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Join Date: Nov 2007
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Vaccum Distillation

12/13/2007 8:50 AM

While separating two solvents from a mixture, heating under vaccum is done ( Vaccum distillation), What makes the solvent evaporate much before its boiling point under vaccum.......

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Guru

Join Date: Jul 2007
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#1

Re: Vaccum Distillation

12/13/2007 9:17 AM

Do you have an azeotrope?

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#2

Re: Vaccum Distillation

12/13/2007 10:44 AM

HI Bmadu srap

Vapour pressure. Above a liquid will be some vapour from that liquid.

When you reduce the pressure above a liquid if becomes easier for the molecules to escape from the surface of that liquid, so it boils more easily.

Consider these two examples.

Climbers on mount Everest cannot boil an egg and make it solidify. This is because at that altitude (28,000 feet or so) the pressure is much lower than it is at sea level, so the water boils at around 90C, which is not high enough to coagualte the protiens in the egg, so it stays mushy (Yuck!)

A pressure cooker can cook food faster than boiling it in a pot because the increase in pressure inside the pressure cooker raises the boiling point of the water over 100C, so the food is made hotter, and so cooks faster.

This is also why chemical properties are stated at 'Standard temperature and pressure' (STP), because variations in either temperature or pressure will affect things like boiling and freezing points, vapour density etc.

Vacuum distillation allows the distillation to occur at lower termperature.

Hope this helps

IPG

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Commentator

Join Date: Aug 2007
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#7
In reply to #2

Re: Vaccum Distillation

12/17/2007 2:13 AM

It is new to me you gave me the knowledge that one cannot boil egg on Mount Everest.

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#8
In reply to #7

Re: Vaccum Distillation

12/17/2007 10:10 AM

Oh you can boil it all right, but it will never get hard!

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#3

Re: Vaccum Distillation

12/13/2007 11:21 PM

No solvent can evaporate before its B.P.

But if specific vacuum and temperature is acheivedthen it may evaporates with repsect to its flast point temp.

Regards,

Vijay

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#4
In reply to #3

Re: Vaccum Distillation

12/14/2007 3:27 AM

Boiling temperature varies with pressure.

Take water, for example.

  • Reduce the pressure to 100mBarA and it boils at 46degC.
  • At sea level pressure it boils at 100degC.
  • At 20BarA it boils at 212degC.

It's the same principle with any solvent (though the numbers are different).

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#6
In reply to #3

Re: Vaccum Distillation

12/16/2007 12:52 PM

Vijay,

Of course solvents evaporate below the boiling point. Other wise, your clothes would never dry after washing (as an example).

I don't understand your flashpoint comment. Flashpoint is the temperature at which a compound has enough vapor pressure to be ignited.

Tad

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#5

Re: Vaccum Distillation

12/14/2007 10:19 AM

Good answer Zaphod. Water boiling temp at altitudes always seems to be the best way to describe that principle.

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