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Mixed melting point

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Question: A compound melting at 134 degrees C was suspected to be either aspirin (mp 135C) or urea (mp 133C) Explain how you could determine whether one of these two suspected compounds was identical to the unknown compound without using any form of spectroscopy.

This seems the same type of problem as my other most recent posting. It seems that the identity could be determined by a mixed melting point procedure if you had aspirin and urea available. A narrow melting point range would verify the substance and eliminate the other.


That's right. Another solution would be odor. Most aspirin smells faintly of acetic acid, expecially in moist air. It hydrolyzes. Urea, with most air and hydrolysis, smells like NH3. Urea is a base, aspirin is an acid so that will distinguish them also. But the mixed melting point is the correct answer here since that is what you are studying.

  • Mixed melting point -

    The unknown compound could be aspirin,this can only happen if the aspirin contains a little impurity and the heating rate is moderate,then its possible for aspirin to melt at a temperature a little less than 135 degree celcius.
    Also,the unknown compound could also be Urea(132-133) if a pure sample of urea is heated at a high rate thus a little addition to the thermometer value when it reads the melting point of urea (ie 135 degree celcius).

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