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A sealed bag on a mountain

Air and fluids · level 3, lesson 5 of 6

A question to start

Take a sealed bag of crisps up a high mountain, or on a plane, and it puffs up like a pillow. Nobody has added any air. Why does it swell?

The idea

The higher you go, the less air there is above you, so the air pressure is lower. At the top of Mount Everest it is only about a third of the pressure at the seaside.

The sealed bag holds the air it was packed with at the bottom. That air keeps pushing out just as hard as before. But outside, the air pushes in less. So the air inside wins, and the bag swells.

bottom: a loose bagtop: puffed up

It also works the other way. Drink from a plastic bottle at the top of a mountain, close the lid, and carry it down. At the bottom, the bigger air pressure outside squashes the bottle in.

Worked examples

1. On a plane. The air in a plane's cabin is kept at a lower pressure than on the ground. Your crisps puff up, and your ears may "pop" as the air inside them evens out.

2. A balloon going up. A helium balloon that floats away grows bigger and bigger as it rises, until it bursts.

3. Going down. A sealed empty bottle from a mountain top gets crumpled by the time you are back at the seaside.

Try it

  1. Is the air pressure higher or lower at the top of a mountain?
  2. Why does a sealed bag puff up on a mountain?
  3. What happens to a sealed bottle carried from a mountain top down to the sea?
  4. Why does a helium balloon that floats away get bigger?
Answers
  1. Lower.
  2. The air inside pushes out as hard as before, but the air outside pushes in less.
  3. It gets squashed in.
  4. As it rises, the air pressure outside falls, so the gas inside spreads out.

Common mistake

"The crisps make gas." No new air appears. The same air inside spreads out because there is less air pushing on the bag from outside.

Where it leads