Here's a quick experiment you can perform to prove to yourself that hot air is less dense than cool air.
What You'll Need:
Dry, empty 2-L soda bottle
Balloon
Hair dryer
Stretch the mouth of the balloon over the top of the soda bottle, and let the balloon hang down over the side. Heat the bottle with the hair dryer. What happens to the balloon?
Pretty quickly, you'll notice that the balloon starts to inflate. Now, you're not blowing into it. The hair dryer isn't blowing into it. There's no chemical reaction going on to create gas. Why is the balloon inflating?
The collapsed balloon, bottle, and the air inside the bottle weigh a certain amount, and there's a certain volume of air in the bottle. If we knew what the exact numbers were, we could calculate the density. When you heat the air, you make it less dense. You can't make it weigh less, because there are the same number of molecules in the cool air as in the hot air. The only way to change the density is to make the volume larger. Since the balloon is free to inflate, that's where the extra volume goes, and you see it begin to grow larger!
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