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Solids, Liquids and Gases

Compare how particles are arranged and move in each state.

  • Define and explain Solids, Liquids and Gases in your own words
  • Use key terms such as particle accurately
  • Apply what you have learned to new examples and questions
  • Avoid the common mistakes learners make with this topic

This lesson focuses on Solids, Liquids and Gases: compare how particles are arranged and move in each state.

Definition: Solids, Liquids and Gases

Compare how particles are arranged and move in each state.

Key ideas

Arrangement decides the state

In a solid, particles are packed tightly in fixed positions and only vibrate, which is why solids keep their shape. In a liquid, particles are still close but can slide past each other, so liquids flow. In a gas, particles are far apart and move quickly in all directions, so gases spread out to fill any container.

Particles are always moving

In the particle model, every substance is made of tiny particles with empty space between them, and the particles are in constant motion. The hotter the substance, the faster its particles move — heating does not make new particles, it makes the existing ones move more. This one idea explains expansion, diffusion and changes of state.

Key term — particle: An extremely small piece of matter; everything is made of particles that are constantly moving.

Worked example: Solids, Liquids and Gases

Describe the arrangement and movement of particles in a solid.

Particles are packed tightly in fixed positions and vibrate; they cannot move around, which is why solids keep their shape.

Answer: Particles are packed tightly in fixed positions and vibrate; they cannot move around, which is why solids keep their shape.

Common mistakes
  • Saying particles melt Particles do not melt, expand or shrink — the substance does. Heating makes particles move faster and spread out; the particles themselves stay the same size.
  • Thinking particles are still in solids Even in a solid, particles vibrate about fixed positions. They only stop moving at absolute zero (−273 °C), a temperature nothing in everyday life reaches.

Practice

Explain why gases spread out to fill their container.
How much space is between gas particles?

Gas particles are far apart and move rapidly in all directions, so they spread until they fill the whole container.

A liquid is heated in a sealed, rigid container and the pressure rises. Explain using particles.
What happens when faster particles hit the walls?

Heating makes particles move faster, so they hit the container walls harder and more often, which increases the pressure.

Why does diffusion happen faster in hot water than in cold water?
What does heating do to particle motion?

Particles move faster when hot, so they spread and mix more quickly.

A puddle dries up on a warm day. Explain what happens to the water particles.
Where do the fastest particles go?

The fastest water particles escape from the surface into the air as a gas; this is evaporation.

Quick check

Solids, Liquids and Gases — quick check

Which of these best defines "particle"?

An extremely small piece of matter; everything is made of particles that are constantly moving.

A thermometer in melting ice reads 0 °C the whole time. Explain why the temperature does not rise.

The energy goes into loosening the particles from their fixed positions rather than raising their speed, so the temperature stays constant until all the ice has melted.
Key takeaways
  • Solids, Liquids and Gases: compare how particles are arranged and move in each state.
  • Arrangement decides the state: In a solid, particles are packed tightly in fixed positions and only vibrate, which is why solids keep their shape.
  • diffusion: The spreading of particles from an area of high concentration to an area of low concentration.
  • Watch out for: saying particles melt
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