It may not seem like it, since it tends to completely fill every available space when it has the chance, but water does have small pockets of open space in it. Granted, this is on the molecular level, and so cannot be seen with the naked eye, but these pockets do exist. The follow Demo Science science demo will help you demonstrate this fact to your students (or whoever you choose to do science experiments with).
CAUTION: Minds may be blown!
Not to Be Confused with “Holy Water”
The required supplies for this one include: one clear glass jar capable of holding at least two cups of liquid; three cups of water, divided (8 oz. each, carefully measured); one cup of rubbing alcohol; masking tape; and a writing utensil. If you use a permanent marker, and don’t mind marking up your jar, you can skip the tape.
First, place a vertical strip of masking tape on the outside of the jar—or, make a thick vertical line with your marker. Pour one cup of water into the jar, and mark the top of its volume in the jar with a mark on the tape or the side of the jar; designate this as line “1”. Add another cup of water, and again mark its volume, this time designated as “2”.
Empty the jar and dry it (I guess you need a dish towel, too). Pour your third and final cup of water into the jar. It should, of course, reach the “1” line you made earlier. Now for the interesting part—pour your single cup of rubbing alcohol into the jar. Where does the fill level end at? Not “2”. What the heck?!
How Is 2 Cups Less Than 2 Cups?
So, you’ve added two cups of liquid, marked off its height, drained the jar, added two more cups of liquid, and missed the previous mark. How did that happen?
Well, for starters, when water molecules form, small pockets form betwixt the molecules due to the molecules’ polar nature and the angles at which they form. Molecules of rubbing alcohol are smaller than water molecules, so they squeeze in between and fill the empty pockets. By doing so, the combined volume of one cup water and one cup rubbing alcohol is less than that of two cups of water (or two cups of rubbing alcohol).
Crazy, right?! Just like your mother-in-law.