Smoke fills the jar

The idea

Gases have neither a fixed shape nor a fixed volume and fill the entire available space, because their particles move freely in all directions and the interparticle attractions are negligible.

Incense smoke

A teacher lets the smoke from a burning incense stick collect in a gas jar.

What will the smoke do inside the jar?

• Fill the whole jar
• Stay as a small cloud at the bottom
• Stay in one corner

Let the smoke spread

Fill the jar with smoke. Then open the tap to the bigger jar.

The gaseous state

Gases have neither a fixed shape nor a fixed volume. They fill the entire available space, because their particles move freely in all directions and the interparticle attractions are negligible.

The tiny smoke particles are constantly hit by the invisible particles of the gases in air. Their movement helps us see how gas particles move. A teacher can also show this with iodine vapour in a closed gas jar.

• What we see | Why (particles)
• Smoke fills the whole jar | Gas particles move freely in all directions
• It spreads into the bigger jar too | The attractions between them are negligible

Notes

Gases have neither a fixed shape nor a fixed volume and fill the entire available space: their particles move freely in all directions and the attractions are negligible.

Check yourself

A gas in a small jar is let into a bigger jar. What happens to its volume?

Why does a gas fill all the space available to it?

Which describes a gas?

• It stays the same as before. A gas has no fixed volume. It fills all the space it gets.
• It spreads to fill the bigger space — correct. Yes. A gas has no fixed volume.
• It turns into a liquid. Letting it spread does not turn it into a liquid.
• Its particles move freely in all directions and their attractions are negligible — correct. Yes. Nothing holds them together.
• Its particles are held in fixed positions. That describes a solid.
• Its particles are pulled strongly towards the walls. There is no such pull. The particles move freely and the attractions are negligible.
• No fixed shape and no fixed volume — correct. Yes. It takes the shape of its container and fills it.
• A fixed shape and a fixed volume. That describes a solid.
• A fixed volume but no fixed shape. That describes a liquid.