Organic Techniques - Purity (Edexcel International A Level Chemistry): Revision Note

Melting & Boiling Temperature Determination

Melting point determination

• The melting point of a solid is indicative of its purity and identity

• A melting point can be matched to a known substance as a means of identification or confirmation of a desired product

• The proximity of a melting point to the actual data book value can express purity

  • Impurities tend to lower the melting point of a solid

• The melting point range also reveals the degree of purity

  • Pure substances have sharp well defined melting points

  • Impure substances have a broad melting point range, i.e. a large difference between when the substance first melts until it completely melts

• The skills needed in performing a melting point test are largely dependent on the specific melting point apparatus you are using

• However, there are some common key skills:

  • Correctly preparing the melting point tubes

  • Heating the tubes very slowly

  • Repeating to get a range of measurements (three would be normal)

• The sample solid must be totally dry and finely powdered - this can be achieved by crushing it with the back of a spatula onto some filter paper or the back of a white tile (this absorbs any moisture)

• Use the first tube to find the approximate melting point range and then repeat using a much slower heating rate 

Melting point test using an oil bath

Melting point test using a Thiele tube

Melting point test using a melt station 

Boiling point determination

• The boiling point of a liquid is indicative of its purity and identity

• Boiling point is determined by distillation

• The sample is gently heated until it boils and this temperature is recorded

  • This boiling point can then be compared against literature / database values

• If the sample contains impurities:

  • The boiling point may appear higher than the literature / database values

  • The sample may boil over a range of temperatures instead of at a single temperature