Properties of Air

Place an empty balloon on a scale and weigh it. Take this same ballon and inflate it. Weigh it again. What do you see?. A really clear way to show this is to make a balance with a stick or coat-hanger suspended by a string in the middle. Tie an empty balloon on each side to prove they weigh the same. Inflate one balloon and rehang it. That side of the balance will be heavier. If air had no mass, there would’ve been no change.

Air is really quite heavy. It is just that it has always been there for you and me so we do not notice. Asking a human if air is heavy is like asking a fish if water is heavy.

ake a metre stick and lay it on a table. Unfold a full page of a newspaper and lay it flat over the metre stick. Push down on the other end of the metre stick. What happens? Why can’t you lift a super-light piece of paper? Air exerts pressure (in all directions).

The air above the paper pushes down on it (pressure). This pressure is what makes the paper lay flat on the table - it’s being pushed down. Even though they’re too tiny to see, all the molecules of air in the atmosphere above your head weigh something. And the combined weight of these molecules causes a pressure pressing down on your body of 10,000 kg per square metre (10,000 kg = 22,000 lbs). This means that the mass of the air above the 0.1 square metre cross section of your body is 1,000 kg, or a tonne.

Take a plastic pop bottle and with the cap off the bottle, hold you hand above the mouth of the bottle and squeeze.  What do you feel?  Screw the cap on tightly and squeeze again.  What happens when you squeeze the bottle now?  Now, fill the bottle completely with water, replace the cap and squeeze again.  What do you feel now?

When you squeezed the open bottle, you forced some of the air out of the mouth.  When you placed the cap on the bottle and squeezed again, there was no place for the air to go, but you were able to squeeze the bottle together.  In other words, you were able to compress (or squeeze together) the air inside the bottle.  However, when you filled the bottle with water and capped it, you could not squeeze the bottle very much at all because you could not compress the water inside.

Gases such as air can be compressed, but liquids such as water, cannot be compressed.

Take a balloon and place it over the top of a pop bottle (2L is best with a little bit of water in it). Observe the size of the balloon now (@ room temperature). Now place it in a freezer for 10 minutes, remove it and observe size of balloon. Now take the bottle and hold it in a baking dish of almost-boiling water for 10 minutes. Now let the bottle sit on the table for 10 minutes. You should now see the balloon return to the same size as it was to start.

The greater the temperature, the faster the air particles move (increasing pressure), hitting the sides of the balloon more often and harder, making the balloon inflate more. The colder the air becomes though, the slower the air particles move (lowering pressure), resulting in the same amount of air now taking up less space. This is why the beach-ball you left in the garage over night will be “smaller” in the morning that it was during the day (when it was warmer).