Melting and boiling data: identify and assess purity
Compare supplied reference data under matching conditions, interpret sharp or broadened changes, and avoid treating one temperature as conclusive identity proof.
Section 12.4 · Separation and purification
Exam scope and assessment notes
Core and Extended · Chemistry 0971, 2026–2028. These paper-based method exercises support supervised laboratory learning; they do not certify practical competence. Cambridge states that IGCSE 0971 has the same subject content as IGCSE 0620, with 9–1 grading. This StudyForA pathway maps the 2026–2028 syllabus only. This 9–1 graded qualification is available only in a limited number of administrative zones. Confirm that your centre can enter you for this code and exam series with Cambridge International. Cambridge has published a separate syllabus for 2029. StudyForA has not reconciled that version yet; this page supports exams in 2026, 2027 and 2028 only.
Before you begin
- Distinguish melting from boiling.
- Read a range and compare values with a supplied reference.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which comparison is most defensible for boiling-point identification?
What you will learn
- Use supplied melting and boiling points to support identification.
- Evaluate purity evidence while recognising conditions and limits of a single measurement.
Match the conditions and the type of measurement
A pure substance has a characteristic melting point and boiling point under specified conditions. Compare a measured value with supplied reference data using the same relevant pressure, appropriate calibration and measurement method. A close match supports a candidate identification; it is not unique proof, because different substances can have similar values.
Pressure affects boiling point, so a value measured at one pressure cannot be compared blindly with a reference at another. A flat region on a heating curve can indicate a change of state under the supplied model, but the full investigation still needs the substance, conditions and measurement quality. For an unknown sample, combine suitable evidence rather than identifying it only from its colour or one thermometer number.
Interpret a range as evidence, not a universal verdict
In the usual school solid-mixture example, an impure solid can melt over a range and at a lower temperature than the pure substance. A sharp melting point close to the reference is consistent with purity under those conditions. It does not prove every impurity is absent, and a broad result also calls for considering the method rather than automatically naming one contaminant.
Impurities can alter boiling behaviour; avoid the universal claim that all impurities must change the boiling point in the same direction. A changing boiling temperature can support a mixture interpretation in a supplied simple model. State the observation first, then the inference and its limit. Purity for a particular use may also require evidence beyond melting or boiling data.
Pause and explain
A solid melts below the pure reference over a range. What is a justified inference in the supplied school model?
Worked example
Reference candidate A melts sharply at 80°C and candidate B at 95°C under the supplied conditions. A sample melts from 76°C to 79°C. What can the data support?
Show the worked solution
- The measured range lies much closer to A's reference than B's.
- The lower, broadened melting behaviour is consistent with an impure A sample in this school model.
- It does not conclusively prove A or a particular contaminant; check the measurement method and seek further suitable evidence.
Answer Consistent with impure A in the supplied model, but not conclusive identity or contaminant proof.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
A sample melts sharply at 80°C, matching candidate A's supplied pure reference under the same conditions. State what is supported and what is not proved.
Give me a hint
Separate a useful match from an absolute claim.
Compare my reasoning
- The measured point agrees with candidate A's reference.
- Sharp melting is consistent with a pure A sample under the stated conditions.
- The one result does not establish unique identity or absence of every possible impurity.
Consistent with pure A under the supplied conditions; unique identity and total absence of impurities are not proved.
Look for these in your work
- The reference match and purity evidence are recognised.
- The conclusion stays within the measurement's limits.
Reference A melts at 80°C and B at 95°C. The sample melts over 76–79°C. Interpret the data without naming an unsupported contaminant.
Give me a hint
Compare the range with both references and the expected school impurity pattern.
Compare my reasoning
- The range is closer to A than B.
- The broadened lower range is consistent with an impure A sample.
- Check calibration/method and use supporting evidence before asserting identity or a specific impurity.
Consistent with impure A in the supplied model; a particular impurity is not identified by this range alone.
Look for these in your work
- Both supplied references are considered.
- Observation, interpretation and uncertainty are distinct.
An unknown liquid boils steadily at 95°C at a reduced pressure. A reference candidate boils at 100°C at normal atmospheric pressure. A student rejects it solely from the 5°C difference. Evaluate that conclusion.
Give me a hint
Ask whether the two conditions make the reference directly comparable.
Compare my reasoning
- The measured and reference pressures differ.
- Boiling temperature can change with pressure, so the 5°C difference alone is not a like-for-like rejection test.
- Use a reference at the relevant pressure or a controlled comparable measurement, then combine suitable evidence.
The rejection is not justified by the difference alone because pressure conditions differ; obtain comparable data and supporting evidence.
Look for these in your work
- The confounding condition is identified.
- The improvement makes the comparison relevant rather than forcing an unsupported identity.
Common mistakes
- Using one temperature as unique proof of identity.
- Ignoring pressure when comparing boiling points.
- Claiming every impurity changes boiling point in the same direction.
- Confusing a measured range with a named contaminant.
What can you explain now?
Does a sharp melting point close to a reference prove every impurity is absent?
Compare with the explanation
No; it is evidence consistent with purity under those conditions.
A single physical measurement has limits and can need supporting tests.
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Make it stick
Explain what sharp, broadened and pressure-dependent phase-change data can support, and why one value is not absolute identity or purity proof.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.