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Particle Model

Compressing gases, and the force a gas exerts

  • 3 things to know
  • Paper 2

Overview

Why gases compress and expand as pressure changes, syringes and balloons carried up a mountain, why liquids cannot be compressed, and gas pressure pushing at right angles to every surface, which gives an inflated balloon its rounded shape.

What you need to know

Answer each one before you reveal it.

Your recallNot started
Key term

Explain why a sealed balloon of air gets bigger when it is carried up a mountain.

Key term

Explain why the air inside an inflated balloon pushes outwards on every part of its skin.

Key term

Explain why an inflated balloon takes up a rounded shape.

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    Explain why a gas can be compressed into a smaller volume.

    Model answer

    Its particles are widely spaced, so raising the pressure pushes them closer together.

  2. 2

    Describe what happens to a gas when the pressure acting on it is reduced.

    Model answer

    It expands, because its particles spread out to fill a larger volume.

  3. 3

    The plunger of a sealed syringe of air is pushed in. Describe what happens to the spacing of the air particles inside.

    Model answer

    They are pushed closer together.

  4. 4

    Explain why a sealed balloon of air gets bigger when it is carried up a mountain.

    Model answer

    The air pressure outside falls, so the gas inside expands until the pressures balance again.

  5. 5

    Explain why a liquid cannot be compressed in the way that a gas can.

    Model answer

    Its particles are already touching, so there are no large gaps left to close up.

  6. 6

    State the direction of the force that the pressure of a gas produces on a surface.

    Model answer

    At right angles to that surface.

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