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 bits inside the balloon spread apart as it heats — same number of bits, just more spread out.
4The big test — does it actually rise?
Same air, just hotter. Will it lift?
Here's the trick: we don't add any air and we don't take any away. We only heat the air that's already inside. A scale will compare a balloon-scoop of inside air against the same-sized scoop of the cool air outside.
Guess before you find out
We heat the air inside the balloon without adding or removing a single bit. Compared to the same-sized scoop of cool air outside, what does that heated scoop now weigh?