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Air is all around

Air and fluids · level 1, lesson 1 of 6

A question to start

Push a cup upside down into a bowl of water, with a scrunched-up tissue stuck in the bottom. Lift it out. The tissue is still dry. What kept the water out?

The idea

The cup was not empty. It was full of air.

You cannot see air, smell it or hold it, but it is real stuff, and it takes up space. When the cup goes into the water upside down, the air inside has nowhere to go, so the water cannot get in.

dry tissueair insidewater

There are other ways to tell air is there:

  • Blow into a balloon and it grows. The air you blow in fills it up.
  • Wave your hand fast near your face. You can feel the air move.
  • Blow through a straw into water and bubbles of air come out.

Worked examples

1. The upside-down cup. Air fills the cup. The water pushes up, but the air is in the way, so the tissue stays dry.

2. Tip the cup. Tilt it under the water and a big bubble of air escapes. Now water rushes in and the tissue gets wet. The air had to get out before the water could get in.

3. A "flat" ball. A flat football is not empty either: there is a little air in it. Pumping it up pushes in much more air, and it goes hard and round.

Try it

  1. Can you see air?
  2. Why does the tissue in the upside-down cup stay dry?
  3. What happens if you tilt the cup under the water?
  4. Name one thing that shows air is real, even though you cannot see it.
Answers
  1. No. Air is clear, but it is still there.
  2. The cup is full of air, and the water cannot get in while the air is in the way.
  3. A bubble of air comes out, and water goes in.
  4. Any one of these: a balloon gets bigger when you blow in air; you can feel air when you wave your hand; bubbles come out when you blow through a straw into water.

Common mistake

"An empty cup has nothing in it." An "empty" cup is full of air. It only looks empty because air is clear.

Where it leads