[christina]

What will happen if you choose a solvent whose boiling point is higher than that of the melting point of the compound to be crystalized?


~my thought is that the solvent will boil before the compound you are trying to crystalize will melt and alot of the solution will boil off as you are getting to the melting point of the compound.

Is this correct?

Thanks 

[Custos]

The answer is, it depends. If the compound dissolves in the solvent as you're heating it up, before it reaches the melting point of the compound you should be okay with the recrystallisation. As the solvent cools back down it will pass by the melting point of the compound and then as it cools further to the saturation point the compound will recrystallize.

If however, the compound doesn't dissolve before the solvent reaches the melting point, the compound will melt first and then (perhaps) dissolve. As the solvent cools down the compound will come out of solution as a liquid, which will then solidify as the temperature drops below its melting point. In this case the purification process will not be effective.

Note that if you are trying to recrystallize a compound from a solvent whose boiling point is higher than that of the melting point of the compound to be crystallized, you may not need to heat the solvent to boiling. There's no rule that says recrystallization requires the solvent to be heated to boiling -- all you need do is heat it until the compound dissolves.

[Borek]

my thought is that the solvent will boil before the compound you are trying to crystalize will melt and alot of the solution will boil off as you are getting to the melting point of the compound.

What about a kitchen salt in water?

If you don't like kitchen salt example - what about sucrose?

[christina]

Thanks alot for your help

[rrickert]

Sulfur trioxide can exist in at least 2 crystalline forms.  One of those forms has a melting point above its boiling point.  When that crystalline form is, "melted,"  it instantaneously vaporizes.  I did this with an open, 0.9 kg ampoule.  It was spectacular and I never did that again.  Fortunately, I was 10 feet away from the hood when it, "melted."  No one was injured.