[beheada]

Question:

In a distillation set-up, especially for vacuum, isn't the thermometer adapter supposed to be sealed air-tight to the thermometer? How else are you going to have a lowered pressure? I'm pretty sure we went over WHY the boiling point for something would be lower in a lower atmosphere, but I can't put my finger on it. My thermometer adapter definitely isn't air-tight.

Do water aspirators work on the principle that the water is going so fast that it pulls the air through the side-hole, thus reducing the pressure in the system? It seems pretty ridiculous to just let all that water go down the drain, right?

For drying solutions or making them anhydrous, the basic idea is that you throw in something like magnesium sulfate that's looking for water molecules. Then, once you throw it in, it absorbs the water from solution because it wants those molecules? Then filter the solid out of solution and the solution should be anhydrous?

[jdurg]

1):  Yes, the thermometer adapter should be sealed air-tight so that the pressure can be reduced in the set-up.  In a low pressure situation, the boiling point of a substance decreases because for a substance to boil it's vapor pressure has to equal the atmospheric pressure surrounding it.  The temperature at which this is true is the boiling point.  If you lower the atmospheric pressure, you push the vapor pressure of the substance closer to the atmospheric pressure and thusly lower the boiling point.

2):  Yes, you are correct.  The quick movement of water past the side hole causes a localized decrease in pressure which creates a suction.

3):  Correct again.  The dehydrating agent binds to the free water molecules leaving behind an anhydrous solution.

[Dude]

Physically, the why might be somewhat complicated.  Here is my take.  Boiling point (the tendency of a material to become a vapor and diffuse away) is a primarily a function of the intermolecular interactions of the molecule (polar molecules adhere to each other more tightly and require more heat to overcome the interaction, larger molecules have more sites of intermolecular interaction) and the intrinsic diffusivity (primarily related to the molar mass) of the molecule.  Reducing the pressure would not be expected to change the intermolecular interactions.  Air exerts a pressure of approximately 14.7 psi in all directions.  This pressure (force / area) requires a certain amount of energy for the boiling molecule to vaporize and diffuse into.  Lowering the pressure reduces the number of molecules of air above the boiling compound and consequently reduces the force exerted on the liquid, enabling a lower temperature to effect distillation.

Water aspirators are an extreme waste of water.  They should be outlawed.  All scholastic demonstrations involving reduced pressure distillations should be done with pumps.

[beheada]

The thermometer adapter I've seen for a simple/fractional distillation is making me wonder if there are two different types of adapters. This one is kind of shaped like a plug except it has threading at the top with a small black plastic piece that the thermometer slides through. Even though the threading is tight, there is still room around the thermometer which would mean that it's not air-tight. Is there a way to make a rubber gasket or something if I wanted to change the adapter to work for a reduced pressure system or would you need a different adapter?

Also... are there any real world applications for having to use the vacuum distillation? Such as maybe industrial uses? Maybe making it safer to distill volatile solutions by lowering their boiling point?

Thanks in advance.

[Dude]

The vacuum distillation of crude oil to generate the base stock for lubricants is the largest that comes to mind.  There are many industrial uses of vacuum distillation.  It is normally used for heavier materials that might thermally degrade before they volatilize. 

Laboratory distillations are done with either a tapered thermometer containing a 10-18 or 10-30 size wedge that fits into a tapered glass slot of the distillation flask.  Silicone grease is normally used to effect a better seal and to prevent sticking of the glass surfaces.  The type you describe contains on o-ring and two plastic pieces.  The plastic pieces mate and squeeze the o-ring into contact with the thermometer to seal.  If it's not tight or the correct size for the thermometer you are using, get another adapter.