Gas — Science definition

A state of matter with no fixed shape or volume; particles are far apart and move freely and quickly.

Where it is taught

In use

Many everyday materials are described using three familiar states — solid, liquid, or gas — and the real difference between them comes down to how the tiny particles inside are arranged and how fast they're moving.

States of Matter & Phase Changes

Questions using gas

Problem 1

Dry ice (solid carbon dioxide) turning directly into a gas without becoming a liquid is an example of…
  1. melting
  2. sublimation
  3. condensation
  4. freezing
Answer and explanation

sublimation

Sublimation is the direct change from solid to gas.

Problem 2

Changing from a liquid to a gas is
  1. condensation
  2. evaporation
  3. freezing
  4. sublimation
Answer and explanation

evaporation

Evaporation (or boiling) takes a liquid to a gas; condensation is the reverse.

Problem 3

A sealed flask of gas is heated. Explain what happens to the particles and to the pressure.
Answer and explanation

The particles gain kinetic energy and move faster, so they strike the walls more often and harder. Because the flask is sealed the volume cannot change, so the pressure rises. The number of particles has not changed.

Response criteria

  • State a scientifically accurate conclusion.
  • Show the data, mechanism, or calculation that supports it.
  • Include units, conditions, or limits where relevant.

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