2.6 - Using melting points to test purity
You will use the start and end temperatures of melting to judge purity, and distinguish evidence about purity from evidence about a substance’s identity.
What melting tells us
A pure substance has a sharp melting point at a given pressure. In a real measurement, this may appear as a very narrow interval, such as 81.8–82.2 °C. An impure sample usually starts melting at a lower temperature and melts over a broader interval than the pure substance.
Melting interval
The melting interval runs from the temperature at which melting starts to the temperature at which the sample is fully liquid. Its width is the end temperature minus the start temperature.
Melting data gives evidence about purity. Do not invent a universal cutoff between “narrow” and “wide”: consider the measurement precision and the contrast between samples. A high temperature alone does not imply purity, and a low temperature alone does not imply a mixture.
A sharp melting point is also useful for checking identity against a reference value. Two different pure substances can have different sharp melting points. A sharp result at the wrong reference temperature could be a different pure substance.
Using melting data
Compare these samples, which are not assumed to be the same substance:
| Sample | Starts melting / °C | Fully liquid / °C | Interval width / °C |
|---|---|---|---|
| A | 82.0 | 82.2 | 0.2 |
| B | 76.0 | 84.0 | 8.0 |
| C | 92.8 | 93.1 | 0.3 |
Continue this lesson.
You’re previewing NovaLearn without an account. Create a free account to read the rest.
Create free account