Organic Techniques - Purity (Edexcel International A Level Chemistry)

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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 using an oil bath, downloadable AS & A Level Chemistry revision notes

Melting point test using an oil bath

 

Melting point test using a Thiele tube

 

Melting point using a melt station, downloadable AS & A Level Chemistry revision notes

Melting point test using a melt station 

Examiner 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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Richard

Author: Richard

Expertise: Chemistry

Richard has taught Chemistry for over 15 years as well as working as a science tutor, examiner, content creator and author. He wasn’t the greatest at exams and only discovered how to revise in his final year at university. That knowledge made him want to help students learn how to revise, challenge them to think about what they actually know and hopefully succeed; so here he is, happily, at SME.