Cambridge IGCSE Chemistry (0620) — Past Papers, Revision Notes & Exam Prep

Common mistakes and how to avoid them

1. Reactivity series

   MistakeYou forget the order of the reactivity series, so you choose copper reacting with dilute hydrochloric acid, or aluminium extracted by heating its oxide with carbon.

   Fix

   Most reactive first: K Na Ca Mg Al C Zn Fe H Cu Ag Au.

   Cu, below H: no hydrogen with dilute acid.

   Above C: extraction by electrolysis, not carbon.

2. What acids produce

   MistakeYou expect hydrogen from every reaction of an acid, so you miss the carbon dioxide from a carbonate or the water from a metal oxide or hydroxide.

   Fix

   Potassium hydroxide + nitric acid → potassium nitrate + water.

   Potassium carbonate + nitric acid → potassium nitrate + water + carbon dioxide.

   Neither gives hydrogen: only a metal above hydrogen does.

3. Precipitate colours

   MistakeYou link a precipitate colour to the wrong metal ion, for example naming copper(II) or zinc ions from a green precipitate with aqueous sodium hydroxide.

   Fix

   Copper(II): light blue precipitate with aqueous sodium hydroxide.

   Iron(II): green.

   Chromium(III): green, soluble in excess.

   Iron(III): red-brown.

   Zinc: white, soluble in excess.

   Blue-green is copper's flame colour, not its precipitate.

4. Ionic compounds

   MistakeYou choose an option saying a solid ionic compound conducts electricity.

   Fix

   Ions carry a current only when they can move.

   Solid magnesium chloride: ions held in place, no current.

   Molten or dissolved in water, its ions move and it conducts.

5. Exothermic and endothermic

   MistakeYou mix up exothermic and endothermic, for example linking an endothermic reaction to a rise in the temperature of the reaction mixture.

   Fix

   Exothermic: thermal energy goes out to the surroundings, so the temperature rises.

   Endothermic: energy is taken in, so the temperature falls.

   Mixture cools from 23 °C to 16 °C: endothermic.

6. Atoms and ions

   MistakeYou add electrons when a metal atom forms a positive ion, so you choose the wrong electronic configuration or electron count for the ion.

   Fix

   Metal atoms lose electrons to form positive ions.

   Non-metal atoms gain electrons to form negative ions.

   Lithium (2,1) loses one electron: Li⁺ is 2 (like helium), not 2,2.

7. Graphite and diamond

   MistakeYou accept an option giving graphite the wrong property or the wrong reason for its use, such as 'electrode, because it is giant covalent'.

   Fix

   Electrode: graphite conducts through mobile electrons.

   Lubricant: weak forces between layers let them slide.

   'Giant covalent' is true of graphite but explains neither use.

   Diamond is very hard: cutting tools.

8. Tests for water

   MistakeYou mix up the colour changes, or the words hydrated and anhydrous, so you choose the wrong result for a test for water.

   Fix

   Anhydrous means without water.

   With water, white anhydrous copper(II) sulfate turns blue.

   With water, blue anhydrous cobalt(II) chloride turns pink.

   Heating the hydrated salt reverses each change.

More multiple-choice traps: Top 10 IGCSE Chemistry MCQ Mistakes

Common mistakes and how to avoid them

1. Equilibrium and yield

   MistakeYou assume a faster reaction gives a higher yield, or predict how temperature or pressure changes the yield without checking ΔH and the moles of gas on each side.

   Fix

   Heating favours the endothermic direction.

   High pressure favours fewer gas moles.

   In exothermic PCl₃(g) + Cl₂(g) ⇌ PCl₅(g), heating raises rate but lowers yield.

   High pressure (2 moles to 1) raises yield.

2. Activation energy

   MistakeYou think a higher concentration gives a greater proportion of particles enough energy to react, rather than just more collisions per second.

   Fix

   Concentration changes collision rate, not particle energy.

   Double the concentration: twice the collisions per second, same proportion with enough energy.

   Heating raises that proportion; a catalyst lowers Ea.

3. Polymer linkages

   MistakeYou call a polymer addition when its chain has amide or ester links, or mix up nylon, protein and PET, so you choose the wrong monomers.

   Fix

   Carbon-only backbone: addition polymer, like poly(ethene).

   Amide links (-CONH-): nylon (diamine + dicarboxylic acid) or protein (amino acids).

   Ester links (-COO-): PET (diol + dicarboxylic acid).

   Polyamides and polyesters are condensation polymers.

4. Electrolysis of solutions

   MistakeYou choose sodium, or another reactive metal, forming at the cathode from an aqueous solution, or mix up the anode product from dilute and concentrated halide solutions.

   Fix

   At the cathode: hydrogen, unless the metal is below hydrogen, like copper.

   Aqueous calcium chloride: hydrogen at the cathode, never calcium.

   At the anode: chlorine if concentrated, oxygen if dilute.

5. Limiting reactants

   MistakeYou assume the reactants react one to one, or work from the reactant in excess, so you pick the wrong amount of product.

   Fix

   2Na + Cl₂ → 2NaCl with 0.30 mol Na and 0.20 mol Cl₂.

   0.30 mol Na needs 0.15 mol Cl₂, so Na is limiting.

   NaCl = 0.30 mol, not 0.20 or 0.40 mol.

6. Strong and weak acids

   MistakeYou mix up strong and weak acids, for example saying a weak acid fully dissociates, or that the stronger acid has the higher pH at equal concentration.

   Fix

   Hydrochloric acid is strong: HCl → H⁺ + Cl⁻, every molecule dissociates.

   Ethanoic acid is weak: CH₃COOH ⇌ H⁺ + CH₃COO⁻, only some do.

   At equal concentration, the strong acid has the lower pH.

7. Oxidising and reducing agents

   MistakeYou pick the substance that is reduced as the reducing agent, mixing up 'reduced' with 'reducing agent'.

   Fix

   Zn + 2Ag⁺ → Zn²⁺ + 2Ag.

   Ag⁺ gains electrons and is reduced, so it is the oxidising agent.

   Zn loses electrons and is oxidised, so it is the reducing agent.

8. Addition and substitution

   MistakeYou treat an alkene reaction as substitution, such as bromine swapping places with hydrogen, or forget that an alkane substitutes with chlorine in UV light, giving hydrogen chloride.

   Fix

   Alkenes add across C=C, giving one product: C₇H₁₄ + Br₂ → C₇H₁₄Br₂, not C₇H₁₃Br + HBr.

   Alkanes substitute in UV light, Cl replacing H: C₇H₁₆ + Cl₂ → C₇H₁₅Cl + HCl.

More multiple-choice traps: Top 10 IGCSE Chemistry MCQ Mistakes

Common mistakes and how to avoid them

1. Particle model

   MistakeYou mix up arrangement, separation and motion, answering a different one from the one asked, or take 'separation' to mean filtration.

   Fix

   Answer each feature the question names.

   Separation is how far apart particles are, not filtration.

   Liquid mercury, separation: close together, not 'small gaps'.

   Arrangement: random.

   Motion: slide past each other.

2. Tests for ions

   MistakeYou give the test for another ion, or the right reagent with the wrong colour or without the word 'precipitate'.

   Fix

   Give the named ion's own test: reagent, then what you see.

   Aluminium ions: aqueous ammonia gives a white precipitate, insoluble in excess.

   Write 'precipitate': 'white' alone is not enough.

3. Making dry crystals

   MistakeTo get crystals from a salt solution, you heat it until all the water has gone, leave out filtering, or dry the crystals in a hot oven.

   Fix

   Potassium nitrate solution: heat only until a drop on a cold glass rod forms crystals.

   Leave to cool, then filter.

   Pat dry between filter papers, not in an oven.

4. Pollutants and effects

   MistakeYou give a vague effect such as 'harmful' or 'air pollution', or the effect of a different pollutant, such as acid rain for nitrates in water.

   Fix

   Give each pollutant its own effect.

   Carbon monoxide: toxic gas, not 'air pollution'.

   Nitrates in water: deoxygenation, not acid rain.

   Oxides of nitrogen: acid rain, breathing problems.

5. Crude oil processes

   MistakeYou mix up fractional distillation and cracking, for example calling cracking the way petroleum is separated into fractions.

   Fix

   Fractional distillation separates petroleum into fractions by boiling point; no molecules break.

   Cracking (high temperature, catalyst) breaks long alkanes into shorter alkanes and alkenes.

   C₁₇H₃₆ → C₉H₂₀ + 4C₂H₄.

Common mistakes and how to avoid them

1. Displayed formulae

   MistakeYou draw a carbon atom with three or five bonds, or leave out the O-H bond in an alcohol or carboxylic acid.

   Fix

   Count bonds at every atom: C four, O two, H one.

   The COOH carbon: C=O (two), C-O and one other bond make four.

   Draw the O-H as a separate bond.

2. Bond energies

   MistakeYou subtract the wrong way round, giving ΔH the wrong sign, or miss bonds, such as counting a bond once when there are two.

   Fix

   ΔH = bonds broken − bonds made.

   H₂ + F₂ → 2HF: break H-H and F-F, 436 + 158 = 594 kJ.

   Make two H-F: 2 × 568 = 1136 kJ.

   ΔH = 594 − 1136 = −542 kJ/mol, not +542.

3. Ionic equations

   MistakeYou write the full equation, not the ionic one, get the number of an ion wrong, or mix up which salts dissolve, for example calling a nitrate the precipitate.

   Fix

   All nitrates dissolve; most hydroxides do not.

   Zinc nitrate + sodium hydroxide: Na⁺ and NO₃⁻ stay dissolved, so leave them out.

   Zn²⁺(aq) + 2OH⁻(aq) → Zn(OH)₂(s): two OH⁻ balance the 2+ charge.

4. Halogens and halides

   MistakeYou mix up a halogen with its halide ion, such as writing iodine for iodide ions, or you call the Group VII elements halides.

   Fix

   Halogen: a Group VII element, such as bromine, Br₂.

   Halide: its ion, bromide, Br⁻, as in sodium bromide.

   Write 'bromide ions lose electrons to form bromine', not 'bromine loses electrons'.

5. Key definitions

   MistakeYour bond definitions are loose, such as a covalent bond 'between two or more atoms', or an ionic bond as 'strong forces' or 'intermolecular forces'.

   Fix

   Covalent bond: a pair of electrons shared between two atoms, never 'two or more'.

   Ionic bond: strong electrostatic attraction between oppositely charged ions.

   Not 'forces' alone, and not 'intermolecular forces'.

Common mistakes and how to avoid them

1. Recording readings

   MistakeYou round a thermometer or burette reading to a whole number, or record readings in one column to different numbers of decimal places.

   Fix

   Thermometers: nearest 0.5 °C.

   34.0 °C, not 34 °C.

   Burettes: nearest 0.1 or 0.05 cm³.

   17.4 cm³, not 17 cm³.

   Same decimal places down each column.

2. Drops, then excess

   MistakeWhen testing with aqueous sodium hydroxide or ammonia, you write one combined observation instead of what happens with a few drops and then in excess.

   Fix

   Give two results, in order.

   'Few drops: white precipitate'.

   'Excess: precipitate dissolves, giving a colourless solution'.

   If nothing changes in excess, write 'precipitate remains'.

3. Gas tests

   MistakeYou write only the gas name, with no test and result, or record a gas test result that the reactants could not give.

   Fix

   Write the test and what you saw, not just the name.

   'A lighted splint popped', not 'hydrogen'.

   A metal and dilute hydrochloric acid: no carbon, so limewater cannot turn milky.

4. Pipettes and burettes

   MistakeYou mix up a volumetric pipette with a dropping pipette, or lose track of which solution was in the burette.

   Fix

   A volumetric pipette delivers one fixed volume, such as 25.0 cm³, not drops.

   If the burette held solution P, rinsing it with water removes P, not the flask's solution.

5. Drawing graphs

   MistakeYou join the points dot to dot with a ruler, or force a straight line through points that clearly lie on a curve.

   Fix

   Draw one best-fit line, never dot to dot.

   Gas volumes read a minute apart: 0, 20, 32, 38 cm³.

   Rises of 20, 12, 6 shrink, so draw a smooth curve.

Common mistakes and how to avoid them

1. Observations, not conclusions

   MistakeYou write what you think formed, such as 'a gas is given off', instead of what you saw, or record a change when a test is negative.

   Fix

   Write what you see, not what formed.

   'Bubbles', not 'a gas is given off'.

   If the ion tested for is absent, write 'no visible change', not its positive result.

2. Describing precipitates

   MistakeYou write 'cloudy' or 'cloudy white' when two solutions form a solid, instead of 'precipitate' and its colour.

   Fix

   When mixed solutions go cloudy, a solid has formed: write 'precipitate' and its colour.

   'White precipitate', not 'cloudy white' or 'it turned cloudy'.

3. Naming apparatus

   MistakeYou give a vague name such as 'stirrer' or 'tube', or say a volumetric pipette can measure out any volume you need.

   Fix

   Use the full standard name.

   'Filter funnel', not 'cone'.

   'Test-tube holder', not 'clip'.

   A volumetric pipette gives one fixed volume, so measure 16.5 cm³ with a burette.

4. Rate by mass loss

   MistakeYou compare rates using the total mass lost, or gas collected, when the reaction stops, but that total depends on the amounts used, not the rate.

   Fix

   Mass falls as gas escapes.

   Compare mass lost, or gas collected, in the same time.

   'In 2 minutes, flask A lost 0.82 g and B 0.41 g: A reacted faster.'

5. Explaining method steps

   MistakeYou give a stock answer that does not fit this experiment, such as 'stir to dissolve it' or 'do not stir with the thermometer'.

   Fix

   Fit the reason to this experiment.

   'To dissolve' fits only if a solid is present.

   Stirring with the thermometer does not harm accuracy: 'use a glass rod' is no improvement.