SPM Additional Science common mistakes
SPM Additional Science · Understand the traps and how to avoid them.
When tabulating results, the candidate writes raw readings straight into the table with no units in the headers and no consistent decimal places, then plots them directly.
Candidates copy numbers from the instrument display without deciding in advance how many decimal places the instrument justifies. Because SPM marks are given for consistency and correct units in table headers, this single habit loses marks across every practical question.
What to do instead
Before recording, decide the precision the instrument allows and state it once. Put the quantity and unit in each column header (e.g. 'Length / cm'), keep every entry to the same number of decimal places, and calculate derived quantities (average, rate) after the table, showing the formula used.
The candidate states a hypothesis as a question or a vague statement such as 'the higher the temperature, the better', without naming the manipulated and responding variables.
Hypotheses are often written from memory of everyday phrasing rather than from the operational variables of the experiment, so the relationship is never made testable.
What to do instead
Use the frame 'The greater the [manipulated variable], the greater/less the [responding variable]', and check it names both variables and a direction. Then state the constant variables separately. Test the sentence by asking whether a single measurement could contradict it.
When drawing electron arrangements, the candidate fills shells as 2, 8, 8 for every atom regardless of atomic number, and writes ions by adding or removing protons.
The '2, 8, 8' pattern is memorised as a rule rather than as a consequence of shell capacity and proton number, and the difference between proton number and electron number in an ion is easily confused under exam pressure.
What to do instead
Always start from the proton number and build outward: 2 in the first shell, 8 in the second, 8 in the third, filling until electrons run out. For ions, change only the electron count and state which shell gained or lost electrons. Practise with chlorine, potassium and the oxide ion, whose patterns break the naive rule.
The candidate balances an equation by changing subscripts in a formula (writing H2 + O2 → H2O2 or Fe + O2 → Fe2O3 with altered formulae) instead of placing coefficients.
Balancing is treated as making the numbers match visually, and subscripts look like the adjustable numbers, so the candidate edits the chemical identity rather than the quantity.
What to do instead
Adopt a fixed order: write correct formulae first, list atoms per element on each side, then place coefficients only in front of formulae. Never touch a subscript. Finish by recounting every element, and check charge if ions are involved.
The candidate calculates speed or acceleration by substituting distance and time into v = s/t without checking that the time is in seconds, and reports the answer without direction when the quantity is a vector.
Unit conversion is skipped because the numbers look convenient, and speed, velocity and acceleration are treated as interchangeable labels.
What to do instead
Write the given values with units first, convert to SI before substituting (minutes × 60, km × 1000), and carry units through the working. Whenever the question mentions direction or displacement, state magnitude and direction, and distinguish distance from displacement explicitly.
In circuit questions the candidate adds resistances in parallel by simple addition, or applies V = IR to the whole circuit while ignoring that one component has a different current.
The series rule is learned first and over-generalised, and candidates do not redraw the circuit to identify which quantities are shared (current in series, voltage in parallel).
What to do instead
Before calculating, label the circuit: mark where current is the same and where voltage is the same. For parallel branches use 1/R = 1/R1 + 1/R2 and check that the combined resistance is smaller than the smallest branch. Verify with the total current from the supply.
The candidate describes reflection or refraction by saying 'the ray bends' without drawing or stating the normal, the angle of incidence and the angle of refraction, and measures angles from the surface rather than the normal.
Textbook diagrams are remembered as pictures rather than as constructions, so the normal is omitted and the protractor is placed against the mirror or surface.
What to do instead
Always draw the normal as a dashed line perpendicular to the surface at the point of incidence, then mark both angles between the ray and the normal. State the rule being applied (angle of incidence equals angle of reflection; light bends towards the normal entering a denser medium).
When explaining osmosis, the candidate writes that water moves 'from high concentration to low concentration' without specifying water potential across a partially permeable membrane, or claims the solute moves.
Osmosis is conflated with diffusion of solute particles, and the phrase 'high concentration' is ambiguous between solute concentration and water concentration.
What to do instead
Write the full chain: water moves from a region of higher water potential (more dilute solution) to lower water potential (more concentrated solution) through a partially permeable membrane. Name the membrane and the direction, and state the consequence for the cell (turgid, flaccid, plasmolysed, or lysed).
In homeostasis questions the candidate describes only the corrective response and omits the detection and coordination steps, or describes negative feedback as 'the body stops the change' with no receptor, control centre or effector named.
The response is the most memorable part of the loop, so the receptor and coordinating role of the brain or gland are dropped, leaving an incomplete pathway that cannot earn the sequence marks.
What to do instead
Write every answer as a loop: stimulus, receptor, control centre, effector, response, restoration of the set point. For temperature, name the thermoreceptors in the skin and hypothalamus, and the effectors (sweat glands, arterioles, skeletal muscle). Repeat for blood glucose with insulin and glucagon.
The candidate answers ecology questions by naming organisms and their food links but never quantifies energy transfer, and describes a food chain as if energy passes on in full.
Food chains are learned as labels rather than as energy-flow diagrams, so the 10 per cent figure, the loss as heat and the role of decomposers are omitted.
What to do instead
For every food chain, annotate the energy at each level and state that most energy is lost through respiration, movement and undigested material, so only about 10 per cent reaches the next trophic level. Explain why chains are short and why a pyramid of energy never inverts, unlike a pyramid of numbers.