A-Level Chemistry revision guide: the spec, the maths, the plan
A-Level Chemistry splits into physical, inorganic and organic chemistry, examined in three papers with at least 20% of marks for maths skills and a separate practical endorsement. The subject rewards two things above all: precise mechanisms drawn with correct curly arrows, and fluent mole-based calculation. Revision should drill mechanisms and calculations as skills, learn mark-scheme phrasing for explanations, and use past papers to find which of the three branches is weakest.
A-Level Chemistry is really three subjects sharing an exam. Physical chemistry is calculations and models; inorganic is patterns and explanations; organic is a visual language of mechanisms. Students who revise it as one subject keep having the same problem — strong in one branch, bleeding marks in another — and only find out which when a mark scheme tells them.
What you're actually sitting
| Board | Papers | The quirk |
|---|---|---|
| AQA (7405) | P1 physical+inorganic (105 mk, 2h) · P2 physical+organic (105 mk, 2h) · P3 all content (90 mk, 2h) | Paper 3 includes a multiple-choice section and synoptic questions |
| Edexcel (9CH0) | P1 inorganic+physical (90 mk, 1h45) · P2 organic+physical (90 mk, 1h45) · P3 synoptic (120 mk, 2h30) | Paper 3 is the longest chemistry exam on offer — stamina matters |
| OCR A (H432) | P1 modules 1,2,3,5 (100 mk, 2h15) · P2 modules 1,2,4,6 (100 mk, 2h15) · P3 'Unified' (70 mk, 1h30) | Short synoptic paper that crosses every module |
All three boards split assessment the same underlying way — a practical endorsement assessed by teachers (pass/fail, reported separately) plus written papers that examine practical methods as ordinary questions. At least 20% of marks across the papers are for maths skills.
The topic breakdown
Physical chemistry
- Atomic structure and the amount of substance — moles, Avogadro, ideal gases.
- Bonding and structure — ionic, covalent, metallic, shapes, intermolecular forces.
- Energetics — Hess's law, enthalpy changes, bond enthalpies, calorimetry.
- Kinetics — rates, orders, rate equations, Arrhenius.
- Equilibria — Kc and Kp, Le Chatelier, and the calculations that decide the grade.
- Thermodynamics, electrode potentials, acids and bases — pH, Ka, titrations, buffers.
Inorganic chemistry
- Periodicity — trends in ionisation energy, melting points, and their explanations.
- Group 2 and Group 7 — reactions, trends, and the tests examiners repeat.
- Period 3 oxides and their reactions.
- Transition metals — colours, complex ions, catalysis, redox titrations.
- Reactions of ions in aqueous solution — the precipitate colours that come up every year.
Organic chemistry
- The functional groups and their reactions — alkanes to amines, in order.
- Mechanisms — nucleophilic substitution, elimination, electrophilic addition and substitution, drawn with curly arrows.
- Isomerism — structural, E/Z, and optical.
- Organic synthesis — multi-step routes between compounds.
- Analysis — mass spec, IR, NMR, and the spectra questions worth easy marks.
How the marking works
Chemistry mark schemes are exacting about three things. Mechanisms: a curly arrow must start at a lone pair or bond and end where the pair goes — an arrow from a charge scores nothing. Calculations: the method marks are in the moles and the units, and a final answer without working scores almost nothing. Explanations: the mark scheme wants the named concept — 'increased nuclear charge', 'more shielding' — not a description of the trend.
Worked questions
For the equilibrium N₂ + 3H₂ ⇌ 2NH₃, 0.4 mol of NH₃ is present at equilibrium in a 2 dm³ vessel. If Kc = 0.5, find the equilibrium concentration of N₂ (take [H₂] = 0.3 mol dm⁻³).
- Kc = [NH₃]² / ([N₂][H₂]³).
- [NH₃] = 0.4/2 = 0.2 mol dm⁻³.
- Rearrange: [N₂] = [NH₃]² / (Kc × [H₂]³) = 0.2² / (0.5 × 0.3³).
- = 0.04 / (0.5 × 0.027) = 0.04 / 0.0135 ≈ 2.96 mol dm⁻³.
- Marks are in the rearranged expression and the units — write both before computing.
[N₂] ≈ 2.96 mol dm⁻³. The Kc expression, rearranged before substituting, is the whole question.
Draw the mechanism for the reaction of bromomethane with hydroxide ions.
- Identify the type — nucleophilic substitution (SN2 for a primary halogenoalkane).
- The OH⁻ lone pair attacks the δ+ carbon — the curly arrow starts at the oxygen's lone pair.
- The C–Br bond breaks — the second curly arrow goes from the bond to the bromine.
- Products: methanol and Br⁻. One step, concerted — not a two-step carbocation.
- Every arrow is a mark: start at the electron pair, end where it goes. Arrows from charges are the classic fail.
OH⁻ → Cδ+, C–Br bond → Br. Two arrows, one step, methanol + Br⁻ out.
Where marks get dropped
- Curly arrows starting at a charge instead of a lone pair or bond — the single commonest organic error.
- Calculation answers without working or units — the method marks are most of the question.
- Explaining a trend without the named concept — 'it gets bigger' instead of 'shielding increases'.
- Forgetting state symbols and conditions where they are part of the answer.
- In mechanisms, drawing SN1 when the substrate is primary — know which mechanism applies when.
- Signs in energetics and electrochemistry — ΔH and E values dropped or flipped.
Your revision checklist
- I can draw every mechanism on the spec with correct curly arrows.
- I can do Kc, Kp, pH, Ka and titration calculations without hesitating on the rearrangement.
- I can explain periodic trends using nuclear charge, shielding and distance.
- I know the ion tests and precipitate colours by heart.
- I can plan a multi-step organic synthesis between two given compounds.
- I have done timed papers per paper code and checked my explanations against the mark schemes.
Chemistry's wall is usually a mechanism or a rearrangement that the textbook explains once, in one way. Lumi's job is the second and third way: it draws the mechanism step by step, animates where the electrons go, and lets you interrupt at the arrow that does not make sense — then gives you a variant to check the fix.
What to take from this
- Physical, inorganic and organic are different skills — revise them as three subjects, not one.
- At least 20% of marks are maths: moles, Kc, pH, rates — calculation fluency is a grade skill.
- Curly-arrow mechanisms are drawn, not described — the arrows are where the marks live.
- The practical endorsement is separate, but practical methods are examined on the written papers.
- Explanations need the technical term — 'electronegativity', 'shielding', 'activation energy' — not approximations.
Questions people also ask
At least 20% of marks across the papers are for maths skills — logs, rearranging, moles, standard form, and the physical chemistry calculations like Kc and pH. It is the branch where maths fluency most directly buys marks.
Most students name organic synthesis (multi-step route planning) or the physical chemistry calculations — pH buffers and electrode potentials. Both are skills rather than content, so both respond to drilling faster than to rereading.
Yes — but as types, not as individual cases. Learn the six or seven mechanism types and which functional groups use which; the exam applies them to unfamiliar molecules, so memorising specific examples fails where understanding the type transfers.
A pass/fail assessment of lab competence across the required practicals, reported separately from your grade. The written papers examine the practical methods as questions — technique, variables, errors — regardless of the endorsement.