Cambridge IGCSE Chemistry 12 Experimental Techniques and Chemical Analysis Exam Questions

About Cambridge IGCSE Chemistry 12 Experimental Techniques and Chemical Analysis

Experimental techniques and chemical analysis in Cambridge IGCSE Chemistry 0620 is the practical heart of the course. You need to name and use apparatus, carry out and describe acid-base titrations, run and interpret paper chromatography, choose separation and purification methods, and identify ions and gases with chemical tests. Extended candidates also use locating agents and Rf values in chromatography. This topic is examined heavily in Paper 5 (Practical Test) and Paper 6 (Alternative to Practical) as well as in the theory papers, and it links to acids and salts, stoichiometry and the Periodic Table. Examiners expect precise observations, such as colour of precipitate and whether it dissolves in excess.

What you need to know

• 12.1 Experimental design Name and use stop-watches, thermometers, balances, burettes, volumetric pipettes, measuring cylinders and gas syringes. Know the terms solvent, solute, solution, saturated solution, residue and filtrate, and suggest advantages and disadvantages of methods and apparatus.
• 12.2 Acid-base titrations Use a volumetric pipette to measure the alkali into a conical flask, add a suitable indicator, then add acid from a burette until the colour changes. Repeat until concordant results are obtained and use the average.
• 12.3 Chromatography Draw the baseline in pencil and keep it above the solvent level. A pure substance gives one spot; a mixture gives several. Rf = distance moved by substance / distance moved by solvent front. Extended candidates use locating agents for colourless substances.
• 12.4 Separation and purification Filtration separates an insoluble solid from a liquid; crystallisation obtains a soluble solid from solution; simple distillation separates a solvent from a solution; fractional distillation separates miscible liquids with different boiling points. Pure substances have sharp melting and boiling points.
• 12.5 Tests for cations With aqueous sodium hydroxide: Cu2+ light blue precipitate, Fe2+ green, Fe3+ red-brown, Zn2+ and Al3+ white and soluble in excess, Ca2+ white and insoluble in excess, Cr3+ green and soluble in excess. NH4+ gives ammonia on warming. Use aqueous ammonia to tell Al3+ (insoluble in excess) from Zn2+ (soluble in excess).
• 12.5 Flame tests Lithium red, sodium yellow, potassium lilac, calcium orange-red, barium light green, copper(II) blue-green.
• 12.5 Tests for anions Carbonate: add dilute acid, effervescence, gas turns limewater milky. Halides: acidify with dilute nitric acid, add aqueous silver nitrate: chloride white, bromide cream, iodide yellow precipitate. Sulfate: acidify, add aqueous barium nitrate, white precipitate. Nitrate: warm with aqueous sodium hydroxide and aluminium foil, ammonia given off. Sulfite: decolourises acidified aqueous potassium manganate(VII).
• 12.5 Tests for gases Ammonia turns damp red litmus blue; carbon dioxide turns limewater milky; chlorine bleaches damp litmus; hydrogen gives a squeaky pop with a lighted splint; oxygen relights a glowing splint; sulfur dioxide turns acidified potassium manganate(VII) from purple to colourless.

Common mistakes in 12 Experimental Techniques and Chemical Analysis

• Drawing the chromatography baseline in ink or below the solvent level. Use pencil, which does not dissolve, and keep the spots above the solvent so they do not wash into it.
• Writing 'cloudy' or 'goes white' without naming what is seen. Say 'white precipitate' and, for cation tests, state whether it dissolves in excess.
• Adding hydrochloric acid before testing for chloride or sulfuric acid before testing for sulfate. These add the ion you are testing for; use dilute nitric acid.
• Saying the glowing splint test is for hydrogen. A glowing splint relights in oxygen; a lighted splint gives a squeaky pop with hydrogen.
• Using a measuring cylinder when accuracy is needed in a titration. Use a volumetric pipette for the fixed volume and a burette for the titre.

Try three 12 Experimental Techniques and Chemical Analysis exam-style questions