Can you now think of why expelling water might be better than expelling just air?

(a) Correct. Water is heavier than air because it is DENSER. This means that more water molecules are packed into a given volume.


Density = Mass ÷ Volume
Density = kilograms per cubic meter = kg/m3

Examples are:
Air density = 1.29 kilograms per cubic meter
Water density = 1,000 kilograms per cubic meter!!!

The water molecule itself, H2O, is actually less massive than a Nitrogen molecule, N2, which makes up 80% of our air. It’s just that there are more water molecules in one liter than there are air molecules…even when the air is pressurized. Think of it this way: if you pump up the bottle to 140 pounds per square inch (PSI) of air pressure, then for every air molecule expelled out of the bottle, you could have expelled roughly 83 water molecules. That’s a lot more mass!

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