Topic 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)
Use melting and boiling data
A physical measurement supports identity; a single match is not proof.
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 example
Interpret a melting range
Pure substance P melts at 82 degrees C. A sample melts over 76-80 degrees C.
- The lower, broader melting range is inconsistent with a pure sample of P.
- It is consistent with P containing impurities, although more evidence is needed to establish identity.
The sample is impure if it is P. A matching single temperature alone would not uniquely identify an unknown.
Worked example
Match 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.
- The melting point alone cannot distinguish P and Q: both have the same value.
- The boiling point of X matches Q, not P. Within the supplied candidates, both measurements support pure Q.
- The second sample melts below the reference temperature over a range. If it is Q, the data indicate impurities.
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.