This mini-lesson covers the whole of Cambridge IGCSE Chemistry (0620) Topic 12: measurement, acid–base titrations, criteria of purity, paper chromatography & Rf, methods of purification, and the identification of ions and gases.
Extended only: the lilac Supplement pill marks statements assessed only on the Supplement (extended) papers. Everything else is Core and Supplement.
Work through each screen, answer the questions as you go (some are wordy, some are calculations) and collect ⭐ stars. Press Start when you're ready.
Good chemistry starts with the right tool for the quantity you are measuring:
Watch out: a measuring cylinder reads to ~±1 cm³ — fine for "about 25 cm³", but for a titration you need the precision of a pipette (fixed volume) and a burette (variable, read to 0.05 cm³). Always read the bottom of the meniscus at eye level.
A titration finds the exact volume of one solution that reacts with another. The method:
Suitable indicators: the 0620 syllabus names litmus, thymolphthalein and methyl orange. (A universal indicator is not used for titrations — its gradual colour change gives no sharp end-point.)
A pure substance melts and boils at fixed, sharp temperatures. An impurity:
Watch out: melting and boiling points are used to test purity — they are not a way to purify anything. A truly pure substance has a sharp, single melting point; a range means impurity.
Paper chromatography separates a mixture of soluble, coloured substances (e.g. inks, food dyes):
Watch out: the baseline must be pencil (ink would dissolve and run), the solvent must start below the baseline (or the spots wash off), and distances are measured to the centre of each spot.
To compare chromatograms we calculate each spot's Rf value — a ratio that is the same for a given substance and solvent:
A spot moves 4.8 cm from the baseline; the solvent front moves 6.0 cm.
Rf = 4.8 ÷ 6.0 = 0.80 (no units)
Watch out: Rf has no units and is always less than 1 (the spot can never travel further than the solvent). Matching Rf values against known substances identifies an unknown.
Many substances — such as amino acids and sugars — are colourless, so their spots are invisible. A locating agent is sprayed on to make them show up.
Two-way chromatography separates substances that have similar Rf values in one solvent:
You must match the method to the mixture:
| Method | Separates… |
|---|---|
| Filtration | an insoluble solid from a liquid (e.g. sand from water) |
| Crystallisation | a dissolved solid from its solution (e.g. salt from salt solution), by evaporating to a hot saturated solution, then cooling so crystals form |
| Simple distillation | a solvent from a solution (e.g. pure water from salt water) |
| Fractional distillation | two or more miscible liquids with different boiling points (e.g. ethanol from water) |
Watch out: to get the solid back from a solution you crystallise (don't boil dry — that decomposes hydrated crystals). To get the solvent back you distil. Use fractional distillation only when the liquids both evaporate (similar, but different, boiling points).
Tap a mixture on the left, then tap the method on the right that separates it.
Hold a sample on a clean wire in a hot, blue Bunsen flame. The metal cation gives a characteristic flame colour:
Watch out: sodium's strong yellow easily masks other colours, so the wire must be clean (dip in acid first). Flame colour identifies the metal ion only.
Add a few drops, then excess, of aqueous sodium hydroxide or aqueous ammonia. Note the colour of the precipitate and whether it dissolves in excess:
| Cation | with NaOH(aq) | with NH₃(aq) |
|---|---|---|
| NH₄⁺ | no ppt; on warming gives ammonia gas (red litmus → blue) | — |
| Cu²⁺ | light blue ppt, insoluble in excess | light blue ppt, dissolves in excess to give a dark blue solution Supplement |
| Fe²⁺ | green ppt, insoluble in excess | green ppt, insoluble in excess |
| Fe³⁺ | red-brown ppt, insoluble in excess | red-brown ppt, insoluble in excess |
| Ca²⁺ | white ppt, insoluble in excess | no ppt (or very slight) |
| Zn²⁺ Supplement | white ppt, dissolves in excess (colourless) | white ppt, dissolves in excess (colourless) |
| Al³⁺ Supplement | white ppt, dissolves in excess (colourless) | white ppt, insoluble in excess |
Watch out: Zn²⁺ and Al³⁺ both give a white ppt that dissolves in excess NaOH — tell them apart with ammonia: Al³⁺'s ppt stays, Zn²⁺'s redissolves. Supplement The Cu²⁺ ppt redissolving in excess ammonia to a deep blue is a Supplement detail.
| Anion | Test | Positive result |
|---|---|---|
| Carbonate CO₃²⁻ | add dilute acid | fizzes; CO₂ given off (turns limewater milky) |
| Chloride Cl⁻ | acidify with dilute nitric acid, add silver nitrate | white precipitate (AgCl) |
| Sulfate SO₄²⁻ | acidify with dilute nitric acid, add barium nitrate | white precipitate (BaSO₄) |
| Nitrate NO₃⁻ Supplement | add aqueous NaOH then aluminium foil, warm | ammonia gas given off (red litmus → blue) |
| Sulfite SO₃²⁻ Supplement | add dilute acid and warm (or acidified potassium manganate(VII)) | SO₂ given off; turns acidified KMnO₄ from purple to colourless |
Watch out: always acidify first with dilute nitric acid before the chloride/sulfate tests — this removes carbonate ions, which would also give a precipitate and mislead you. Supplement The nitrate and sulfite tests are Supplement (extended) only.
| Gas | Test & positive result |
|---|---|
| Ammonia NH₃ | turns damp red litmus paper blue |
| Carbon dioxide CO₂ | turns limewater milky (cloudy white) |
| Chlorine Cl₂ | bleaches damp litmus paper (turns it white) |
| Hydrogen H₂ | lighted splint → "squeaky pop" |
| Oxygen O₂ | relights a glowing splint |
| Sulfur dioxide SO₂ Supplement | turns acidified aqueous potassium manganate(VII) from purple to colourless |
Watch out: a lighted splint (pop) tests for hydrogen; a glowing splint (relights) tests for oxygen — don't mix them up. Supplement The sulfur dioxide test is Supplement (extended) only.
Tap a gas on the left, then tap its correct test result on the right.
Measurement: pipette/burette = accurate volume; gas syringe = gas volume; balance = mass.
Titration: pipette + burette + a single indicator (litmus, thymolphthalein, methyl orange) → sharp end-point.
Purity: a pure substance has a sharp m.p./b.p.; impurity lowers & broadens m.p., raises b.p.
Chromatography: pencil baseline, measure to spot centre; Rf = substance ÷ solvent (no units, < 1).
Supplement: locating agents, two-way chromatograms.
Purification: filtration · crystallisation · simple & fractional distillation — match to the mixture.
Identifying ions: flame colours; NaOH/NH₃ precipitates (cations); acid + AgNO₃/Ba(NO₃)₂ (anions).
Identifying gases: NH₃, CO₂, Cl₂, H₂, O₂ (+ SO₂ Supplement).
You've covered all of Cambridge IGCSE Chemistry (0620) Topic 12. Press Finish to see your score.
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