Re: If you mix 1 gallon of whater and 1 gallon of alcohol why don't you get 2?

First a quick kitchen experiment you can do to illustrate how two 
substances can mix together and take up less volume.

Measure 1 cup of uncooked rice and one cup of dried beans. Mix the two 
together and measure how much volume the combination takes up. It will be 
considerably less than 2 cups! In this case, the rice fits in the spaces 
between the beans and the result is a smaller volume.

Moving to alcohol and water:

The molecules of a liquid are held together by several types of 
intermolecular forces. In both water and alcohol, the most important of 
these are called "hydrogen bonds" which are strong interactions between the 
–OH groups on neighboring molecules.

(If you view the next 7 lines in font like Courier, you should get a 
reasonable approximation of the chemical structure of water and alcohol.)

water:   H-O           alcohol:    H H
            \                      | |
             H                   H-C-C-O
                                   | |  \
                                   H H   H
 
The result of these interactions are that water-alcohol mixtures are held 
together more tightly than alcohol alone, and thus take up less space than 
the sum of the two volumes.

In the following analysis, I’m going to use metric units (they’re easier!). 
All densities came from either the Merck Index or the CRC Handbook of 
Chemistry and Physics. Both references should be available in the reference 
section of your local library.

One liter of alcohol at 25 degress C (77 degrees F) has a mass of 785 
grams. One liter of water at 25 degrees C has a mass of 997 grams. If you 
mix the two together, you might expect to get 2 liters, with a mass of 1782 
grams, or 891 grams per liter. But if you do this, the actual mass of a 
liter of the mixture will be about 930 grams, or in other words, the 
combined volume will be only 1.9 liters, not 2 liters.