Blog

Here’s a great excuse for a trip up a mountain! Grab some bottles and learn why it’s hard to breathe at the top of tall mountains.

You will need

• 2 empty and dry soft drink bottles of the same size
• A bag to carry the bottles in
• A mountain, at least 100 metres taller than the surrounding area
• A way to get to the top of the mountain (e.g. you could climb, or ask an adult to drive you)
• Marker (optional)

Safety: Ask an adult to go with you, and remember to be sun smart! If you’re going to climb the mountain take plenty of water to drink.

What to do

1. This activity is sensitive to temperature. Start with the bottles at room temperature. For example, if you just got out of a hot car, wait five minutes for the bottles to cool down.
2. At the bottom of the mountain, put a lid on one of the bottles and keep the other one open.
3. Go to the top of the mountain.
4. Put a lid on the other bottle. (You may wish to label the bottles with a marker, so you know which is which.)
5. Feel both bottles. Do they feel like they’re at the same pressure? (Don’t worry if the difference is not very noticeable – we found it was a lot better on the way back down.)
6. Go back to the bottom of the mountain. Do the two bottles feel the same as each other, or different? Do they feel the same as they did at the top of the mountain? How do they look?

What’s happening?

Earth’s atmosphere is heavy. It might not seem like it, but air weighs something, and Earth has quite a lot of air. If you drew a one-metre square on the ground, there would be about 10 000 kilograms of air directly above that square. That’s about the weight of two elephants!

If you go upwards, by climbing a mountain or hopping in a plane, there will be less air above you pushing down, and that means less air pressure. For every 100 metres you climb, the air pressure drops about 1%.

When air pressure drops, the volume of that air increases by the same percentage. That means 100 metres up, what was a litre of air is now about one litre and 10 mL. That should be enough of a difference for you to feel it when you squeeze the bottle.

On the way down, the process reverses. For every 100 metres you go down, a litre of air becomes about 990 mL. Depending on how tall your mountain is, you might be able to see the bottle being crushed a bit by the increased pressure.

These percentages keep adding up the further you go. The top of Mount Everest is above more than two-thirds of Earth’s air, and the air there is not being squeezed very much at all so it’s thin and hard to breathe. One litre of air captured at Everest’s summit would shrink to about 330 mL if you took it back to sea level!

If you’re after more science activities for kids, subscribe to Double Helix magazine!