Draw a well labelled diagram of Vacuum still and describe its theory.
Theory :-
Liquid boils at lower temperature when pressure on the surface of the liquid is reduced.
Therefore, when distillation is carried out under reduced pressure the rate of distillation increases.
Thus liquids which are unstable at their boiling points , at atmospheric pressure may be distilled at much lower temperature under reduced pressure.
Ex: In case of water, if external pressure is reduced to 70mm it will boil at 40 ºc, instead of its normal boiling point 100 ºc, at atmospheric pressure (760 mm of Hg ).
Diagram: