Squeeze air, squeeze water

The idea

Gas particles have a lot of space between them, which external pressure can reduce, so gases can be compressed; water and other liquids are practically incompressible.

A closed syringe

Take a syringe without a needle. Close its nozzle with a finger and push the plunger. First it has air in it, then water.

Which one lets the plunger go in more?

• Air
• Water
• Both the same

Push the plunger

Push the plunger with air inside, then let go. Then try it with water.

Compressing gases

Gas particles have a lot of space between them. External pressure can reduce this space, so gases can be compressed. Water is practically incompressible.

• In the closed syringe | When we push the plunger
• Air | It goes in: the particles are forced closer. Let go, and it moves back
• Water | It hardly moves: water is practically incompressible

Notes

Gas particles have a lot of space between them; external pressure reduces it, so gases can be compressed. Water is practically incompressible.

Check yourself

Why can air be compressed?

The nozzle of a syringe full of water is closed. You push the plunger. What happens?

You stop pushing the plunger of a closed syringe of air. What happens?

• Air has no particles. Air is made of particles, with a lot of space between them.
• Its particles get smaller when pushed. The particles do not shrink. The spaces between them get smaller.
• Its particles have a lot of space between them — correct. Yes. Pushing reduces that space.
• It hardly moves — correct. Yes. Water is practically incompressible.
• It goes in easily. That happens with air. Water is practically incompressible.
• The water turns into a gas. Pushing does not turn water into a gas.
• The air leaks out. The nozzle is closed, so no air leaks. The particles spread again.
• The particles spread and the plunger moves back — correct. Yes. The air goes back to its original volume.
• The plunger stays pushed in. The air particles spread again and push the plunger back.