Experimental Techniques and Chemical Analysis: IGCSE Chemistry 0620
Experimental Techniques and Chemical Analysis for IGCSE 0620: apparatus, separation methods, chromatography and Rf values, and purity checks.
The IGCSE Chemistry Specialist Team · founded by Rig
Written to the Cambridge IGCSE Chemistry (0620) syllabus and mark-scheme conventions. Last updated 2026-06-11.
This topic is examined twice: directly on Paper 6 (the Alternative to Practical, worth 20% of the whole qualification) and indirectly every time another topic asks how you would separate, purify or identify something. Examiner reports for Paper 6 make uncomfortable reading: candidates who know the chemistry still lose marks for choosing a measuring cylinder where a burette is needed, drawing chromatography baselines in ink, or writing “heat it” where the method demands evaporation to the point of crystallisation. Technique is content here, and it is markable content.
Measurement and apparatus
Match the quantity to the apparatus, and the apparatus to the precision required:
| Quantity | Apparatus | Notes |
|---|---|---|
| Time | Stopwatch | Read in seconds |
| Temperature | Thermometer | Read to 0.5°C, at eye level |
| Mass | Electronic balance | Typically to 0.01 g |
| Fixed volume | Volumetric pipette | One precise volume, e.g. 25.0 cm³ |
| Variable volume, precise | Burette | Read to 0.05 cm³, at eye level |
| Variable volume, approximate | Measuring cylinder | When precision is not critical |
Gases are collected over water (for gases not very soluble in water, like hydrogen and oxygen), in a gas syringe (any gas, measured volume), or by downward delivery for dense gases such as carbon dioxide. Method-choice questions are marked on the match between the gas’s properties and the method. Say why: “collected over water because hydrogen is insoluble in water.”
Purification and separation methods
Each separation works because of one property difference. Lead with that property and the method mark follows:
- Filtration separates an insoluble solid from a liquid: the residue stays in the paper, the filtrate passes through. Use those two words; they are marking points.
- Crystallisation recovers a dissolved solid: heat the solution to the point of crystallisation (test by dipping a glass rod: crystals form on it), then cool, filter off the crystals and dry between filter paper. Evaporating to dryness wrecks hydrated crystals, exactly as in salt preparation.
- Simple distillation separates the solvent from a solution: the solution boils, vapour rises into the condenser, cools and condenses, and pure solvent is collected. Water flows into the condenser at the bottom and out at the top.
- Fractional distillation separates miscible liquids with different boiling points using a fractionating column. The liquid with the lower boiling point (ethanol, 78°C) reaches the top of the column and distils first; the thermometer reading holds at 78°C while ethanol comes over.
The “which method and why” question is the staple. The why is always a property: solubility, boiling point difference, particle size. Why pure substances boil sharply while mixtures boil over a range comes straight from states of matter.
Chromatography and Rf values
Paper chromatography separates substances by how strongly they are attracted to the paper versus the solvent. Method details that carry marks: draw the baseline in pencil (ink would dissolve and run), spot the samples on the line, keep the solvent level below the baseline, and remove the paper before the solvent front reaches the top, marking the front immediately.
Interpretation: substances that travel the same distance as a known reference are the same substance; a pure substance gives one spot, a mixture gives more than one.
(S) The Rf value makes the comparison quantitative:
Rf = distance moved by substance ÷ distance moved by solvent
Measure both distances from the baseline; Rf has no units and is always less than 1 (the spot cannot outrun the solvent). A calculated Rf above 1 means the two measurements were swapped. Check before writing it down.
Colourless substances (amino acids, sugars) need a locating agent, a spray that reacts with the spots to make them visible. The definition is a one-mark regular on Paper 6.
Tests for ions, gases and water
The identification toolkit. Gases first: hydrogen pops with a lighted splint; oxygen relights a glowing splint; carbon dioxide turns limewater milky; chlorine bleaches damp litmus paper; ammonia turns damp red litmus paper blue. Cations are identified with aqueous sodium hydroxide and aqueous ammonia: copper(II) gives a light blue precipitate, iron(II) green, iron(III) red-brown, and ammonium salts release ammonia on warming with sodium hydroxide. Anions: carbonates fizz with dilute acid (CO2 turns limewater milky); sulfates give a white precipitate with acidified barium nitrate; halides give precipitates with acidified silver nitrate: white for chloride, cream for bromide, yellow for iodide. Water turns anhydrous copper(II) sulfate from white to blue. Every test answer needs reagent, condition and observation, all three. The full test-by-test drill lives in our qualitative analysis guide.
Identifying unknown substances
Paper 6 ties everything together with an unknown: you run the tests in sequence and deduce the substance. The logic is eliminative: a white solid that fizzes with acid and gives a lilac flame colour is potassium carbonate. Purity checks close the loop: a pure substance melts sharply at a fixed temperature and boils at a fixed temperature; an impure substance melts over a range, melting lower and boiling higher than the pure value. Quote the data in your answer: “the sample melted between 118°C and 124°C, so it is impure.”
Worked exam question
A student uses paper chromatography to analyse a food colouring. The solvent front moves 8.0 cm from the baseline. One spot in the colouring moves 6.0 cm. (a) Calculate the Rf value of this spot. (b) State two precautions the student should take when setting up the chromatogram. [4]
Model answer: (a) Rf = distance moved by substance ÷ distance moved by solvent = 6.0 ÷ 8.0 (1) = 0.75 (1). (b) Draw the baseline in pencil, because ink would dissolve in the solvent and run (1). Keep the solvent level below the baseline, otherwise the samples dissolve into the solvent instead of moving up the paper (1).
Mark-by-mark: the first mark in (a) is the working: the correct ratio written down, substance over solvent. The second is the value, 0.75, with no units (writing “0.75 cm” loses it). In (b), each precaution mark needs the precaution and its reason; “use pencil” alone is half the sentence. Other accepted precautions: use a small/concentrated spot, mark the solvent front as soon as the paper is removed.
The mistakes that cost marks
- Inverting the Rf ratio. Solvent distance over spot distance gives a number above 1, which is impossible. Substance on top, solvent on the bottom, and show the working line.
- Ink baselines. Ink is a mixture of dyes; it dissolves and runs up the paper with the samples. Pencil, with the reason stated.
- Wrong apparatus for the precision. Measuring 25.0 cm³ of alkali for a titration with a measuring cylinder loses the apparatus mark. That volume needs a pipette.
- Observations replaced by conclusions. Writing “CO2 is produced” when the question asks what you see. You see effervescence/bubbles, and limewater turning milky. Observation first, conclusion second.
- “Evaporate to dryness” for crystallisation. Heat to the point of crystallisation, then cool and crystallise. Overheating decomposes hydrated salts and the mark scheme penalises it explicitly.
How to phrase it for full marks
| Student wording | Mark-scheme wording |
|---|---|
| ”Filter out the solid" | "Filter; the (named) insoluble solid is the residue, the solution passes through as the filtrate" |
| "Use a line at the bottom of the paper" | "Draw the baseline in pencil so it does not dissolve in the solvent" |
| "The spot moved quite far" | "Rf = 6.0 ÷ 8.0 = 0.75" |
| "It’s pure because it melted" | "It melts sharply at a fixed temperature, so it is pure; an impure sample melts over a range" |
| "Boil off the water" | "Heat to the point of crystallisation, then leave to cool so crystals form” |
The pattern: name the apparatus, name the property the method exploits, and give observation before conclusion. Most of these marks sit on Paper 6, and our Paper 6 Alternative to Practical guide walks through the paper question type by question type.
The Malaysia note
Paper 6 is where Malaysian students lose grades they did not know they were losing. International schools in KL, Penang and JB run good labs, but lab time gets squeezed in Year 11, and the Alternative to Practical rewards written technique that students never rehearse: apparatus choice, precaution-plus-reason sentences, Rf working. The 20% weighting means a shaky Paper 6 can drag an A to a B on its own. We treat this topic as an exam paper, not a lab manual: past-paper drills, model diagrams, phrasing practice. Book a free trial lesson and ask the tutor to run a Paper 6 diagnostic. Twenty minutes shows exactly how much of that 20% is currently at risk.
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Frequently asked questions
How do I calculate an Rf value?
Rf = distance moved by the substance ÷ distance moved by the solvent. Measure both from the baseline (the pencil start line): the spot distance to the centre of the spot, the solvent distance to the solvent front. Rf is always between 0 and 1 and has no units.
When do I use fractional distillation instead of simple distillation?
Simple distillation separates a solvent from a solution: water from salt water. Fractional distillation separates two miscible liquids with different boiling points (ethanol from water) using a fractionating column. If the question has two liquids mixed together, the answer is fractional.
How do I choose between a burette, pipette and measuring cylinder?
A pipette delivers one fixed volume precisely (25.0 cm³). A burette delivers a variable volume precisely, read to 0.05 cm³, used for titrations. A measuring cylinder is for volumes where precision matters less. Choosing the wrong one is a standard Paper 6 mark.
What is a locating agent in chromatography?
A substance sprayed onto the chromatogram to make colourless spots visible, needed for substances such as amino acids and sugars that you cannot see. One sentence, one mark, and it appears regularly on Paper 6.
How does melting point show whether a substance is pure?
A pure substance melts sharply at a fixed temperature. An impure substance melts over a range of temperatures, and impurities lower the melting point and raise the boiling point. Quote both halves: sharp for pure, range for impure.