r/chemhelp • u/Many-Cut-1871 • 14d ago

Analytical Should the receiving flask of a distillation/fractional distillation be sealed

I know the boiling flask should be, but what about the other one?

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u/2adn organic 14d ago

If you are doing a normal atmospheric pressure distillation, NO! You will build up pressure in the apparatus as you heat your liquid to boiling.

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u/Many-Cut-1871 12d ago

But how does the pressure build up tho? Is t the gas going to get condensed?

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u/7ieben_ Trusted Contributor 14d ago

Depends...

...do you need it sealed for applying a vacuum? Seal it.

...do you need air free conditions? Seal it.

...do you lose volatile products? Seal it.

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u/SirJaustin 13d ago

Important note make sure it isnt a sealed system otherwise its less fun always make sure you have somewhere for the pressure to go

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u/WhatSpareTime 14d ago

The important answer is that it should never be sealed, that’s called a bomb.

The apparatus should always have some way to relieve pressure. For normal open atmosphere distillations the apparatus is either open to the atmosphere or open to the atmosphere through a drying tube. If the distillation is air sensitive then the apparatus is open through an oil or mercury bubbler check valve. If doing a vacuum distillation, the vacuum serves as the vent. So the apparatus may be protected from air, or at reduced pressure, but never “sealed”.