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K326 / K328 / 2027
Experimental Chemistry

Lesson 6 of 6

Use melting and boiling data

A physical measurement supports identity; a single match is not proof.

O-Level 5086 / 5088 (2026) / SEC G3 K326 / K328 (2027)

  • I can infer identity and purity.Syllabus

    Syllabus K326 / K328, 1.2(d). Infer identity and purity

Use melting and boiling data

A physical measurement supports identity; a single match is not proof.

Make a guess. You are not marked.

Pure aspirin melts sharply at a fixed temperature. How does a sample of aspirin with a little impurity melt?

  1. Higher, and over a range of temperatures
  2. Lower, and over a range of temperatures
  3. At the same temperature, but more slowly
Show the answer

Lower, and over a range of temperatures

Impurities lower the melting point and spread the melting over a range. A sharp, correct melting point is evidence of purity.

At a specified pressure, a pure substance melts sharply and boils at a characteristic temperature. Soluble impurities usually lower and broaden the melting range and can raise the boiling point. Compare measured data with reference values taken under comparable conditions; a mixture may have a changing boiling temperature.

Worked exampleInterpret a melting range

Pure substance P melts at 82 degrees C. A sample melts over 76-80 degrees C.

Try it first. Then show the worked stepsHide the worked steps
  1. The lower, broader melting range is inconsistent with a pure sample of P.
  2. It is consistent with P containing impurities, although more evidence is needed to establish identity.
Answer

The sample is impure if it is P. A matching single temperature alone would not uniquely identify an unknown.

Worked exampleMatch two measurements to reference data

At the same pressure, pure P melts at 18 degrees C and boils at 126 degrees C; Q melts at 18 degrees C and boils at 154 degrees C. Unknown X melts sharply at 18 degrees C and boils steadily at 154 degrees C. A second sample, claimed to be Q, melts over 12-16 degrees C. Interpret both samples.

Try it first. Then show the worked stepsHide the worked steps
  1. The melting point alone cannot distinguish P and Q: both have the same value.
  2. The boiling point of X matches Q, not P. Within the supplied candidates, both measurements support pure Q.
  3. The second sample melts below the reference temperature over a range. If it is Q, the data indicate impurities.
Answer

X is consistent with pure Q. The second sample is impure Q if its claimed identity is correct; its melting range alone cannot establish identity.

Check your understanding

A liquid is clear and colourless. Is it pure?

  1. Not necessarily. A colourless substance could be dissolved in it.
  2. Yes. An impurity would make the liquid cloudy or coloured.
  3. Not necessarily. Pure liquids are slightly cloudy, so clear means it has been filtered.
Show the answer

Not necessarily. A colourless substance could be dissolved in it.

Many dissolved substances, such as salt or sugar, are colourless. Use a fixed boiling point, chromatography or another test, not appearance.