1Two things to know about air
Air is tiny bouncing bits — and heat makes them spread
You can't see them, but air is made of zillions of little bits zooming around. Watch what they do when things warm up:
Air = tiny bits
Air is stuff. It's made of tiny bits bouncing everywhere. They're light, but they're real — and they take up room.
Heat = more spread
Warm them up and they zoom faster and bump farther apart. Same number of bits, but now they fill more space.
2So a balloon can hold air two ways
Crowded balloon vs roomy balloon
The bits are packed in tight
When the air inside is cool, its bits are just as packed as the air outside. The balloon is crowded — and it's holding just as much air as the same-sized scoop of air around it.
The bits spread out and leave
Heat the inside air and its bits spread apart. The balloon is the same size, so some bits get pushed out the bottom. Now the balloon is roomy — it holds fewer bits than the same scoop of cool air outside.
3Your turn — work the burner
Turn up the flame and watch the bits
Slide the flame up and down. Watch the air inside spread out as it heats. The balloon keeps roughly the same size, so some warm air leaves through the open bottom.
4The big test — does it actually rise?
Can a hot balloon carry any load?
The burner and balloon are unchanged, but now the basket carries extra cargo. Compare the weight of everything inside the outline — warm air, balloon, basket and cargo — with the outside air displaced.
Guess before you find out
The balloon can lift a light basket. With a much heavier load and the same maximum heat, must it still rise?
First test: heavy load, 50 weight units. Heat it fully and watch. The numbers are an illustrative model.