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What are the properties of gases?

Drifting smoke particlesDrifting smoke particlesThe molecules in gases are much more widely spaced than those in liquids, so they move about more quickly. Gases have no shape of their own, so they spread to fill whatever container they are in. Gases can also be squashed to take up less space.

Water evaporating in the morning sunWater evaporating in the morning sun

A hot-air balloonA hot-air balloon


When a gas is heated, it expands: its molecules spread out and become less dense. As a result, warm or hot gas becomes lighter than cold gas. Lighter, warm air rises above heavier, colder air. This is why hot-air balloons rise. The air inside them is heated by a burner in the basket hanging underneath the envelope.

Nitrogen and oxygen molecules in the air zip about at an average speed of around 450 metres per second (about 1500 feet per second).

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