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

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

   Below H: no reaction with dilute hydrochloric acid.

   Above C: extracted by electrolysis.

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.

   Fix

   Acid + metal above hydrogen → salt + hydrogen.

   Acid + metal oxide or hydroxide → salt + water.

   Acid + carbonate → salt + water + carbon dioxide.

   Of these, only carbonates give carbon dioxide.

3. Precipitate colours

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

   Fix

   With aqueous sodium hydroxide: copper(II) gives a light blue precipitate, iron(III) red-brown, iron(II) green, chromium(III) green that dissolves 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, because you know ionic compounds contain charged particles.

   Fix

   Ions carry a current only when they can move.

   Solid ionic compounds do not conduct; molten or aqueous ones do.

   Metals and graphite do conduct when solid.

5. Exothermic and endothermic

   MistakeYou mix up the two words, so you pick an option calling melting, or a reaction that makes the test tube feel cold, exothermic.

   Fix

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

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

   Melting and boiling take in energy.

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.

   Sodium, 2,8,1, loses one electron, so Na⁺ is 2,8.

7. Graphite and diamond

   MistakeYou accept an option linking graphite to the wrong property, such as being an electrode because its layers slide, or one saying diamond conducts.

   Fix

   Graphite conducts because it has free electrons; weak forces between its layers let them slide, so it is a lubricant.

   Diamond has no free electrons, so it does not conduct.

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.

   Water turns anhydrous copper(II) sulfate from white to blue, and anhydrous cobalt(II) chloride from blue to 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 N₂ + 3H₂ ⇌ 2NH₃, heating raises rate but lowers yield; high pressure (4 moles to 2) raises yield.

2. Activation energy

   MistakeYou think a higher temperature or concentration lowers the activation energy, or read Ea on a reaction pathway diagram as the overall energy change.

   Fix

   Only a catalyst lowers Ea.

   Hotter or more concentrated reactants collide successfully more often; Ea is unchanged.

   Reactants 100, peak 250, products 40 kJ: Ea = 150 kJ, ΔH = −60 kJ.

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.

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

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

   Polyamides and polyesters are condensation polymers.

4. Electrolysis of solutions

   MistakeYou choose an option with sodium, or another reactive metal, forming at the cathode when its aqueous solution is electrolysed.

   Fix

   In solution, hydrogen forms at the cathode unless the metal is less reactive than hydrogen, such as copper.

   Concentrated chloride solutions give chlorine at the anode; dilute ones give oxygen.

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

   Change each mass to moles, then use the balanced equation's ratio to see which reactant runs out first.

   Work out the product from that limiting reactant only.

6. Strong and weak acids

   MistakeYou read 'strong' as meaning concentrated and 'weak' as meaning dilute, so you accept a wrong description of an acid.

   Fix

   A strong acid, like hydrochloric acid, fully dissociates into ions; a weak acid, like ethanoic acid, only partly does.

   Concentration means how much acid is in each dm³.

7. Oxidising and reducing agents

   MistakeYou pick the substance that is reduced as the reducing agent. The agent is the substance that causes the change in the other one.

   Fix

   An oxidising agent gains electrons, so it is itself reduced.

   A reducing agent loses electrons and is oxidised.

   Acidified potassium manganate(VII), purple to colourless, is an oxidising agent.

8. Addition and substitution

   MistakeYou choose a product where bromine swaps places with hydrogen on an alkene, or where chlorine simply adds on to an alkane.

   Fix

   Alkenes add: both bromine atoms join across the C=C bond, giving one product.

   Alkanes substitute in UV light: a chlorine atom replaces a hydrogen atom, also forming hydrogen chloride.

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

Common mistakes and how to avoid them

1. Particle model

   MistakeYou describe the particles in a solid, liquid or gas by only one feature, usually motion, or mix up how they are arranged with how they move.

   Fix

   Give three things every time: separation (close together or far apart), arrangement (regular or random) and motion (vibrate in place, slide past each other, or move fast and randomly).

2. Tests for ions

   MistakeYou give the test for a different ion from the one asked about, or name the reagent but leave out what you would see.

   Fix

   Learn each test as reagent plus result.

   For halides, add dilute nitric acid then aqueous silver nitrate: white precipitate means chloride, cream means bromide, yellow means iodide.

3. Making dry crystals

   MistakeTo get crystals of a salt from its solution, you heat it until all the water has gone, or you leave out filtering and drying.

   Fix

   Heat gently until the solution is saturated, then leave it to cool so crystals form.

   Filter off the crystals and pat them dry between filter papers.

4. Pollutants and effects

   MistakeYou call a pollutant 'harmful' or 'bad for the environment' without naming the pollutant and its exact effect.

   Fix

   Match each one: sulfur dioxide causes acid rain; carbon monoxide is toxic; oxides of nitrogen cause acid rain and breathing problems; nitrates and phosphates lead to deoxygenation of water.

5. Crude oil processes

   MistakeYou mix up fractional distillation and cracking, or name cracking or filtration as the way petroleum is separated into fractions.

   Fix

   Fractional distillation separates petroleum into fractions by boiling point; nothing is broken down.

   Cracking breaks long-chain alkanes into shorter alkanes and alkenes, using a high temperature and a catalyst.

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

   Show every atom and every bond.

   Check each carbon has four bonds, each oxygen two and each hydrogen one.

   In -COOH, draw C=O and a separate C-O-H.

2. Bond energies

   MistakeYou subtract the wrong way round, count a bond once when the equation has several of it, or give the wrong sign for ΔH.

   Fix

   ΔH = energy to break all bonds in the reactants minus energy released making all bonds in the products.

   Count every bond in every molecule.

   Negative ΔH means exothermic.

3. Ionic equations

   MistakeYou write the full equation instead of the ionic one, get the number of each ion wrong, or choose a nitrate as the precipitate.

   Fix

   Find the insoluble product using the solubility rules (all nitrates dissolve).

   Write only its ions as reactants, balanced for charge: Cu²⁺(aq) + 2OH⁻(aq) → Cu(OH)₂(s).

4. Halogens and halides

   MistakeYou write 'more reactive' without saying what it is compared with, or mix up a halogen such as chlorine with a halide ion such as chloride.

   Fix

   Name both: 'chlorine is more reactive than bromine, so it displaces bromine from aqueous bromide'.

   Halogens are elements (Cl₂); halides are ions (Cl⁻) in compounds.

5. Key definitions

   MistakeYour definitions are loose, such as a covalent bond as 'atoms joined together' or a hydrated salt as 'a salt mixed with water'.

   Fix

   Covalent bond: a pair of electrons shared between two atoms.

   Ionic bond: electrostatic attraction between oppositely charged ions.

   Hydrated: chemically combined with water, as water of crystallisation.

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

   Record every reading as precisely as the scale allows: thermometers usually to 0.5 °C, burettes to 0.05 cm³.

   Keep the same number of 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

   Record two results: first with a few drops, then with excess.

   For example, zinc ions give a white precipitate that dissolves in excess, giving a colourless solution.

3. Gas tests

   MistakeYou name the gas you expected instead of doing the test and writing what happened, or record a result that cannot match that gas.

   Fix

   Write the test and what you saw: limewater turns milky (carbon dioxide), lighted splint pops (hydrogen), glowing splint relights (oxygen), damp red litmus turns blue (ammonia).

4. Pipettes and burettes

   MistakeYou describe a volumetric pipette as something that just 'measures volume', or lose track of which solution was in the burette.

   Fix

   A volumetric pipette delivers one fixed volume accurately.

   The burette adds a variable volume; note which solution is in it.

   Titre = final reading minus initial reading.

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

   Plot each point as a small, neat cross.

   Then draw one best-fit line, straight or a smooth curve, that follows the trend.

   It need not pass through every point.

Common mistakes and how to avoid them

1. Observations, not conclusions

   MistakeYou write what you think formed, such as 'ammonia given off', instead of what you saw, or leave a gap when nothing happened.

   Fix

   Describe only what you see or measure, like 'bubbles' or 'no visible change'.

   A negative result, such as 'no bubbles with acid', is a valid observation.

2. Describing precipitates

   MistakeYou call the result of mixing two solutions 'cloudy', or give a colour without saying that a solid has formed.

   Fix

   When two solutions make a solid, write 'precipitate' and its colour, such as 'white precipitate'.

   'Cloudy' alone does not say that a solid formed.

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 standard name, such as glass rod, conical flask, measuring cylinder or gas syringe.

   A measuring cylinder is quicker but less accurate than a volumetric pipette or burette.

4. Rate by mass loss

   MistakeWhen a reaction in an open flask loses mass, you say a reactant has been used up, rather than that a gas has escaped.

   Fix

   Mass falls because a gas, such as carbon dioxide from a carbonate and acid, leaves the flask.

   Plot mass lost against time: a steeper line means a faster rate.

5. Explaining method steps

   MistakeYou give a stock reason for a step, such as 'stir to dissolve it', when the reason does not fit this experiment.

   Fix

   Say what the step does here.

   Stirring mixes the reactants and spreads heat evenly, so one thermometer reading represents the whole mixture.

   Dissolving matters only if a solid is present.