Drifting smoke particles show how the surrounding gas (which is invisible) moves about.The 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.
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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