Chemistry 2.5 - 2.6 - Purity and melting ranges

Chemical purity and mixtures

In everyday language, pure may mean natural, high quality, clean or with nothing deliberately added. A carton labelled pure orange juice can still contain water, sugars, acids and many other substances. The everyday label does not make it chemically pure.

A sample of pure copper contains only the element copper. A sample of pure water contains only the compound water; containing two different elements does not make a compound a mixture. The word one refers to one chemical substance, not one type of atom.

A mixture contains two or more different substances together. The substances are not chemically joined to one another, the proportions can vary, and no new substance has to form when the mixture is made. Air is a mixture of gases, and orange juice is a mixture even if its everyday label calls it pure.

Purity is different from identity and concentration. A purity test asks whether other substances are mixed into the sample. An identity test asks which substance is present. A concentrated solution can still be a mixture because it contains solute and solvent.

Interpreting melting ranges

At a fixed pressure, an ideal pure substance melts at one characteristic temperature: it has a sharp melting point. In a real experiment, the instrument may show a very narrow interval between the first liquid appearing and the last solid disappearing. A mixture melts progressively over a broader range because it contains different substances rather than one uniform substance.

For an observed sample:

The range width, measured in degrees Celsius, is the difference between the two readings. A narrow interval is consistent with a pure substance; a broad interval is evidence of a mixture. Do not decide purity from whether the melting temperature is high or low: many pure substances have low melting points, and many have high ones.

Worked example

Two batches are expected to contain the same substance.

For A:

For B:

Batch A changes state over the much narrower interval, so its data are consistent with a pure substance. Batch B melts over a broad $7.2 \,^{\circ}\mathrm{C}$ range, so its data indicate a mixture. Each subtraction has a sensible positive answer because the completion temperature must be at least as high as the onset temperature.

A sharp melting result is evidence about purity, but it does not by itself prove identity: two different substances could have similar melting points. Poor technique can also broaden an observed range, so the method matters.

Measuring a melting range

This is not a required core practical for points 2.5-2.6. It is a useful investigation of how melting data are obtained and how strongly those data support a conclusion about purity.

Purpose and apparatus

Compare the melting ranges of a suspected pure sample and a suspected mixture of the same nominal substance. Use teacher-approved dry solids, sealed-end melting-point capillaries, a clean watch glass and spatula, an electric melting-point apparatus with a temperature display, eye protection and a heatproof surface.