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Detailed notes on Experimental Techniques for Cambridge IGCSE Coordinated Science, covering key concepts, explanations, examples, and exam-focused revision points.
A pure substance has a sharp, precise melting point and boiling point. Impurities lower the melting point and raise the boiling point. Cambridge tests how melting and boiling point data are used to determine purity.
Mapped to the Cambridge IGCSE 0654 syllabus (2025-2027).
A pure substance has a fixed, sharp melting point. Any deviation from the known value — or a range rather than a point — indicates impurity.
Pure substance: melts at exactly one temperature (the melting point) and boils at exactly one temperature (the boiling point). No range — sharp transition.
Impure substance:
Why impurities affect melting/boiling points:
Practical use:
Example: Pure aspirin melts at 135°C. If a sample melts at 130–134°C, it is impure.
Chromatography separates mixtures based on how strongly components interact with a stationary and mobile phase. Rf values identify components.
Chromatography separates mixtures based on the different affinities of components for:
Paper chromatography — procedure:
Rf value (retardation factor):
Rf = distance moved by spot / distance moved by solvent front
Applications:
Verbatim phrases and definitions Cambridge mark schemes credit.
Paper 6: Chromatogram diagrams — 'Calculate the Rf value of spot X' (must show: distance spot / distance solvent front, with values substituted). 'A student found the melting point of a sample to be 118–124°C. The known melting point is 132°C. What does this suggest?' (2 marks — sample is impure; impurities have lowered the melting point and widened the range). Rf calculations are nearly guaranteed in Paper 6.
Sources: Cambridge IGCSE Coordinated Sciences 0654 syllabus 2025-2027 (C2); 0654 Examiner Reports 2022-2024. Last reviewed 2026-05-14.
Step-by-step solutions to past-paper-style questions on criteria of purity, written exactly the way a tutor would explain them at the board.
Question
A substance melts sharply at 78°C. A second sample of the same substance melts over the range 72°C to 80°C. Suggest which sample is purer and explain your reasoning.
Step-by-step solution
Step 1
A pure substance melts at a single, sharp temperature because all particles have the same intermolecular forces.
Step 2
Impurities disrupt the regular structure, requiring less energy to separate particles in some regions — lowering the onset of melting — and creating a broader range.
Step 3
The first sample (sharp melting at 78 °C) is purer; the second (range 72–80 °C) contains impurities.
Answer
The first sample is purer; it has a sharp melting point. The second sample contains impurities, which depress and broaden the melting range.
Examiner tip
Cambridge mark schemes award marks for both 'sharp melting point = pure' and 'broad range = impurities'. Include both points.
Question
In a paper chromatography experiment, a spot moves 4.5cm from the baseline and the solvent front moves 9.0cm. Calculate the Rf value.
Step-by-step solution
Step 1
Apply the Rf formula.
Rf=distance moved by solvent frontdistance moved by substance
Step 2
Substitute values.
Rf=9.04.5=0.50
Answer
Rf=0.50 (no units)
Examiner tip
Rf values have no units and must always be between 0 and 1. State 'no units' or leave blank — do not write cm.
Question
A student runs a chromatogram of an ink and observes three spots. What does this show about the composition of the ink?
Step-by-step solution
Step 1
Each spot represents a different substance that has separated because it has a different Rf value (different solubility in the solvent and different attraction to the paper).
Step 2
Three spots indicate the ink is a mixture of at least three different coloured dyes/substances.
Answer
The ink is a mixture of at least three different substances/dyes, each with a different Rf value.
Question
A chromatogram is run alongside known dye standards. An unknown spot has an Rf=0.62. Standard dye A has Rf=0.41 and standard dye B has Rf=0.62. Identify the unknown substance and explain your reasoning.
Step-by-step solution
Step 1
Compare the Rf of the unknown spot with the standards run under identical conditions.
Step 2
The unknown Rf (0.62) matches standard dye B (Rf = 0.62). Substances with the same Rf value in the same solvent are identical.
Answer
The unknown substance is dye B; both have Rf=0.62 in the same solvent system.
Examiner tip
Examiners expect the phrase 'same Rf value under the same conditions' for identification by chromatography.
The formulae you need to memorise for criteria of purity on the Cambridge IGCSE 0654 paper, with every variable defined in plain English and a note on when to use it.
Rf=distance moved by solvent frontdistance moved by substance
When to use
Identifying substances by paper or thin-layer chromatography.
Example
Spot travels 3.6 cm; solvent front travels 9.0 cm → Rf=3.6/9.0=0.40
Definitions to memorise and the exact keywords mark schemes credit for criteria of purity answers — sharpened from recent examiner reports for the 2026 0654 sitting.
A substance consisting of only one type of element or compound; it has a fixed, sharp melting and boiling point.
The ratio of the distance moved by a substance to the distance moved by the solvent front in chromatography; a constant for a given substance in a given solvent at a fixed temperature.
A technique for separating and identifying components of a mixture based on their different solubilities in a solvent (mobile phase) and attractions to paper (stationary phase).
The temperature at which a solid changes to a liquid at standard pressure; sharp and fixed for a pure substance, depressed and broadened by impurities.
Impurities lower the melting point below the expected value and cause melting to occur over a range of temperatures rather than at a sharp point.
The traps other students keep falling into on criteria of purity questions — taken from recent Cambridge IGCSE 0654 examiner reports and mark schemes — and how to avoid them.
0654 Examiner Report 2023
Why it happens
Students do not recognise that Rf is a ratio of two distances and is therefore dimensionless.
How to avoid it
Rf values have no units. Write the number alone (e.g. 0.50, not 0.50 cm).
Why it happens
Students confuse the direction of impurity effects on melting and boiling points.
How to avoid it
Impurities LOWER the melting point AND ELEVATE (raise) the boiling point. Remember: melting goes down, boiling goes up.
Why it happens
Students assume chromatography is a definitive purity test; it only detects components that separate in that particular solvent.
How to avoid it
A single spot suggests the substance may be pure or is a mixture of components with the same Rf in that solvent. Confirm with melting point test as well.
The things students keep getting wrong in this sub-topic, answered.