Using evidence to assess purity
Purity tests: use melting-point and boiling-point data to identify substances and assess purity, and explain why purity matters in food and drugs.
On this page
Purity evidence is useful only when you connect the observation to a conclusion. Compare a measured melting or boiling behaviour with the expected pure value, or interpret the number of chromatogram spots.
Start with the sample and the goal
A pure substance consists of only one element or one compound. An impurity is an unwanted substance mixed into the sample.
A mixture contains two or more substances mixed together but not chemically combined.
Purity can be tested by comparing results with known data:
- Melting point (for solids)
- Boiling point (for liquids)
- Chromatography (to check if there is more than one substance present)
What you need to know
- A pure solid has a sharp melting point: it melts at one fixed temperature.
- An impure solid usually starts melting at a lower temperature and melts over a range of temperatures.
- A pure liquid has a sharp boiling point: it boils at one fixed temperature (at a stated pressure).
- A changing boiling temperature at constant pressure can indicate a mixture. A dissolved non-volatile impurity raises the boiling point of a solvent; other mixtures can behave differently.
- One chromatogram spot shows one visible component under those conditions; two or more spots prove that the sample is a mixture.
- Purity must be checked where impurities could change safety, effectiveness or quality, especially in food and medicines.
Choosing and explaining a method
- Compare the observed temperature and melting or boiling range with reliable reference data.
- Check the apparatus and conditions before attributing a difference to impurity.
- A matching sharp temperature supports purity, but one physical property alone does not prove identity or complete purity.
Melting point (solids)
Use melting point data in this comparison:
For a known crystalline solid measured carefully under the same conditions:
| Observation | What it supports |
|---|---|
| Sharp melting behaviour close to the reference temperature | Consistent with a pure or nearly pure sample of the stated substance. |
| Starts below the reference temperature and melts over a broader range | Supports the presence of a dissolved impurity. |
| A temperature different from the reference, without other evidence | Investigate impurity, incorrect identity, apparatus error or different conditions. |
The key idea: an impurity changes how particles pack in the solid, so the sample usually starts melting below the known value and melts over a range rather than at one sharp temperature.
Same idea shown as a labelled graph (axes + units):
Melting: a pure solid and the same solid with an impurity
Illustrative curves compare a flat melting plateau at 80 °C with a lower melting interval from 75 °C to 79 °C for the same solid with an impurity.
Scroll across the graph to read all labels.
View figure data
| Time (min) | Pure solid | Same solid with impurity |
|---|---|---|
| 0 | 20 | 20 |
| 1 | 40 | 40 |
| 2 | 60 | 60 |
| 3 | 80 | 75 |
| 4 | 80 | 77 |
| 5 | 80 | 79 |
| 6 | 90 | 90 |
| 7 | 100 | 100 |
Boiling point (liquids)
A pure liquid boils at a fixed temperature at constant pressure. Many mixtures show a changing boiling temperature as their composition changes during boiling. However, a constant temperature alone does not rule out every mixture.
Boiling point depends on atmospheric pressure. Compare a measured value with reference data only when the pressure is the same, unless the question tells you how to account for a pressure difference.
A dissolved non-volatile solute, such as salt in water, raises the solvent’s boiling point at the same pressure. As solvent boils away, the remaining solution becomes more concentrated and its boiling temperature can rise. If an impurity is volatile too, use the given data rather than assuming that it must raise the boiling point.
Chromatography (checking for more than one substance)
In paper chromatography, a chromatogram is the paper after the solvent has moved up and the spots have separated.
- One spot: could be a pure substance, or a mixture whose components happen to travel the same distance.
- Two or more spots: the sample is a mixture.
For the method (start line in pencil, solvent level below the line, etc.), see paper chromatography and Rf values.
“One spot” means “only one visible component under these conditions”. It does not guarantee purity because different substances can have the same spot position in the same solvent.
Why measuring purity matters
Purity is not only a laboratory label. The acceptable level depends on how a substance will be used.
| Context | Why purity is measured |
|---|---|
| Medicines | The dose of the active ingredient must be reliable, and harmful contaminants must be controlled. |
| Foodstuffs | Contaminants or undeclared substances can make food unsafe or reduce its quality. |
| Chemical manufacture | Impurities can change reaction results, damage equipment or make the product unsuitable. |
“Purity is important” is incomplete. State the consequence: an impurity may make a medicine unsafe or give an unreliable dose, or may make food unsafe to consume.
Avoiding common mistakes
- Giving a purity conclusion without explaining the observed temperature, range or reference comparison.
- Saying that every impurity raises boiling point. That statement applies to a dissolved non-volatile solute, rather than every possible mixture.
- Ignoring pressure when comparing boiling points (especially altitude questions).
- Claiming “one spot = pure” with no qualification.
- Using ink for the start line in chromatography (it dissolves and gives a fake result).
Using the evidence
Use “melts at a fixed temperature” for pure solids and “melts over a range of temperatures” for impure solids. For liquids, use “boils at a fixed temperature” vs “boils over a range”.
Compare the measured value with the reference at the same conditions. Explain what the difference supports, taking account of the stated identity and reliability of the method. A mismatch alone does not establish which alternative caused it.
Worked examples
Modelled example 1
Interpreting a Melting Range
Problem
Study the worked solution
Inspect the melting behaviour
Method
Identify a four-degree melting range.Reason
A pure solid should melt sharply at one fixed temperature under the same conditions.Working
Observed: 117–121°C, not a sharp point.Compare with reference data
Method
State that the evidence supports an impure sample of Z.Reason
It melts over a range and begins below the 125°C reference value.Working
Conclusion: impure Z; pure Z melts sharply at 125°C.
Guided practice 2
Boiling Point Range (Purity Conclusion)
Problem
Compare the observation with pure-liquid behaviour
Hints
Hint 1: keyword
Hint 2: compare
View solution step by step
Describe the data
Method
State that boiling occurs over 78–82°C.Reason
The thermometer does not remain at one sharp temperature.Working
Boiling range: 4°C.Infer purity
Method
Conclude that the changing temperature supports a mixture.Reason
A pure liquid boils at one fixed temperature at the same pressure.Working
Range of boiling temperatures → impure sample.
Common misconception 3
Chromatography Conclusion
Learner conclusion
Interpret the number of separated components
View solution step by step
Interpret the spots
Method
Count at least three visible components.Reason
Chromatography separates substances that travel differently under the chosen conditions.Working
Three separated spots → at least three components.Classify the sample
Method
Conclude that the dye is a mixture.Reason
A pure substance contains only one element or compound, whereas this sample gives multiple components.Working
Mixture, not pure.
Examiner practice 4
Explaining Why Medicine Purity Matters
Examination question
Link purity to dose and safety
View solution step by step
Dose reliability
1 markMethod
Link purity to the amount of active ingredient in a stated tablet mass.Reason
Incorrect purity can make the delivered dose ineffective or unsafe.Working
Known purity helps ensure a reliable active-ingredient dose.Contaminant safety
1 markMethod
State that harmful contaminants must be detected or controlled.Reason
Even a correct nominal dose may be unsafe if unwanted substances are present.Working
Purity measurement reduces contaminant risk.
Self-mark with the mark scheme
Compare your response with each mark point. Select a point only when your response contains that evidence.
Self-mark the dose and contaminant consequences.
Challenge 5
Identifying a Liquid From Boiling Point
Identity transfer
Use sharpness, reference match and evidential caution
Hints
Hint 1: sharpness
Hint 2: identity
View solution step by step
Use fixed-temperature evidence
Method
State that the constant value supports a pure or nearly pure liquid.Reason
A pure liquid boils sharply at one temperature at fixed pressure.Working
Observed: constant 100°C.Compare reference values
Method
Identify the sample as likely pure water.Reason
The observed temperature matches water rather than ethanol at the same pressure.Working
Data support pure water; use another test if confirmation is required.
Try it independently
Mind stretcher 1: Conflicting EvidenceExtension
Question: In reliable tests under controlled conditions, sample P gives one spot on a chromatogram. However, it melts over a range 122°C to 126°C. Is P pure? Explain.
Show Answer
No. The melting data shows impurity because it melts over a range of temperatures. The chromatogram having one spot does not prove purity (different substances can have the same spot position in one solvent).
Mind stretcher 2: Planning a Purity TestExtension
Question: You are given a white crystalline solid X, a suitable chromatography solvent and locating agent, and reference data for pure X. X melts without decomposing and dissolves in the supplied solvent. Describe two tests that could provide evidence about its purity.
Show Answer
- Melting point test: measure the melting point and compare it with the reference value. A pure sample melts sharply at the reference temperature; an impure sample usually melts over a range and starts below that value.
- Paper chromatography: dissolve a small amount in a suitable solvent and run it alongside a known sample of pure X under the same conditions. Mark the solvent front, dry the paper and apply the suitable locating agent to reveal colourless compounds. Two or more separated sample spots prove that the sample is a mixture. One spot aligned with pure X supports the result but does not prove purity on its own.
Practise and check
Practise using melting points, boiling points and chromatograms to identify substances and assess purity.
Open the topic checkSyllabus and review details
- SEC G3 Pure Chemistry 2027 · 2027
Content structure and subject content, PDF pages 9–24
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