Topic 12
3 subtopics · Cambridge IGCSE Chemistry 0620
Key Idea
Different separation techniques are used depending on the nature of the mixture: filtration, crystallisation, simple distillation, fractional distillation, and chromatography.
Explanation
Separation techniques: 1. FILTRATION: separates an insoluble solid from a liquid. Residue = solid left in filter paper. Filtrate = liquid that passes through. 2. CRYSTALLISATION: obtains a soluble solid from its solution. Evaporate to concentrate, cool, crystals form, filter, dry. 3. SIMPLE DISTILLATION: separates a solvent from a solution. Heat, solvent evaporates, condenses in condenser, collected as distillate. 4. FRACTIONAL DISTILLATION: separates a mixture of liquids with different boiling points. Uses a fractionating column. Liquid with lowest boiling point distils first. 5. CHROMATOGRAPHY: separates mixtures of soluble substances. Rf = distance travelled by substance / distance travelled by solvent front. Rf is always between 0 and 1. A pure substance gives one spot. Impure gives multiple spots.
Worked Examples
Practice Questions
Which separation technique would be most suitable for obtaining pure water from sea water?
Key Idea
Specific chemical tests identify anions and cations in solution. Flame tests and gas tests are also used.
Explanation
ANION TESTS: - Carbonate CO3(2-): add dilute acid, CO2 gas produced, turns limewater milky. - Chloride Cl-: acidify with dilute HNO3, add AgNO3(aq), white precipitate of AgCl. - Bromide Br-: acidify with dilute HNO3, add AgNO3(aq), cream precipitate of AgBr. - Iodide I-: acidify with dilute HNO3, add AgNO3(aq), yellow precipitate of AgI. - Sulfate SO4(2-): acidify with dilute HNO3, add Ba(NO3)2(aq), white precipitate of BaSO4. CATION TESTS (with NaOH): - Al3+: white ppt, dissolves in excess NaOH. - NH4+: warm gives NH3 gas (turns damp red litmus blue). - Ca2+: white ppt, insoluble in excess. - Cu2+: blue ppt, insoluble in excess. - Fe2+: green ppt, insoluble in excess. - Fe3+: red-brown ppt, insoluble in excess. - Zn2+: white ppt, dissolves in excess NaOH. FLAME TESTS: - Li+: crimson red. Na+: yellow/orange. K+: lilac/violet. Ca2+: brick red. Ba2+: apple green. Cu2+: blue-green. GAS TESTS: - H2: lighted splint gives squeaky pop. - O2: glowing splint relights. - CO2: limewater turns milky. - Cl2: damp litmus paper bleached white. - NH3: damp red litmus turns blue. - SO2: acidified KMnO4 decolourised.
Practice Questions
A student performs a flame test on an unknown ionic compound and observes a yellow-orange flame. Which cation is present?
Describe how you would test a solution to determine whether it contains sulfate ions (SO4 2-). Include the reagents used and the expected observation for a positive result. [3 marks]
Key Idea
Titration measures the exact volume of one solution needed to react completely with a known volume of another. Concordant results (within 0.1 cm3) are required.
Explanation
Titration procedure: 1. Use volumetric pipette to measure exact volume of alkali into conical flask. 2. Add a few drops of indicator. 3. Fill burette with acid. Record initial reading. 4. Add acid slowly from burette, swirling flask, until indicator just changes colour (end-point). 5. Record final burette reading. Calculate volume of acid used (titre). 6. Repeat to get concordant results (within 0.1 cm3). 7. For pure salt: repeat without indicator, then evaporate to crystallise. End-point: the point at which the indicator permanently changes colour. Indicator choices: - Phenolphthalein: colourless in acid, pink in alkali. - Methyl orange: red in acid, yellow in alkali.
Practice Questions
Describe the procedure for carrying out an acid-base titration to find the volume of hydrochloric acid needed to neutralise 25.0 cm3 of sodium hydroxide solution. [5 marks]