Re: why does a vacumn form when rubbing alcohol and H20 are mixed?

To begin with... it was not a vacuum for long.

Huh?  Well lets begin at the beginning.  Water; a polar molecule, structure

O-H
|
H

The oxygen is negative and gloms onto positive particles, while the 
hydrogens are positive and glom onto negative things.  When water is just 
with water all the molecules are a certain distance from each other giving 
water its particular density.

O-C4H9  (see your book or science teacher for a more explicit structure)
|
H

In the isoprpyl alcohol the OH end is like water, and therefore acts just 
like water,  glomming on to passing positives and negatives.  The "organic" 
portion - the C4H9 is non-polar and is ignored and repelled by the water 
and/or the other OH parts.  And again when isopropyl alcohol is with just 
other isopropyl alcohol, all the molecules are a certain distance from each 
other giving the alcohol its particular density.

Now when the water and alcohol are first poured into the test tube they 
haven't really mixed, so the test tube is filled up with a layer of water 
and a layer of alcohol.  The test tube is shaken, and the water and alcohol 
mix.  The organic end of the alcohol allows the water and alcohol to snuggle 
in closer with each other than they would with themselves (water to water or 
alcohol to alcohol).  The result; the water alcohol mix takes up less space 
than the two separate liquids... makeing that space at the top of the test 
tube.

Now why did I say that the space wasn't a vacuum for long?  Well,  the very 
instant it formed... it was a vacuum.  But any liquid faced with a vacuum 
will instantly turn to vapor to fill up the vacuum. So as soon as the vacuum 
formed,  a little bit of the water and alcohol turned to gas and filled in 
the space. You couldn't see it because gas is invisible to the naked eye If 
you were to get curious about liquids filling in vacuums... go look up 
Raoult's law.  I will concede that the pressure inside the space was 
probobly quite a bit lower than the air pressure outside the testtube 
(regular air pressure). So - closer to a vacuum.

That was a great demonstration you saw... it deals with a huge number of 
chemical and physical principles.  Good Luck!

Greta Hardin