BGCSE Chemistry 0570 common mistakes
BGCSE Chemistry 0570 · Understand the traps and how to avoid them.
When asked to explain why a gas can be compressed but a liquid cannot, the candidate writes that gas particles 'are smaller' or 'weigh less', instead of describing the large empty spaces between gas particles and the particles being far apart and moving randomly.
Candidates confuse the macroscopic property (compressibility) with particle size or mass, because the particle diagram in textbooks shows small dots and they reason that small dots can be squeezed. They have memorised 'particles are far apart' as a phrase without linking it to the absence of significant intermolecular forces and the large volume of empty space.
What to do instead
Practise a fixed three-part answer structure for every kinetic-theory explanation: (1) state the arrangement and separation of particles, (2) state their motion/energy, (3) link that to the named property. For gases always write 'particles are far apart with large empty spaces between them, so the volume can be reduced by pushing particles closer'. Contrast with liquids in the same sentence. Draw and label the three states side by side before writing the answer.
In electron configuration questions the candidate writes 2,8,9 for an atom of potassium or fills the 3d subshell before the 4s, and then predicts the wrong group and ion charge from that configuration.
Candidates apply the simple shell-filling rule (2,8,8) as if it were universal and have not learned that the fourth shell begins to fill before the third is complete, or they memorise the order of subshells incorrectly. The error propagates into group number and ionic charge.
What to do instead
Learn the filling order as a written sequence (1s 2s 2p 3s 3p 4s 3d 4p) and practise writing configurations for the first 20 elements plus K, Ca, Fe and Cu. Always count outer-shell electrons from the correct shell to find the group, and derive ion charge from the group number, then check the total electron count equals the proton number for the ion.
The candidate draws a dot-and-cross diagram for sodium chloride showing a shared pair of electrons between Na and Cl, or draws ionic bonding with overlapping circles instead of a transferred electron and square brackets with charges.
Candidates do not distinguish consistently between the three bonding models and default to the covalent diagram they practised most. They also omit the charge labels because they see bonding as 'joining atoms' rather than as electrostatic attraction between ions.
What to do instead
Before drawing, force an explicit decision step: metal plus non-metal gives ionic, two non-metals give covalent, metal plus metal gives metallic. For ionic diagrams, draw the electron transfer with a curved arrow, put each ion in square brackets with its charge, and write a one-line statement that the bond is the electrostatic attraction between oppositely charged ions.
When balancing an equation the candidate changes a subscript in a formula (turning H2O into H2O2 or FeCl2 into FeCl3) in order to make the atom counts agree, rather than placing coefficients in front of the formulae.
Candidates treat balancing as an arithmetic puzzle to be solved by any means, and they have not internalised that a subscript is part of the identity of the substance. The habit is reinforced when the equation 'looks balanced' after the illegal edit.
What to do instead
Adopt a rule stated aloud before every balancing task: subscripts are fixed, only coefficients may change. Work through a systematic method: list each element and its count on both sides, balance one element at a time starting with elements that appear in only one compound, leave hydrogen and oxygen until last, and finish by re-checking every count including charges for ionic equations.
In mole calculations the candidate mixes units, for example substituting a mass in grams into n = c x V without converting cm3 to dm3, or uses the relative formula mass in place of the molar mass in kg, and reports an answer with no unit or the wrong unit.
Candidates learn the three mole formulae as isolated strings and do not track the units attached to each symbol. Volume in cm3 is common in practical work, so the conversion factor of 1000 is easily forgotten under time pressure.
What to do instead
Write the unit next to every quantity before substituting (m in g, M in g/mol, V in dm3, c in mol/dm3) and convert cm3 to dm3 by dividing by 1000 as an explicit written step. Use a triangle or rearranged formula sheet for n = m/M, n = cV and n = V/24, and always finish with a unit check and a sensible-magnitude check.
The candidate describes the rate of reaction as 'the time taken for the reaction to finish' or claims that adding a catalyst increases the yield of product, rather than stating that rate is the change in amount of reactant or product per unit time and that a catalyst increases rate without changing the position of equilibrium or the final yield.
Everyday language equates 'fast' with 'short time', and candidates have not separated the concepts of rate, yield and equilibrium. The catalyst misconception comes from a vague idea that catalysts 'help the reaction more'.
What to do instead
Define rate in writing as amount of reactant used or product formed per unit time, and practise describing the gradient of a mass-loss or gas-volume graph. For catalysts, learn the fixed statement: a catalyst provides an alternative pathway of lower activation energy, increasing the proportion of collisions that are successful, so equilibrium is reached sooner but the position of equilibrium and the final yield are unchanged.
In electrolysis questions the candidate assigns the wrong product to an electrode, for example predicting hydrogen at the anode or chlorine at the cathode in concentrated sodium chloride solution, or writes the electrode equations with the electrons on the wrong side.
Candidates memorise 'cathode' and 'anode' without anchoring them to reduction and oxidation, and they do not apply the reactivity series or the rule that the less reactive ion is discharged. Sign conventions for electrons in half-equations are also shaky.
What to do instead
Anchor with the mnemonics that cathode is the negative electrode where reduction (gain of electrons) occurs and anode is positive where oxidation occurs. For each cell, list the ions present, then apply the discharge rules in order: reactive-metal ions stay in solution, halides give halogen unless the solution is dilute, and aqueous solutions give hydrogen at the cathode unless a less reactive metal such as copper is present. Write half-equations with electrons on the reactant side for reduction and check charge balance.
When asked whether a reaction is exothermic or endothermic, the candidate answers from the sign of the temperature change alone ('the temperature went up so it is endothermic') or confuses bond breaking with energy release, stating that breaking bonds releases energy.
The everyday meaning of 'endothermic' as 'taking in heat' is applied to the thermometer rather than to the system, and the bond-enthalpy convention (breaking requires energy, forming releases energy) is counter-intuitive. Candidates also do not distinguish the system from the surroundings.
What to do instead
Fix the definition: exothermic means the system releases energy to the surroundings, so the surroundings' temperature rises. Learn the bond rule as a pair: breaking bonds is endothermic, forming bonds is exothermic, and the overall sign is the balance of the two. Practise with energy-level diagrams, labelling reactants, products, activation energy and the overall enthalpy change with its sign.
In questions on the blast furnace or the extraction of aluminium the candidate writes a word equation that omits a reactant or product, or states that carbon reduces aluminium oxide in the electrolytic cell, and cannot explain why aluminium is extracted by electrolysis rather than by carbon reduction.
Candidates learn the two processes as separate lists of facts without the underlying principle that the method of extraction depends on the position of the metal in the reactivity series. They also reproduce equations from memory without checking atom balance.
What to do instead
Build a single reactivity-series framework: metals above carbon are extracted by electrolysis of the molten compound, metals below carbon by reduction with carbon. For each named process, write the balanced equation from scratch and check it, and add one sentence on why that method is used. For aluminium, note that aluminium is more reactive than carbon and that the ore is dissolved in molten cryolite to lower the operating temperature.
When naming or drawing organic compounds the candidate gives a name that ignores the functional group or the position of the substituent, for example calling CH3CH2OH 'ethane' or naming but-2-ene as 'butene' without a number, and draws displayed formulae with the wrong number of bonds on a carbon.
Candidates treat organic nomenclature as a set of arbitrary labels rather than a rule system, and they do not check that each carbon has four bonds. The general formula for alkenes is applied without locating the double bond.
What to do instead
Use a fixed naming routine: find the longest carbon chain, identify and name the functional group suffix, number the chain from the end that gives the lowest locant to the functional group or double bond, then add prefixes. After drawing any structure, count bonds on every carbon and every oxygen and hydrogen to confirm valency of four for carbon, and practise converting between displayed, structural and molecular formulae for the same molecule.