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Cambridge IGCSE Physics · 0625

Chapter 2: Thermal physics — Part 1

Topic 2.1.1–2.1.2 · States of matter and the particle model

States of matter

Solids have a fixed shape and volume. Liquids have a fixed volume but take the shape of the container. Gases fill the container and are easily compressed.

Changes of state: melting (solid to liquid), freezing (liquid to solid), boiling or evaporation (liquid to gas), condensation (gas to liquid). Direct solid-gas transfers are not required.

Particle model

Solid
Particles in a regular arrangement, close together, vibrating about fixed positions.
Liquid
Particles close together, random arrangement, able to move past each other.
Gas
Particles far apart, random, moving at high speed in all directions.

Higher temperature means particles move faster (more kinetic energy). There is a lowest possible temperature, absolute zero (-273 °C), where particles have least kinetic energy.

Gas pressure is due to particles colliding with a surface. More frequent or harder collisions mean higher pressure.

Particle diagrams of solid (regular close lattice), liquid (close, random, able to move past each other) and gas (far apart, random, high speed), with arrows showing vibration or movement.
Diagram 1: Particle diagrams of solid (regular close lattice), liquid (close, random, able to move past each other) and gas (far apart, random, high speed), with arrows showing vibration or movement.

Forces between particles and Brownian motion

Forces and distances between particles, and their motion, affect the properties of solids, liquids and gases (for example why gases are compressible).

Gas pressure can be described as a force per unit area from particles hitting surfaces.

The random motion of microscopic particles in a suspension (smoke, pollen) is evidence for the kinetic model. Fast, light molecules collide with the larger specks and knock them about. Call the specks microscopic particles; call the colliding particles atoms or molecules.

Brownian motion: a large microscopic particle in a gas or liquid being jostled by much smaller, faster, unseen molecules, producing an irregular path.
Diagram 2: Brownian motion: a large microscopic particle in a gas or liquid being jostled by much smaller, faster, unseen molecules, producing an irregular path.

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