correctly for a cross between two heterozygotes, giving offspring ratios of 1:1 instead of 3:1.
The vocabulary of allele, gene, genotype and phenotype is learned as a glossary rather than applied. Candidates also forget that meiosis halves the chromosome number, so each gamete carries one allele of each pair.
What to do instead
Practise the full routine every time: state the parental genotypes, write the gametes (one letter each), draw the square, then read off genotype ratio (1 BB : 2 Bb : 1 bb) and phenotype ratio (3 dominant : 1 recessive) separately. Always define the symbols at the start (B = brown eyes, b = blue) and check that the total number of squares equals the number of possible offspring combinations.
Candidates balance chemical equations by changing subscripts in formulae, for example writing H2 + O2 → H2O2 to make the atoms balance, and then carry the altered formula into mole calculations.
Balancing is taught as 'make the numbers match' without the rule that formulae are fixed by valency and only coefficients may change. The distinction between a coefficient and a subscript is not made explicit.
What to do instead
State the rule before every balancing exercise: formulae are fixed, only the large numbers in front may change. Balance by counting each element in a table, adjust coefficients, and re-count. Check by working out the relative formula mass on each side, which must be equal. If a candidate is tempted to alter a subscript, that is the signal to use a different coefficient or a fractional coefficient doubled up.
When calculating moles, candidates mix up mass, molar mass and concentration, for example dividing moles by volume in cm³ rather than dm³, or using Ar instead of Mr for a compound such as CO2.
The three quantities have similar symbols and the unit conversions are treated as an afterthought. Candidates also plug numbers into a remembered triangle without checking which quantity the question actually asks for.
What to do instead
Always write the quantity, its symbol and its unit before substituting: n = m ÷ Mr (mass in g, Mr in g/mol), and n = c × V (concentration in mol/dm³, volume in dm³, so divide cm³ by 1000). Compute Mr explicitly by adding Ar values for every atom in the formula. Finish by checking the order of magnitude: a few grams of a small molecule should give a fraction of a mole, not hundreds.
In qualitative analysis candidates identify a salt from a colour alone, for example calling any white precipitate 'lead chloride', or they forget to acidify with dilute nitric acid before adding silver nitrate when testing for halides.
Colour is the most memorable observation, so candidates stop there. The purpose of acidification (to remove carbonate ions that would also give a precipitate) is rarely understood, so the step is omitted.
What to do instead
Learn tests as a decision tree with the reagent and the observation paired: flame tests for metal cations, dilute sodium hydroxide for coloured hydroxide precipitates (and ammonia for the soluble ones), dilute nitric acid then silver nitrate for halides (white chloride, cream bromide, yellow iodide), and barium chloride with hydrochloric acid for sulfate. Practise writing 'add X, observe Y, therefore Z' as a three-part sentence so the conclusion is always justified by the test.
In electricity questions candidates add resistors in parallel as if they were in series, or they confuse current and voltage when using V = IR, for example saying 'current is shared in a series circuit' or 'voltage is the same across each parallel branch' without checking which quantity is conserved.
The two circuit rules have parallel wording and candidates learn them as sentences rather than as consequences of charge and energy conservation. The formula triangle then lets them substitute confidently into the wrong rearrangement.
What to do instead
Anchor the rules in conservation: charge is conserved, so current is the same everywhere in a series loop and splits between parallel branches; energy per unit charge is conserved, so voltage splits across series components and is the same across parallel branches. Draw the circuit, label the known values, then choose the rule before touching the formula. For parallel resistance, use 1/R = 1/R1 + 1/R2 and check that the total is smaller than the smallest branch.
In forces and motion questions candidates read a distance-time graph as if it were a velocity-time graph, or they calculate speed as distance ÷ time using the total distance but the final instant of time, and they omit units or the direction when asked for velocity.
Both graph types share axes labelled with time, and the gradient means different things in each. Candidates also treat average speed and instantaneous speed as the same quantity, and the vector nature of velocity is forgotten under time pressure.
What to do instead
Before reading any graph, label the vertical axis aloud: distance-time gradient gives speed, velocity-time gradient gives acceleration and area under the line gives distance. For calculations, write the formula, substitute with units, and state whether the answer is average or instantaneous. When a question asks for velocity, give magnitude and direction, and check the sign convention for deceleration and for motion in the opposite direction.