Measuring the melting point of a substance is a good way to test for purity
In this experiment, you use the melting point as a way of investigating the purity and identity of laboratory prepared aspirin samples.
A pure substance usually has a sharp melting point – ie a narrow temperature range during which it changes from a solid to a liquid. A substance which contains impurities often melts over a range of several degrees.
- Melting point tubes
- Watch glass
- Bunsen burner and heatproof mat
- Melting point apparatus: this may consist of a small beaker containing oil or glycerol in which is supported a 0–360 °C thermometer with small rubber bands for attaching the melting pointing tubes, or a commercial apparatus.
Health, safety and technical notes
- Read our standard health and safety guidance here.
- Wear eye protection
- 2-Hydroxybenzoic acid (salicylic acid): DANGER: this substance is harmful if swallowed, is suspected of damaging the unborn child, is harmful to aquatic life with long-lasting effects and may cause an allergic skin reaction. See CLEAPSS Hazcard HC052
- If you do not have sealed melting point tubes, heat the end of a capillary tube in a Bunsen burner flame until the glass softens and the end is sealed. Do not heat the tube so strongly that it bends. Leave it on a heatproof mat to cool.
- Make sure that your samples of solid are dry, by leaving them in a desiccator or an oven at 50 °C overnight. Fill the melting point tube to a depth of about 0.5 cm with dry impure aspirin sample.
- Seal a second tube and fill it to a depth of about 0.5 cm with dry crystals of purified aspirin.
- Place each tube in the melting point apparatus, slowly increase the temperature and note the temperature range over which the substances melt. A simple but effective apparatus consists of a beaker of oil or glycerol in which is supported a 0–360 °C thermometer. The two melting point tubes are attached to the thermometer close to the bulb using a rubber band. This apparatus makes it easy to compare the behaviour of the two solids. An electric melting point apparatus can also be used.
Record the melting point ranges of the pure and impure samples and include a description of the melting process in each case.
- On the basis of melting point is it reasonable to conclude that the substances tested contain aspirin?
- Account for any difference between the melting points of the crude and recrystallised samples of aspirin.
- What other impurities could there be in the aspirin made in the laboratory?
- Describe in molecular terms your ideas of what happens when a substance melts.
- Why does this explanation support the fact that aspirin has a lower melting point than 2-hydroxybenzoic acid?
- This answer will depend on the students’ results.
- The crude product may have a significantly lower melting point (126–132 °C) than pure aspirin as it contains certain impurities.
- Possible impurities are water, ethanoic anhydride, ethanoic acid, 2-hydroxybenzoic acid and polymerised 2-hydroxybenzoic acid.
- Forces within crystals of 2-hydroxybenzoic acid may include hydrogen bonding, dipole-dipole forces between polar molecules, and van der Waals forces. These are listed in order of decreasing strength. These forces hold the molecules together in a crystal lattice. When a substance melts these forces have to be overcome. By heating the solid the molecules are given sufficient kinetic energy to overcome these intramolecular forces.
- The two molecules are the same apart from one having a phenolic OH group and the other having an ester group.
An extension to this activity might include determining mixed melting points, using 2- hydroxybenzoic acid, the crude aspirin from activity 3 and the purified aspirin
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These resources were compiled by David Lewis and edited by Colin Osborne and Maria Pack.
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The melting point of aspirin