Question

how does a vacuum reduce atmospheric pressure?

boiling chips and vacuum distillation, respectively, are used in distillations to:

A. provide nucleation sites that give the liquid a place to start forming bubbles to prevent super-heating; lower the boiling points of the substances to be distilled.

B. lower the boiling points of the substances to be distilled; to work synergistically with the vacuum system to further lower the boiling points.

C. lower the boiling points of the substances to be distilled; provide nucleation sites that give the liquid a place to start forming bubbles to prevent super-heating.

D. provide nucleation sites that give the liquid a place to start forming bubbles to prevent super-heating; speed up the distillation process by vacuuming the first distillate out of the apparatus.

A is correct. When transitioning from liquid to gas during boiling, the liquid needs a place to start forming bubbles. The most common way to give the solution those places (nucleation sites) is to either scratch the inside of the flask, or put in a boiling chip. To boil a substance, the Pvap of the substance must equal the Patm. One way to lower the boiling point is to reduce Patm by introducing a vacuum. Thus you need not heat the substance and raise its Pvap as much, since Patm is lower.

Answer 1

Creating vacuum means that gaseous pressure would be much less than atmospheric pressure over the surface of liquid which is under investigation. As there is an equal and opposite pressure between vapour pressure of liquid (caused by liquid molecules) and atmospheric presssure (caused by gases) maintains the equilibrium at normal temperature. Creation of vacuum removes the atmospheric gases, hence breakes the equilibrium which in turn this facilitates the liquid molecules to come out as vapour phase easilly.