SPM Science common mistakes
SPM Science · Understand the traps and how to avoid them.
Writing the final answer to a calculation question without the correct unit or with a unit that does not match the quantity asked (e.g., giving a density in g instead of g cm⁻³, or a speed in m instead of m s⁻¹).
Candidates focus all attention on getting the number right and treat the unit as decoration; they also copy the unit of the data given rather than deriving it from the formula used.
What to do instead
After every calculation, write the working formula with its quantities, then cancel units symbolically before substituting numbers. Make a habit of writing the unit in the same line as the final value and re-reading the question to confirm which quantity was requested (mass vs weight, energy vs power, concentration vs amount).
Confusing mass with weight and using the wrong quantity in force or pressure calculations (e.g., substituting 5 kg directly into W = mg as 5 N, or using mass instead of weight when finding pressure on a surface).
Everyday language treats 'weight' and 'mass' as synonyms, and the textbook distinction is learned as a definition to recite rather than as an operational rule applied in problems.
What to do instead
Before substituting, label each given value as mass (kg) or weight (N). If the problem gives kg and asks for a force, insert the g = 10 N kg⁻¹ conversion explicitly as a separate line. Practise a mixed set of items where the same object is asked for mass, weight, and pressure, and check that each answer's unit matches the quantity.
Describing an observation and an inference as if they were the same thing - writing 'the plant released oxygen' or 'the acid reacted with the metal' when only bubbles or a colour change were seen.
Candidates answer from memory of the expected result of the experiment rather than from the evidence in the data given, because revision has emphasised conclusions over the process of reasoning from observations.
What to do instead
For every data-response item, draw two columns headed 'What I can see/measure' and 'What it means'. Only statements in the first column may be written as observations; conclusions must be phrased as inferences and must cite the specific observation that supports them. Practise converting conclusions back into observations until the distinction is automatic.
Treating a controlled variable as a manipulated variable when interpreting an experiment, or failing to identify which variable was kept constant, so the conclusion drawn about cause and effect is wrong.
Questions present a table of results without labelling the variables, and candidates describe 'what changed' without checking which change was deliberately imposed and which was merely measured.
What to do instead
Before reading the results, write three labels above the table: manipulated (what the experimenter chose to change), responding (what was measured), and at least one controlled variable. Then phrase the conclusion in the template 'As [manipulated variable] increases, [responding variable] increases/decreases, while [controlled variable] is kept constant.'
Balancing a chemical equation by changing subscripts in a formula (e.g., turning H₂O into H₂O₂ or Fe₂O₃ into FeO) instead of placing coefficients in front of the formulae.
The goal is perceived as 'make the numbers on both sides equal', and altering a subscript is the quickest visual way to do that; the conservation of atoms within a compound is not firmly linked to the meaning of a formula.
What to do instead
Adopt a fixed procedure: list each element and count atoms on both sides, then adjust only the large numbers written before each formula, and finish by re-counting every element. Add a checking step that rewrites each formula unchanged from the question, so any accidental subscript edit is visible.
Describing the reaction of an acid with a metal carbonate as simply 'a gas is produced' and naming the gas as hydrogen, or writing the salt as a hydroxide rather than identifying the correct salt and gas.
The hydrogen test (pop with a lighted splint) is over-practised relative to the carbon dioxide test (limewater turns milky), and the pattern of acid–carbonate reactions is memorised as a single generic statement.
What to do instead
Build a comparison table of acid reactions: metal → salt + hydrogen; carbonate → salt + water + carbon dioxide; base/alkali → salt + water. For each, record the test for the gas and the ions that form the salt. When answering, name the specific salt (e.g., sodium chloride from hydrochloric acid and sodium carbonate) rather than writing 'a salt'.
Explaining a biological process by naming the organ or structure only, without stating the function or the direction of transfer (e.g., 'the alveolus absorbs oxygen' with no mention of diffusion down a concentration gradient, or naming the xylem without saying water and mineral ions move upward).
Marking schemes reward structural–functional links, but candidates revise structure diagrams as labels to be recalled and never rehearse the explanatory sentence that earns the mark.
What to do instead
For each structure on the syllabus, memorise a three-part sentence: structure → property (thin wall, large surface area, rich blood supply) → function/consequence. Answer explanation questions using that chain, and always state the direction of movement and the mechanism (diffusion, osmosis, active transport) where relevant.
Mixing up mitosis and meiosis, or stating that meiosis produces identical cells, when answering questions on cell division, growth, or gamete formation.
Both are 'cell division' and are learned side by side, so the distinguishing outcomes (chromosome number, number of daughter cells, genetic variation, purpose) blur together under exam pressure.
What to do instead
Create a four-row comparison for each: purpose, number of divisions, chromosome number in daughter cells (diploid/haploid), and genetic similarity. Attach each to a concrete example - mitosis for growth and repair, meiosis for gamete formation - and answer by naming the process and then justifying it with the chromosome number.
Reading a distance–time or speed–time graph as if the vertical axis were speed in both cases, so a horizontal line on a distance–time graph is described as 'moving at constant speed' instead of 'stationary'.
The shape of the line is memorised for one graph type and then transferred to the other; candidates look at the gradient's appearance rather than at what the axes represent.
What to do instead
Before interpreting any graph, write the quantity and unit of each axis at the top of the page and state in words what the gradient means for that graph (distance–time gradient = speed; speed–time gradient = acceleration; area under speed–time = distance). Then describe each segment in words using that definition before choosing an answer.
Answering 'state the relationship' or 'suggest why' questions with a restatement of the data or a single vague word ('because it is faster', 'it increases') instead of a precise, comparative, mechanism-based statement.
Candidates run out of time and write the shortest possible response, and they have practised calculations far more than extended prose, so they do not know what a full-mark sentence looks like.
What to do instead
Rehearse three answer templates and apply them deliberately: relationship - 'As X increases, Y increases, but not proportionally'; comparison - 'A is greater than B because …'; explanation - 'because [mechanism], which causes [consequence]'. For each data-response item, aim for one sentence that quotes a value or trend from the data plus one sentence giving the reason, then check that both parts are present.