[DavidF]

My friend asked me this question regarding an experiment she did in a students lab.
she had a sealed beaker containing Benzene and Methanol at room temp. then a sudden drop in pressure has led to freezing (or at least it seemed like it) of the mixture .


now, what i had in mind was reducing the pressure caused the air humidity to condensate on the beaker , which in turn reduced the temp of the beaker. THOUGH i'm not sure if it's possible . the effect is too small.

any suggestions ?? 

Thanks in advance !

[Henry]

When evaporating a benzene-methanol mixture, the liquid phase cools down due to the heat of evaporation (which is needed to convert the liquid into vapor). In addition, benzene and methanol form an azeotrope which boils at approx. 56° C, the bp. of methanol alone is 65° C. When the temperature of the residual liquid falls below 6° C, benzene starts to freeze. The melting point of benzene is 5.6° C.

[DavidF]

thanks for the reply !

so what you're practically saying is that evaporation process under reduced pressure (like in any ordinary ROTAVAP) being an endothermic reaction, transfers heat from the surroundings to the system causing temp. drop below benzene's melting point so the mixture freezes.

am I right ?

now, i'm not quite sure what the azeotrope has to do with it.. (?)
AND what is the effect of a sudden pressure drop in your opinion, as my friend mentioned it
wasn't a continuous change in pressure , but rather an instant pressure drop in the system.

THANKS

[nj_bartel]

If it was instantaneous, I would say it probably had something to do with the phase diagram of that solution.  I.e. Decrease in pressure with constant temperature causes phase change from liquid to solid.

[Borek]

If it was instantaneous

Nucleation?