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4.3.3Properties of Matter

The Kinetic Model — SQA National 5 Physics Revision Notes & Topical Questions

5 min read 4 parts Lesson 9 of 11

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Part 1 of 4The Kinetic Model of a Gas

4.3.3 The Kinetic Model

The model describes a gas in terms of moving particles and lets us predict how it behaves when conditions change.

Particles
  • A gas is made of a very large number of tiny particles with huge spaces between them, so most of a gas is empty space.
Random motion
  • Particles move rapidly in straight lines in random directions until they collide with another particle or with a wall.
Collisions
  • Collisions between particles and with walls are elastic, so no kinetic energy is lost overall.
Forces
  • Apart from collisions, the particles exert almost no forces on each other, so they spread out to fill the whole container.
Pressure
  • The pressure is the combined force of all wall collisions divided by the wall area.

Part 2 of 4Explaining the Gas Variables

Explaining the Gas Variables
Pressure and volume
  • Compressing a gas at constant temperature gives the same particles a smaller space,
    • so they hit the walls more often and the pressure rises.
Pressure and temperature
  • Heating a gas in a fixed volume makes particles faster, so collisions are harder and more frequent and the pressure rises.
Volume and temperature
  • Heating a gas at constant pressure makes it expand,
    • because faster particles push the walls outward until the collision rate falls back.
Cooling
  • Cooling reverses each effectslower particles produce fewer, weaker collisions, so the pressure falls or the volume shrinks.
Average speed
  • The temperature controls the average particle speed, while volume controls how far particles travel between wall collisions.
Change madeParticle behaviourResult
Temperature up, volume fixedFaster, harder collisionsPressure rises
Volume down, temperature fixedMore frequent collisionsPressure rises
Temperature up, pressure fixedFaster particles push walls outVolume rises

Part 3 of 4Writing a Good Kinetic Model Explanation

1
Begin by stating what has changed, such as the temperature rising or the volume being reduced.
2
Describe the effect on the particles: their average speed, or the distance they travel before reaching a wall.
3
State how the collisions change, using both the frequency of collisions and the force of each collision where relevant.
4
Link the change in collisions to the change in pressure or volume that was asked about.
5
Keep the other variables constant in your reasoning so that only one cause is being explained at a time.

Part 4 of 4Exam Checklist & Tips

4.3.3 – Key Points for the Exam

  • Kinetic model A description of matter in terms of the motion and energy of its particles.
  • Elastic collision A collision in which no kinetic energy is lost.
  • Random motion Movement in unpredictable directions with no overall pattern.
  • Average kinetic energy The mean energy of motion of the particles, which depends on temperature.
  • Compression Reducing the volume of a gas so that its particles are closer together.

Common Mistakes

  • Claiming that particles themselves expand when a gas is heated; only the spacing and speed change.
  • Saying particles collide with each other more as the cause of pressure, instead of collisions with the container walls.
  • Forgetting to mention both the frequency and the force of collisions in an explanation.
  • Stating that pressure rises when volume rises at constant temperature.

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