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.
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Make a guess: how can paper pull an ink apart?
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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: use thymolphthalein or methyl orange. (Litmus and universal indicator are not used for titrations — their 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… |
|---|---|
| Dissolving in a suitable solvent | a soluble solid from an insoluble one (e.g. salt from sand: add water, then filter) |
| 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) |
A solvent dissolves a solute to make a solution; a saturated solution holds the maximum concentration of solute at a stated temperature.
Watch out: to get the solid back from a solution you crystallise (don't boil dry — that drives off the water of crystallisation). 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.
Make a guess: how can a few tests name an unknown powder?
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 |
| Cr³⁺ | green ppt, dissolves in excess | green ppt, insoluble in excess |
| Fe²⁺ | green ppt, insoluble in excess (turns brown on standing) | 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⁺ | white ppt, dissolves in excess (colourless) | white ppt, dissolves in excess (colourless) |
| Al³⁺ | 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.
| 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) |
| Bromide Br⁻ / iodide I⁻ | acidify with dilute nitric acid, add silver nitrate | cream ppt (AgBr) / yellow ppt (AgI) |
| Sulfate SO₄²⁻ | acidify with dilute nitric acid, add barium nitrate | white precipitate (BaSO₄) |
| Nitrate NO₃⁻ | add aqueous NaOH then aluminium foil, warm | ammonia gas given off (red litmus → blue) |
| Sulfite SO₃²⁻ | add a little acidified aqueous potassium manganate(VII) | turns 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.
| 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 | 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.
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 (thymolphthalein or 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; compare spots with known substances; a pure substance gives one spot.
Supplement: Rf = substance ÷ solvent, measured to the spot centre (no units, < 1); locating agents for colourless spots; two-way chromatograms.
Purification: suitable solvent · filtration · crystallisation · simple & fractional distillation — match to the mixture.
Identifying ions: flame colours; NaOH/NH₃ precipitates (cations); acid + AgNO₃/Ba(NO₃)₂, Al + NaOH, acidified KMnO₄ (anions).
Identifying gases: NH₃, CO₂, Cl₂, H₂, O₂, SO₂.
You've covered the key ideas in Cambridge IGCSE Chemistry (0620) Topic 12. Press Finish to see your score.
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