Download clean, printable lists of the most common mistakes students make — so you can fix them before they cost marks.
Each sheet is aligned to its exam board and built from recurring student errors highlighted in examiner reports and mark schemes.
What you get
A topic-by-topic mistakes list with a “do this instead” fix and a quick self-check.
How to use it
Review before past papers, then use the quick checks to catch errors under timed conditions.
Why it works
Many marks are lost on predictable slips: rounding, sign errors, units, and misreading commands.
Coverage by topic
Preview (up to 5 per topic)
17 total rows in download
| Topic | Common mistake / misconception | Do this instead | Quick check |
|---|---|---|---|
| Atomic Structure | Writing electronic configurations incorrectly — filling 4s before 3d but then removing from 4s first in ions. | Fill: ...3p, 4s, 3d... Remove electrons from 4s FIRST when forming transition metal ions. e.g. Fe²⁺: [Ar] 3d⁶ not [Ar] 3d⁴ 4s². | Forming ion? Remove 4s electrons before 3d electrons. |
| Atomic Structure | Confusing ionisation energy trend: saying it always increases across a period. | General increase across Period 3, but there are dips: Na→Mg→Al (dip at Al: 3p¹ easier than 3s²) and P→S (dip at S: paired 3p electron repels). | Explain the two dips in Period 3: Al and S. Full marks require explanation of the dip. |
| Bonding | Saying all ionic compounds dissolve in water. | Many ionic compounds are insoluble (e.g. BaSO₄, AgCl). Solubility depends on relative lattice enthalpy vs hydration enthalpies. | Do not assume ionic = soluble. Check solubility rules for common ions. |
| Bonding | Drawing dot-and-cross diagrams with unequal numbers of electrons on each side. | Count valence electrons carefully. Total electrons in diagram must equal total valence electrons available from all atoms combined. | Count: total valence electrons available = electrons drawn in diagram? |
| Energetics | Confusing exothermic and endothermic in terms of sign of ΔH. | Exothermic: ΔH is NEGATIVE (energy released to surroundings). Endothermic: ΔH is POSITIVE (energy absorbed from surroundings). | Exothermic → ΔH < 0. Endothermic → ΔH > 0. |
| Energetics | Using Hess's Law with the wrong direction of arrows. | If you reverse a reaction, change the sign of ΔH. Draw the Hess cycle and ensure all arrows form a consistent loop. | Did you flip the sign when reversing any step in the Hess cycle? |
| Energetics | Confusing bond enthalpies with lattice enthalpies. | Bond enthalpies apply to covalent bonds in gaseous molecules. Lattice enthalpy relates to ionic crystals. Do not mix them. | Ionic compound? Use Born-Haber/lattice enthalpy. Covalent? Use bond enthalpies. |
| Kinetics | Saying increasing temperature increases activation energy. | Activation energy is FIXED for a given reaction. Higher temperature gives more particles enough energy to exceed Ea — it does not change Ea itself. | Temperature changes the energy of particles, NOT the activation energy. |
| Kinetics | Writing the rate equation from the stoichiometric equation. | Rate equations must be determined EXPERIMENTALLY. You cannot read rate = k[A][B] from A + B → C — orders are not stoichiometric coefficients. | Rate equation → always from experiment, never from equation coefficients. |
| Equilibrium | Saying adding a catalyst shifts the equilibrium position. | A catalyst speeds up BOTH forward and reverse reactions equally. It does not shift equilibrium — Kc is unchanged. Equilibrium is reached faster. | Catalyst: faster equilibrium, same position, Kc unchanged. |
| Equilibrium | Confusing Kc and Kp. | Kc uses equilibrium concentrations (mol/dm³). Kp uses partial pressures (Pa or atm). For gases, both are used; Kp = Kc(RT)^Δn. | Concentrations → Kc. Partial pressures → Kp. Units depend on overall order. |
| Acids and Bases | Saying a strong acid has a higher concentration than a weak acid. | Strong/weak refers to degree of dissociation, NOT concentration. A strong acid fully dissociates; a weak acid partially. Same concentration, different pH. | Strong = fully dissociates. Concentration is separate. Same molarity → different [H⁺]. |
| Acids and Bases | Calculating pH of a weak acid using pH = -log[HA] instead of [H⁺]. | For weak acid: Ka = [H⁺]²/[HA]. Find [H⁺] = sqrt(Ka × [HA]), then pH = -log[H⁺]. Never take log of [HA] directly. | pH = -log[H⁺]. Find [H⁺] first from Ka expression. |
| Organic Chemistry | Drawing curly arrows starting from the wrong place in mechanisms. | Curly arrows start from a lone pair or a bond — NEVER from an atom or a positive charge. Arrow points to where electrons move TO. | Arrow starts: lone pair or bond. Never from a + charge or bare atom. |
| Organic Chemistry | Confusing SN1 and SN2 mechanisms. | SN2: one step, 2nd order, primary/secondary halogenoalkane, inversion. SN1: two steps (carbocation), 1st order, tertiary halogenoalkane, racemisation. | Primary → SN2. Tertiary → SN1. Secondary → either, depends on conditions. |
| Organic Chemistry | Forgetting to show a carbocation intermediate in SN1 or E1. | SN1 and E1 go through a carbocation intermediate. Draw this explicitly in the mechanism — it is required for full marks. | SN1/E1: two curved arrows for step 1 (form carbocation), then two more for step 2. |
| Organic Chemistry | Confusing reduction and oxidation in organic reactions. | Oxidation of organic compound = gain of oxygen or loss of hydrogen (C-H bonds → C-O bonds). Reduction = opposite. | Count O and H bonds on the carbon. More O → oxidised. More H → reduced. |