What are the criteria of purity in the context of Cambridge IGCSE Coordinated Science?

In Cambridge IGCSE Coordinated Science, the criteria of purity refer to the methods used to determine the purity of a substance. Pure substances have specific melting and boiling points, whereas impure substances have a range of melting and boiling points. These criteria are essential in experimental techniques to ensure the accuracy and reliability of scientific results.

How can melting and boiling points indicate the purity of a substance?

Melting and boiling points are key indicators of purity. A pure substance will have a sharp, specific melting or boiling point. In contrast, an impure substance will melt or boil over a range of temperatures. This is because impurities disrupt the orderly structure of a substance, causing variations in the energy required to change its state.

Why is it important to determine the purity of a substance in experimental techniques?

Determining the purity of a substance is crucial in experimental techniques because impurities can affect the outcome of experiments. They can alter physical and chemical properties, leading to inaccurate results. Ensuring purity is vital for replicating experiments and achieving consistent, reliable data in scientific research.

What methods are commonly used to test the purity of a substance in the laboratory?

Common methods to test the purity of a substance include measuring its melting and boiling points, chromatography, and spectroscopy. These techniques help identify the presence of impurities by comparing the observed properties with those of a known pure substance, ensuring accurate experimental results.

How does chromatography help in assessing the purity of a substance?

Chromatography is a technique used to separate and identify components of a mixture. In assessing purity, chromatography can reveal the presence of impurities by showing multiple spots or peaks on a chromatogram. A pure substance will typically produce a single spot or peak, indicating uniformity in its composition.

Why is "Writing units (cm or %) after an Rf value" a common mistake in Criteria of Purity?

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). Source: 0654 Examiner Report 2023

Why is "Stating that impurities always raise the boiling point" a common mistake in Criteria of Purity?

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 is "Concluding that a single spot in chromatography proves a substance is 100% pure" a common mistake in Criteria of Purity?

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.

Experimental TechniquesCambridge IGCSE Coordinated Sciences 0654

Criteria of Purity

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Criteria of Purity — Cambridge IGCSE Coordinated Science 0654

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.

What you’ll learn

Mapped to the Cambridge IGCSE 0654 syllabus (2025-2027).

C2.2 — Use melting point and boiling point data to determine purity.
C2.3 — Interpret chromatograms and calculate Rf values.

Melting and Boiling Points as Purity Criteria

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:

Melting point is lower than expected AND the substance melts over a RANGE of temperatures
Boiling point is higher than expected
The wider the range, the more impure the sample

Why impurities affect melting/boiling points:

Impurity particles disrupt the regular lattice → less energy needed to break it → melts at lower temperature
More heating required to drive off solvent + impurity together → boiling point elevated

Practical use:

Published data books list melting points for known substances
If a sample melts at a temperature close to the known value AND over a very narrow range (<2°C) → considered pure
If the melting point is low/range is wide → impure; further purification needed

Example: Pure aspirin melts at 135°C. If a sample melts at 130–134°C, it is impure.

Pure: single sharp melting/boiling point. Impure: range of temperatures, depressed melting point.
Impurities: lower melting point, raise boiling point, widen the range.
Compare measured melting point to known value to assess purity.

Chromatography

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:

Stationary phase: paper or silica (where components stick)
Mobile phase: solvent that moves up (carries components different distances)

Paper chromatography — procedure:

Draw a pencil baseline (not ink — would move with solvent)
Apply spots of mixture on baseline
Dip in solvent (below baseline level)
Solvent rises by capillary action, carrying components at different rates
Remove before solvent reaches top; mark solvent front
Allow to dry; identify spots (UV light or locating agent if colourless)

Rf value (retardation factor):

Rf = distance moved by spot / distance moved by solvent front

Rf is constant for a given substance in a given solvent
Used to identify unknown substances by comparing to known standards run alongside
Pure substance: ONE spot with a definite Rf
Mixture: MULTIPLE spots with different Rf values

Applications:

Identifying dyes in food colouring
Detecting drugs in urine samples
Quality control of pharmaceuticals (HPLC — high-performance liquid chromatography for greater precision)

Chromatography: separates based on interaction with stationary and mobile phases.
Rf = distance moved by spot / distance moved by solvent front.
Same Rf as known standard = same substance. Multiple spots = mixture.

How it’s examined

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.

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Step-by-step worked examples — Criteria of Purity

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.

1Melting point as a test of purity
Extended
Question
A substance melts sharply at $78\,°\text{C}$ . A second sample of the same substance melts over the range $72\,°\text{C}$ to $80\,°\text{C}$ . Suggest which sample is purer and explain your reasoning.
Step-by-step solution
1. Step 1
  A pure substance melts at a single, sharp temperature because all particles have the same intermolecular forces.
2. 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.
3. 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.
2Calculating Rf value
Core
Question
In a paper chromatography experiment, a spot moves $4.5\,\text{cm}$ from the baseline and the solvent front moves $9.0\,\text{cm}$ . Calculate the $R_f$ value.
Step-by-step solution
1. Step 1
  Apply the Rf formula.
  $R_f = \frac{\text{distance moved by substance}}{\text{distance moved by solvent front}}$
2. Step 2
  Substitute values.
  $R_f = \frac{4.5}{9.0} = 0.50$
Answer
$R_f = 0.50$ (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.
3Interpreting a chromatogram
Core
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
1. 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).
2. 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.
4Identifying a substance by Rf
Extended
Question
A chromatogram is run alongside known dye standards. An unknown spot has an $R_f = 0.62$ . Standard dye A has $R_f = 0.41$ and standard dye B has $R_f = 0.62$ . Identify the unknown substance and explain your reasoning.
Step-by-step solution
1. Step 1
  Compare the Rf of the unknown spot with the standards run under identical conditions.
2. 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 $R_f = 0.62$ in the same solvent system.
Examiner tip
Examiners expect the phrase 'same Rf value under the same conditions' for identification by chromatography.

Key Formulae — Criteria of Purity

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.

Retention factor (Rf)
$R_f = \frac{\text{distance moved by substance}}{\text{distance moved by solvent front}}$
$distance moved by substance$
from baseline to centre of spot (cm)
$distance moved by solvent front$
from baseline to leading edge of solvent (cm)
When to use
Identifying substances by paper or thin-layer chromatography.
Example
Spot travels 3.6 cm; solvent front travels 9.0 cm → $R_f = 3.6/9.0 = 0.40$

Key Definitions and Keywords — Criteria of Purity

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.

Pure substance
Examiner keyword
A substance consisting of only one type of element or compound; it has a fixed, sharp melting and boiling point.
Rf value
Examiner keyword
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.
Paper chromatography
Examiner keyword
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).
Melting point
Examiner keyword
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.
Effect of impurities on melting point
Examiner keyword
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.

Common Mistakes and Misconceptions — Criteria of Purity

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.

✕Writing units (cm or %) after an Rf value
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).
✕Stating that impurities always raise the boiling point
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.
✕Concluding that a single spot in chromatography proves a substance is 100% pure
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.

Practice questions

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Criteria of Purity

Short Study Notes in the form of Slides

Short Study Notes — Criteria of Purity

Detailed Notes

What you’ll learn

How it’s examined

1Melting point as a test of purity

2Calculating Rf value

3Interpreting a chromatogram

4Identifying a substance by Rf

Retention factor (Rf)

Pure substance

Rf value

Paper chromatography

Melting point

Effect of impurities on melting point

✕Writing units (cm or %) after an Rf value

✕Stating that impurities always raise the boiling point

✕Concluding that a single spot in chromatography proves a substance is 100% pure

Practice questions

Past paper style quiz

Assess your understanding

Criteria of Purity — frequently asked questions