This is an important technique used to check the purity and identity of a solid product. A pure substance will usually have a sharply defined melting point. We always quote the melting point range of the substance in question - that is the temperature at which it just starts to melt, up to the temperature at which melting is complete. Substances that are impure tend to have a wide melting point range. For this reason, it is important that our sample is thoroughly dry and free of any solvent. This is usually ensured by placing the sample for some time in a desiccator, making sure the sample container is open so that any water can escape. A pure, dry sample will usually melt over a range of one or two degrees centigrade.

The equipment used to measure melting points varies considerably from place to place. A fully automatic melting point apparatus can be obtained for several thousand pounds, and this will judge the melting points for you. The video shows a semi-automatic apparatus in which you have to decide on the melting temperatures, but the machine controls the temperature change. If you are using the same model of melting point apparatus as seen in the video you can check on the details of its operation. You may be using a manual apparatus or a heated oil bath . In this case you will still need the skill of identifying the melting point range, but you will also need to be careful with the temperature control. Students usually heat far too quickly, which is particularly important near to the melting point. This results in inaccurate melting points, as the sample temperature is lagging behind the temperature reading (the heat has to conduct through the glass wall of the melting point tube). The measured melting point will be too high. You should aim to have the temperature rising by about one degree centigrade per minute as the melting point approaches.