Organic Techniques - Purity (Edexcel A Level Chemistry)

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