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What is a gas?

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Quick answer

A gas is a state of matter with no fixed shape or volume — it spreads out to fill whatever container it's in. Its particles are far apart and move quickly in all directions, so a gas can be compressed or expanded far more than a solid or liquid.

Key things to understand

  1. A gas has neither a fixed shape nor a fixed volume — it expands to fill any space available.
  2. Its particles are far apart and zoom around freely, colliding with each other and the container walls.
  3. Those collisions with the walls are what we feel as gas pressure.
  4. Because there's so much empty space between particles, gases are easily compressed.
  5. Cooling a gas enough condenses it into a liquid; the air around us is a mix of gases.

People also ask

Why does a gas fill its container?
Its particles have lots of energy and are barely held together, so they fly apart in all directions until they spread evenly through whatever space they're in.
What is gas pressure?
It's the force from gas particles constantly colliding with the walls of their container. More particles or more heat means more collisions — and higher pressure.
Why can gases be compressed but liquids can't?
A gas is mostly empty space between far-apart particles, so squeezing pushes them closer. A liquid's particles are already close together, leaving little room to compress.