MATSEC SEC Chemistry common mistakes
MATSEC SEC Chemistry · Understand the traps and how to avoid them.
Writing formulas such as Na₂O as 'NaO' or 'NaO₂', or balancing by changing subscripts in a formula rather than placing coefficients in front of it.
Candidates memorise valencies as isolated numbers and then pair them up without applying the 'swap and cancel' rule, or they treat the subscript as a free knob to make atom counts match on both sides of an equation.
What to do instead
For ionic compounds, write the ion charges above each symbol, cross the numerical values down as subscripts, then divide by the highest common factor (e.g. Ca²⁺ and O²⁻ gives Ca₂O₂, which cancels to CaO). For balancing, always check that the formula of each substance is correct before touching coefficients, and practise a fixed routine: list each element, count atoms on each side, adjust only coefficients, recount.
Calculating moles with the wrong molar mass, for example using 16 instead of 18 for water, or using the relative atomic mass of an element when the question gives a compound.
Candidates grab the first number they see in the periodic table or add atomic masses from memory without listing the atoms in the formula; they also forget that a diatomic gas such as O₂ has two atoms.
What to do instead
Before any calculation, write the formula, then write out the atom count explicitly (H₂O = 2×1 + 16 = 18) and only then divide mass by this value. Always state units (g mol⁻¹) next to the molar mass. Do three or four mole calculations per topic area so the routine becomes automatic.
Drawing dot-and-cross diagrams that show the wrong number of outer electrons, omit lone pairs, or show covalent bonding as complete transfer of electrons.
Candidates confuse the group number with the total electron count, or they learn one example (usually NaCl) and over-apply the ionic pattern to covalent molecules such as H₂O or CO₂.
What to do instead
First determine the group number of each atom to get outer electrons, then count electrons needed to reach eight (or two for hydrogen). Draw only outer shells, use different symbols for electrons from different atoms, and check the total electron count is conserved before and after bonding. Practise one ionic, one simple covalent, one double-bond and one dative-covalent example side by side.
Describing trends in the periodic table or reactivity series as lists to memorise, without linking them to electron configuration or atomic structure, so explanations in structured questions score no marks.
Candidates revise facts as isolated statements ('sodium is more reactive than magnesium') and never learn the causal chain, so they cannot answer 'explain why' parts which carry most of the marks.
What to do instead
For every trend, learn a three-step explanation: state the trend, refer to outer electrons and shell number, then state the effect on the ease of losing or gaining electrons. Write these explanations out in full sentences for Group 1, Group 7 and the halide displacement reactions, and self-test by covering the answer.
Writing word equations instead of balanced symbol equations, or giving an equation with correct formulae but no state symbols where they are required.
Candidates are more confident with words and avoid symbols under time pressure; state symbols are often treated as optional decoration rather than part of the answer.
What to do instead
Convert every word equation in past-paper practice into a symbol equation immediately, and add state symbols as a separate final pass. Learn the solubility rules needed (most sodium, potassium, ammonium and nitrate salts are soluble; most carbonates are insoluble) so you can assign (s), (l), (g) and (aq) confidently, especially for precipitation and neutralisation reactions.
Confusing the test and the result for common gases and ions, for example saying 'a glowing splint relights for hydrogen' or using limewater to test for carbon dioxide but describing the result as 'it goes cloudy' without saying the limewater turns milky.
Candidates memorise pairs of words without a clear mental image of the observation, and under exam pressure the gas names get swapped.
What to do instead
Build a two-column table of test and observation for hydrogen (lighted splint gives a squeaky pop), oxygen (glowing splint relights), carbon dioxide (limewater turns milky/cloudy), chlorine (bleaches damp litmus paper) and ammonia (damp red litmus turns blue). For ions, learn the precipitate colours for hydroxide and halide tests. Recite the table aloud until the pairs are automatic.
In titrations and volumetric calculations, using the wrong volume (reading the burette instead of the pipette volume, or forgetting to convert cm³ to dm³) and then applying the mole ratio in the wrong direction.
Candidates follow a memorised sequence of button presses without tracking what each number represents, and the cm³-to-dm³ conversion (divide by 1000) is frequently skipped.
What to do instead
Write the calculation as labelled steps: moles of known substance = concentration × volume in dm³; then moles of unknown = moles known × (ratio from the balanced equation); then concentration = moles ÷ volume in dm³. Always write the equation first and box the ratio. Check the answer is sensible (a titration concentration is usually between 0.05 and 2 mol dm⁻³).
Treating electrolysis as a single memorised set of products, so predicting hydrogen at the cathode in copper(II) sulfate solution or oxygen at the anode in molten lead(II) bromide.
Candidates learn one example (often sodium chloride solution) and do not distinguish between molten electrolytes, dilute aqueous solutions and concentrated aqueous solutions, where the rules differ.
What to do instead
Classify the electrolyte first: molten (ions only), dilute aqueous or concentrated aqueous. Then apply the rules in order: at the cathode, the less reactive metal or hydrogen is discharged; at the anode, halides are discharged if concentrated, otherwise oxygen from water. Write the half-equations for each case and practise one example of each class, including copper refining and brine electrolysis.
Describing rates of reaction only as 'faster' or 'slower' without referring to collision frequency or activation energy, and drawing rate graphs with axes unlabelled or with the curve rising instead of falling.
Candidates learn the practical effect (powder reacts faster than lumps) but not the particle-level explanation, and graph-drawing skills are practised rarely.
What to do instead
For each factor (concentration, temperature, surface area, catalyst), write a two-sentence explanation naming collision frequency and, where relevant, the proportion of collisions with energy greater than the activation energy. When drawing graphs, label both axes with quantity and unit, mark the initial steep section and the plateau, and show the effect of a catalyst as a curve reaching the same final volume sooner.
Giving vague or incomplete answers to 'explain', 'suggest' and 'describe' questions, for example writing 'it is more reactive' when the question asks for a reason, or omitting the observation when asked for a conclusion.
Candidates answer the question they expect rather than the command word given, and they run out of time so they write short answers that cannot earn the available marks.
What to do instead
Underline the command word and the number of marks before writing. Treat each mark as one distinct point and write that many separate statements. For 'explain', always include a because-clause linking cause to effect. Practise by marking your own answers against the mark scheme and counting how many points you actually made.