Edexcel A Level Chemistry:复习笔记7.6.6 Organic Techniques - Purity

Melting & Boiling Point 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

 

8.3.1-Melting-point-using-an-oil-bath

Melting point test using an oil bath

 

 

8.3.1-Melting-point-using-a-Thiele-tube

Melting point test using a Thiele tube

 

 

8.3.1-Melting-point-using-a-melt-station

Melting point test using a melt station

Exam Tip

Always quote a melting point as a range + or - and reference a data book value if you have one.

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

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