Alcohol

[Johnfp]

I was wondering today. If you took 100% Isopropyl Alcohol and left it out in the room;
1. How long would it take to equalize to whatever amount of water it wanted to absorb?
2. At what % of alcohol/water would it finally end up as?

I imagine it is temp dependent, but I was curious. I doubt it has anything to do with the relative humidity unless that was really low. Anyone have any ideas?

[Dinesh]

1. How long would it take to equalize to whatever amount of water it wanted to absorb?

How long to equilibrium? Probably a matter of a few minutes, depending on the various factors, unless the conditions are extreme

At what % of alcohol/water would it finally end up as?

Depends on local factors. Let's say you have the alcohol in a container like a beaker, open to the atmosphere. Then some of the alcohol molecules on the surface have enough energy to escape from the surface and some of the water molecules in the air above the alcohol have enough energy to penetrate the surface of the alcohol. Finally, the amount of alcohol molecules leaving the surface come into equilibrium with the amount of water entering the surface. The nett result is that either a certain volume of water enters the alcohol, diluting the alcohol, or a certain volume of alcohol leaves the surface and the alcohol evaporates.

The amount of alcohol that leaves the surface depends on the surface area - the greater the surface area, the more molecules are available on the surface to "boil off", and the temperature of the alcohol - the hotter the alcohol the more energetic are it's molecules. Some of these alcohol molecules strike air/water molecules and are driven back into the alcohol, so not all of the ones that leave the surface actually escape.

The amount of water that enters the surface depends on the temperature of the air - the warmer the air, the more energetic are the water molecules in the air, the relative humidity - the more humid the air, the more the number of water molecules and the local pressure - the higher the pressure, the more molecules are driven into the alcohol surface. In the end, all of these factors create an equilibrium state in which the number entering per second and the number leaving per second stabilises. If the equilibrium state is such that n molecules of water are driven into the alcohol per second and m molecules are driven into the air from the alcohol per second, then if n>m the alcohol dilutes, while if m>n then the alcohol evaporates.

How much water? Well, 18 gms of water contain the Avagadro number of molecules. So, if n-m molecules enter the water/second at equilibrium (the difference between what's going into the alcohol and what's going out), then the amount of water entering the alcohol is:

(n-m)*18/Avagadro's number = W grams of water

The density of water is 1 gm/cc, so W cc's of water enter the alcohol per second and if the alcohol is 100%, then the alcohol is diluting at a rate of W% per second.

The actual numbers depend on the factors stated above, but usually the stuff evaporates faster than it dilutes if it starts off at 100%. However, the more dilute the alcohol, the fewer are the number of alcohol molecules in the liquid. In this case, the number of water molecules entering will exceed the number of alcohol molecules leaving and the stuff will dilute. So, there is a dilution at which it remains stable. Here in San Diego, with typical temperatures in the 70's and typical humidity in the 30% range, my 75% alcohol left overnight in a 4x5 processing tray seems to go down to about 65% while my 50% seems to remain stable.

However, if you covered the alcohol container with a glass plate, for example, then no water can enter (except, of course, for the water in the air gap between the surface of the alcohol and the glass plate) and alcohol conc. remains pretty stable.

[holorefugee]

You may find this link helpful:

http://blowers.chee.arizona.edu/201proj ... n.pg1.HTML