GCE A/L Science for Technology common mistakes
GCE A/L Science for Technology · Understand the traps and how to avoid them.
When converting units, candidates multiply instead of divide (or vice versa) because they memorise 'move the decimal point' without tracking the direction; for example, converting 5.0 cm to m they write 500 m instead of 0.050 m, or converting 2.5 kW to W they write 0.0025 W instead of 2500 W.
Rote learning of prefixes without a dimensional check; under time pressure they recall a rule like 'small unit to big unit, move left' but apply it backwards, especially with squared or cubed units where the factor must be applied twice or three times.
What to do instead
Always write the conversion as a fraction with units that cancel: 5.0 cm × (1 m / 100 cm) = 0.050 m. For areas and volumes, square or cube the factor: 1 cm² = (10⁻² m)² = 10⁻⁴ m². After any conversion, do a plausibility check: a length of 5 cm cannot become 500 m.
In projectile and free-fall problems, candidates treat the horizontal and vertical motions as linked (e.g., using the same time for both but applying the vertical acceleration to the horizontal component), or they set the final vertical velocity to zero at the top of the trajectory but then use that zero for the whole flight.
The independence of horizontal and vertical motion is counter-intuitive; candidates often apply a single kinematic equation to the resultant motion without resolving, and they confuse the condition at the apex (v_y = 0) with the condition at landing.
What to do instead
Draw a clear diagram with separate columns for x and y. Write the knowns for each direction: horizontal a = 0, vertical a = g (downward). Use the vertical motion to find the time of flight, then substitute that time into the horizontal equation. At the apex, state explicitly v_y = 0 but v_x is unchanged. Practise a standard sequence: resolve → time from vertical → horizontal range.
When calculating heat transfer, candidates add latent heat and sensible heat incorrectly: they use Q = mcΔT for a phase change, or they forget that during melting/boiling the temperature stays constant and use the temperature change across the whole process.
The formula Q = mcΔT is over-practised, so it is applied to every situation; candidates do not visualise the heating curve and do not distinguish between 'energy to change temperature' and 'energy to change state'.
What to do instead
Sketch the temperature–time (or temperature–energy) graph for the process before calculating. Identify each segment: sloped segments use Q = mcΔT with the correct specific heat capacity for that phase; flat segments use Q = mL with the correct latent heat. Sum the segments. Check that the units of L (J kg⁻¹) are consistent with m in kg.
In ray optics, candidates draw ray diagrams without applying the law of reflection or Snell's law correctly: they draw the normal at the wrong point, measure angles from the surface instead of the normal, or assume light bends towards the normal when going from a denser to a less dense medium.
The normal is an abstract construction line, and candidates often draw rays from the object to the mirror/lens without constructing the normal first; they also memorise 'bends towards normal' without linking it to the refractive index.
What to do instead
Always draw the normal as a dashed line perpendicular to the surface at the point of incidence before drawing any ray. Mark the angle of incidence between the ray and the normal. Use n₁ sin θ₁ = n₂ sin θ₂ and check: if n₂ > n₁, θ₂ < θ₁ (bends towards normal). For lenses, use the three standard rays (parallel-to-axis, through-centre, through-focus) and extend them to locate the image.
When solving DC circuit problems with series and parallel combinations, candidates add resistances incorrectly (e.g., adding parallel resistors directly, or treating a series combination as parallel) and they assume current is the same everywhere in a parallel circuit.
The reciprocal formula for parallel resistors is easily misremembered, and candidates do not redraw the circuit in simplified stages; they also conflate the rule for current in series (same) with current in parallel (divides).
What to do instead
Redraw the circuit step by step, replacing each simple series or parallel group with its equivalent resistance. Use R_series = R₁ + R₂ + … and 1/R_parallel = 1/R₁ + 1/R₂ + … . After finding the total current, work backwards to find branch currents using the fact that voltage is the same across parallel branches. Check with Kirchhoff's laws: sum of currents at a junction = 0.
When drawing electron arrangements and Lewis structures, candidates fill the 3d subshell before the 4s subshell, or they place the wrong number of electrons in the outermost shell for transition metals, leading to incorrect ionic charges and bonding predictions.
The Aufbau order is often memorised as a simple sequence without understanding the (n+l) rule or the stability of half-filled and fully filled subshells; the 4s/3d energy crossover is a common trap.
What to do instead
Learn the order using the diagonal rule or the (n+l) rule: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p… . Remember that 4s fills before 3d but empties first when forming cations (e.g., Fe → Fe²⁺ loses 4s electrons first). For main-group elements, use the group number to predict valence electrons. Practise writing configurations for Cr and Cu (exceptions with half-filled/fully filled stability).
In organic chemistry, candidates misidentify functional groups and reaction types: they call an ester an ether, they fail to distinguish between addition and substitution, or they draw a polymer repeat unit with the wrong connectivity (e.g., showing the double bond still present in polyethene).
Functional groups are often learned as isolated names without linking to reactivity; polymerisation is taught abstractly, and candidates do not practise drawing the repeat unit from the monomer.
What to do instead
Build a functional-group table with structure, name, and a characteristic reaction. For polymers, practise the rule: addition polymerisation breaks the C=C to form a single-bond backbone, with substituents attached; condensation polymerisation loses a small molecule (e.g., water) and forms a linkage (ester or amide). Draw the repeat unit in brackets with the continuation bonds through the backbone atoms.
In genetics problems, candidates confuse genotype and phenotype ratios, or they fail to set up a Punnett square correctly for a dihybrid cross, leading to incorrect probabilities (e.g., stating 9:3:3:1 for a monohybrid cross or mixing up parental and recombinant types).
The terminology is dense, and candidates often memorise ratios without understanding independent assortment; they also forget to define alleles clearly before crossing.
What to do instead
Always define alleles with clear symbols (e.g., A = dominant, a = recessive). For a monohybrid cross, draw a 2×2 Punnett square and count genotypes; for a dihybrid cross, use a 4×4 square or the product rule (multiply independent probabilities). State whether the question asks for genotype or phenotype ratio. Check that the sum of ratios equals 16 for a dihybrid cross.
When describing experimental procedures in applied biology or biotechnology, candidates give vague steps such as 'sterilise the equipment' without specifying the method (e.g., autoclave at 121 °C for 15 min) or they omit controls and replicates, making the investigation invalid.
Candidates learn procedures as a list of words rather than as a logical sequence with a purpose; they do not link each step to the need for aseptic technique or reliable results.
What to do instead
Write protocols as numbered steps with quantities, temperatures, and times. For each step, state the reason (e.g., 'flame the loop to prevent contamination'). Always include a control group, at least three replicates, and identify the independent, dependent, and controlled variables. For microbiology, specify the sterilisation method and the incubation conditions.
In instrumentation and engineering applications, candidates read analogue scales without accounting for the smallest division, or they quote a measurement with more significant figures than the instrument allows (e.g., recording a length as 12.345 cm from a ruler marked in mm).
The concept of uncertainty and significant figures is often treated as a mathematical afterthought; candidates do not connect the physical scale to the recorded digits, and they copy all digits from a calculator.
What to do instead
Before taking a reading, identify the smallest division on the scale and estimate to one-tenth of that division. Record the value with the correct number of decimal places and state the uncertainty (e.g., 12.3 ± 0.1 cm). When calculating, round the final answer to the same number of significant figures as the least precise measurement. Use a rule: if the smallest division is 1 mm, record to 0.1 mm.