Gases fill all the space there is

Liquids keep their volume. Do gases? Activity 7.5 shows what a gas does when it's given more room. Be careful with the burning incense stick.

Take two clear gas jars or glasses, A and B. Light an incense stick and hold jar A upside down over the smoke so it fills with smoke. Turn it over and cover it with a glass plate.

Now hold jar B upside down on top of the plate. Slowly slide the plate out, keeping the jars mouth to mouth so no smoke escapes. Watch.

The smoke spreads up into jar B and fills all of the space in both jars. It doesn't stay in the space it had before. So gases have no fixed volume: they spread to fill all the space they can get, and like liquids they take the shape of their container.

That tells us about the particles. In a gas, the particles move freely in all directions and the attractions between them are negligible, almost nothing. So a gas has neither a fixed shape nor a fixed volume.

Why use smoke? You can't see air, so the smoke stands in for it. Tiny smoke particles floating in the air are constantly bumped by the invisible gas particles, so watching the smoke move lets you watch the gas move. Iodine vapour works too (leave a little solid iodine in a closed jar for a while), but be careful, because iodine vapour can irritate your eyes and nose.

Liquids and gases both flow, and neither keeps a fixed shape. Those two properties set them apart from solids and put them in one group: fluids.