2.1Thermal Physics

Kinetic Particle Model of Matter — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions

10 min read 22 parts Lesson 1 of 45 Unit quiz

📚 Cambridge IGCSE Physics 0625 Hub / Topical Revision

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Lesson summary

2.1 Kinetic Particle Model of Matter (Part 1) — IGCSE PHYSICS notes: Describe the properties of solids, liquids and gases, name the changes of state and explain them using particle arrangement and forces.StateShapeVolumeFlowCompressibleSolidDefiniteDefiniteNoNoLiquidTakes the sha…

Freezing (solidification)
Boiling (vaporising)
Condensing
Nature of change
Always in motion
The word particles

Part 1 of 7Properties of the three states

State	Shape	Volume	Flow	Compressible
Solid	Definite	Definite	No	No
Liquid	Takes the shape of its container	Definite	Yes	No
Gas	Takes the shape of its container	Fills the container	Yes	Highly

Part 2 of 7Changes of state

Changes of state

Melting

Melting a solid turns to a liquid.

Freezing (solidification)

Freezing (solidification) a liquid turns to a solid.

Boiling (vaporising)

Boiling (vaporising) a liquid turns to a gas.

Condensing

Condensing a gas turns to a liquid.

Nature of change

Nature of change these are physical changes, so they are reversible.
The number of molecules and the mass do not change; only the energy changes.
All substances do this, but at different temperatures, so do not mention water unless asked.

Part 3 of 7Arrangement and motion of particles

Arrangement and motion of particles

Always in motion

Always in motion all particles move at any temperature.

Expansion

Expansion particles do not expand; the average separation changes.

Property	Solid	Liquid	Gas
Density	High	Medium	Low
Arrangement	Regular pattern	Random, no gaps	Random
Separation	Very close	Close, loosely packed	Widely spaced (about 10 times further apart)
Movement	Vibrate about fixed positions	Slide past each other	Fast and in all directions
Energy	Lowest	Higher	Highest

Part 4 of 7Forces between particles

Forces between particles

Solids

Solids strong intermolecular forces hold particles in place, so they only vibrate.
This gives a rigid shape and fixed volume.

Liquids

Liquids particles have enough energy to move past each other but not to escape.
The volume is fixed, but the liquid flows and changes shape.
Liquids are hard to compress because particles are close and repulsion is large.

Gases

Gases particles have overcome the forces holding them together and move randomly at high speed.
Large gaps let the gas be compressed, expand and flow freely.

The word particles

The word particles it can refer to atoms, molecules, ions or electrons.

Part 5 of 7Temperature and absolute zero

Higher temperature

Higher temperature the average speed and kinetic energy of the particles are greater.

Lowest energy

Lowest energy there is a temperature at which particles have the least kinetic energy, called absolute zero (−273 °C).

Part 6 of 7More key facts: States of Matter and Particle Arrangement

1Same particles

Same particles a substance has the same particles in every state, only the arrangement and energy differ.

2Density

Density solids and liquids are dense because their particles are close, while gases have a low density.

3Compression

Compression gases can be squashed because of the large gaps between particles.

4Flow

Flow liquids and gases flow because their particles can move past each other.

5Condensing and boiling

Condensing and boiling they are opposite processes between gas and liquid.

6Energy change

Energy change heating gives particles more energy and cooling takes it away.

Part 7 of 7More from the specification

1Gas Pressure and Molecular Collisions

Gas pressure arises from continuous, random collisions of gas particles with the container walls; each collision exerts a microscopic force,
and the total force per unit area on the walls is measured as pressure.
- (in units of pascals, Pa)

2Boyle's Law at Constant Temperature

For a fixed mass of gas at constant temperature, pressure is inversely proportional to volume (p·V = constant);
- compressing gas in a balloon or cylinder increases collision frequency with the walls, thereby increasing pressure.

3Absolute Temperature and Brownian Motion

Absolute temperature measured in kelvin (K = °C + 273) is directly proportional to the average kinetic energy of particles;
- Brownian motion provides experimental evidence for this continuous random kinetic motion as visible smoke or pollen particles collide with invisible fluid molecules.

4Inversely proportional

One quantity falls as the other rises in the same ratio.

5Constant

Not changing.

6Kelvin

SI unit of temperature on the absolute scale.

7Collision

Short interaction with large internal forces.

8Brownian motion

Random jiggling of small visible particles hit by fluid molecules.

9Constant volume

The container is rigid and sealed.

10p1V1

p1V1 p2V2

What the symbols mean: p₁, V₁ = pressure and volume before · p₂, V₂ = after; only for a fixed mass of gas at constant temperature

Key terms

Term	Meaning
Solid	A state with fixed shape and volume.
Liquid	A state with fixed volume that takes the shape of its container.
Gas	A state that fills its container and is highly compressible.
Intermolecular force	An attractive force between particles.
Absolute zero	The lowest possible temperature, −273 °C, where particles have least kinetic energy.
Condensing	A gas turning into a liquid.

Avoid these mistakes

Saying particles expand when a substance is heated — the separation increases, not the size of the particles.

Saying particles in a solid do not move — they vibrate.

Writing "water" in answers about states when no substance is named.

Saying mass changes during melting or boiling.

Last updated: 15 October 2026