After thousands of hours tutoring A Level Chemistry across CIE, Edexcel and AQA, I have watched the same errors recur with depressing regularity — not because students lack knowledge, but because nobody has shown them how examiners actually think. This article is about that gap: between knowing chemistry and getting credit for it.
The six mistakes below are not equally damaging. Some cost one or two marks per paper. Others are systemic — built into how a student approaches the entire subject — and can be the difference between an A and a C. I have tried to rank them accordingly, with specific fixes and real worked examples for each.
| Mistake | Typical marks lost per paper | Impact |
|---|---|---|
| Conceptual gaps in core mechanisms | 15 – 25+ | Critical |
| Misreading or misanswering the question | 8 – 15 | High |
| Calculation errors and missing units | 6 – 12 | High |
| Poor diagram labelling and mechanisms | 5 – 10 | Medium |
| Weak practical and experimental answers | 4 – 8 | Medium |
| Poor time allocation | 3 – 10 | Variable |
This is not really a mistake — it is the mistake from which all others flow. Students who have memorised reactions but not understood the mechanisms underneath them are constantly guessing. They can write the products of a nucleophilic substitution reaction, but cannot predict whether it will proceed by SN1 or SN2. They know that increasing concentration increases rate, but cannot explain why — which means they cannot answer the three-mark "explain, with reference to collision theory" question that always appears.
The mechanisms that underpin the entire A Level course are surprisingly few: electronegativity and polarity driving intermolecular forces; electron density governing reactivity in organic chemistry; equilibrium as a dynamic balance between forward and reverse rates; entropy and enthalpy determining whether a reaction is thermodynamically feasible. Master these and the rest of the course becomes a set of worked examples of the same principles.
CIE mark schemes routinely award two or three marks for explaining the "why" behind an observation — why does the rate decrease when you cool the reaction, why does the equilibrium shift left when you increase pressure, why is compound X a stronger acid than compound Y. These marks are specifically designed to distinguish students who understand from students who have memorised. They cannot be drilled without genuine conceptual work.
Memorise that nucleophiles attack electrophiles. Write this in the exam. Receive zero marks on the mechanism question because no curly arrows, no explanation of electron density, no indication of which carbon is attacked and why.
Understand that nucleophiles are electron-rich species attacking electron-deficient carbon. From this one idea, you can draw any nucleophilic mechanism, predict regioselectivity, and explain why tertiary carbons favour SN1 — all without additional memorisation.
I have marked student responses that contained entirely correct chemistry — beautifully explained, logically structured — that received no credit because they answered the wrong question. A student asked to "explain why the rate increases" who writes "the rate increases because more collisions occur" has said something true and earned nothing, because "explain" at A Level requires a mechanistic account: more particles per unit volume, higher frequency of collisions with sufficient activation energy.
Command words are a language, and examiners use them precisely. "State" wants a one-line fact. "Explain" wants a causal chain. "Suggest" invites reasoning beyond the syllabus. "Compare" requires reference to both substances. Ignoring these distinctions costs marks on every single question.
Read your question paper twice before beginning — not once, twice. Underline the command word. Circle the specific substance or condition being asked about. Note the mark allocation: a six-mark question requires six distinct creditworthy points. Students who write three excellent sentences on a six-mark question will not score above three, regardless of quality.
"Describe" vs "Explain." Describing the trend in ionisation energies across a period earns one mark. Explaining it — shielding, nuclear charge, electron–electron repulsion in paired orbitals — earns five. Many students describe when asked to explain.
Before writing a single word, count the marks available and plan your response to match. A three-mark "explain" answer needs three distinct, linked points. If you cannot identify three, you have not thought about it deeply enough yet.
Struggling with exam technique as much as the content itself? A tutor who has been through this curriculum can show you exactly what examiners are looking for.
Book a Free Trial →Mole calculations, titration analysis, equilibrium constants, enthalpy cycles — these are the calculation-heavy sections of A Level Chemistry, and they share a common failure mode: students who understand the method but rush the arithmetic. The tragedy is that examiners award marks for method, so a student who sets up the calculation correctly but makes an arithmetic error at the final step loses only one mark. A student who panics, skips steps, and writes only the final number loses all the method marks too.
Units are not optional decoration. Stating that Kc = 0.025 without units is wrong if units apply, and right only if you understand why Kc is dimensionless in some cases. Stating that enthalpy change = 84 without kJ mol⁻¹ will cost marks every time. Examiners read scripts looking for evidence of understanding — units are evidence.
Write what you're finding first: n(NaOH) = c × V = 0.100 mol dm⁻³ × 25.0 cm³
Convert units before calculating: V = 25.0 ÷ 1000 = 0.0250 dm³
Calculate with units carried through: n = 0.100 × 0.0250 = 0.00250 mol
Use stoichiometry explicitly: ratio NaOH : HCl = 1:1 → n(HCl) = 0.00250 mol
State the final answer with units: c(HCl) = 0.00250 ÷ 0.0200 = 0.125 mol dm⁻³
Forgetting to convert cm³ to dm³ (÷1000). This single error appears in roughly one in three titration calculations I mark and costs at least two marks every time.
Write units at every step. If units do not cancel correctly, you have made an error. The unit trail is your error-detection system — use it.
Organic chemistry mechanisms are one of the highest-yield topics at A Level and one of the most poorly answered. The problem is almost never that students do not know the reaction — it is that they cannot translate their knowledge into a correct mechanism diagram. Curly arrows drawn from the wrong atom, starting from a bond instead of a lone pair, or pointing in the wrong direction all cost marks. A mechanism with one incorrect arrow often receives zero marks for the entire step.
The fix is not complicated, but it requires deliberate practice. Mechanisms must be drawn — not read, not highlighted, not watched on video. Your hand needs to learn the patterns. Every mechanism I teach, I draw on a whiteboard while explaining it, and then immediately ask the student to draw it back from memory. If they cannot reproduce it without prompting, we draw it again. This is the only method that works.
Curly arrows starting from atoms instead of bonds or lone pairs. Drawing half-headed arrows (fishhook arrows for radical mechanisms) where full curly arrows are needed, or vice versa. Omitting charges on intermediates.
Curly arrows always originate from a bond or lone pair and point toward where the electrons go. Full arrows = two electrons moving (ionic/polar mechanisms). Fishhook arrows = one electron moving (radical mechanisms). Always show formal charges on intermediates.
Mechanism questions are the most reliable source of grade-boundary marks. An A* student and a B student often know the same reactions — the difference is that the A* student can draw the mechanism cleanly under pressure. This is a learnable skill, but it requires more than re-reading your notes. Draw every mechanism you study, from memory, at least five times.
"Students who draw mechanisms regularly make fewer errors than students who understand mechanisms perfectly but rarely draw them. The hand builds habits the mind alone cannot."
— Fahad Rafiq, Chemistry & Biology TutorPractical and experimental questions follow different rules from theoretical ones, and most students do not realise this until they have lost marks on several papers. The core rule is this: you report what would actually be observed — colour, precipitate, gas produced, temperature change — not the underlying theory. "A white precipitate forms" earns the mark. "A nucleophilic substitution reaction occurs" does not, even though it is correct.
Similarly, evaluation questions — asking you to identify limitations of an experiment or suggest improvements — require specific, technical answers. "The experiment could be more accurate" earns nothing. "A burette reading should be taken at eye level to avoid parallax error, which could cause an inaccuracy of up to 0.1 cm³" earns marks. Specificity is everything.
"When silver nitrate is added, a halide substitution reaction occurs and silver halide is produced, confirming the presence of a halide ion."
"A cream precipitate forms, which is insoluble in dilute ammonia solution but dissolves in concentrated ammonia solution, confirming the presence of bromide ions."
For observations, use sensory language: what you see, smell (cautiously), or measure. For evaluations, always quantify: "a systematic error of approximately X" beats "some error." For improvements, always give the reason for the improvement, not just the technique.
A Level Chemistry papers are long by design. The time pressure is intentional, and students who have not practised under timed conditions will feel it acutely in the exam hall. The most common failure mode is spending twenty minutes on a four-mark calculation that goes wrong, then rushing the final section — which typically contains the highest-mark questions on organic synthesis, mechanisms, or extended response — and leaving points on the table that a prepared student would have collected.
The simple rule: allocate roughly one minute per mark, plus five minutes at the end for checking. If a question is not yielding after twice its allocated time, leave a blank, move on, and return. A blank can become a mark. A half-finished paper cannot.
Work through the paper in order, spending too long on early calculation questions, then rushing or skipping the organic and extended-response sections at the end where the marks per question are highest.
Skim the whole paper before starting. Identify where your strongest topics appear. In the first pass, answer confidently and quickly. In the second pass, tackle the harder questions with remaining time. Never leave a question blank without attempting one or two lines.
The only way to build exam pace is to time yourself doing past papers under real conditions. Do one complete paper per week in the final six weeks before your exam — no pausing, no checking notes, no extra time. Then mark it, identify the mistake category for each lost mark, and address each one specifically. This is more effective than any other revision method at this stage.
Print this out (or save it) and mark your last practice paper against it. For every mark you lost, decide which of these six categories it belongs to. You will almost certainly find that two or three categories account for the majority of your lost marks. Those are your priorities — not the entire syllabus, not an extra revision guide, just those two or three specific failure modes addressed deliberately and repeatedly over the coming weeks.
Chemistry exams reward preparation that is specific and honest. The students who improve most dramatically in the final months are not the ones who work the hardest in general — they are the ones who identify exactly what is costing them marks and fix precisely those things.
Every mistake on this list is fixable. None of them require extra intelligence or a deeper understanding of chemistry than you already have. They require awareness, practice, and honest self-assessment. The best tutor — whether human or a well-marked past paper — is the one that gives you that honesty clearly and quickly.
In a free trial session, we go through a past paper together, identify exactly which of these mistakes are costing you marks, and build a plan to fix them — before your exam.
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