Describe the particle structure of solids, liquids and gases in terms of the arrangement, separation and motion of the particles and represent these states using simple particle diagrams

2.1 Kinetic Particle Model of Matter

Learning Objective (AO1, AO2)

Describe the particle structure of solids, liquids and gases in terms of arrangement, separation and motion, explain how temperature, pressure and phase changes relate to the particle model, and represent each state with a simple labelled particle diagram.

1. The Particle Model – Overview

The particle model assumes that all matter consists of tiny particles (atoms or molecules) that are in constant motion. Macroscopic behaviour of a material can be explained by three microscopic factors:

• Arrangement – the pattern in which particles are positioned relative to one another.
• Separation – the average distance between neighbouring particles.
• Motion – the type and magnitude of movement each particle exhibits.

2. Temperature, Kinetic Energy & Absolute Zero

• Increasing the temperature of a substance raises the average kinetic energy of its particles, making them move more vigorously and, if enough energy is supplied, causing a change of state.
• At absolute zero (‑273 °C or 0 K) particles possess the minimum possible kinetic energy; in an idealised picture they would be at rest, although quantum‑mechanical zero‑point motion still persists.
• Ideal‑gas kinetic‑energy relation (supplementary):

  \$\langle Ek\rangle = \tfrac{3}{2}\,k{\mathrm B}T,\$

  where \$k_{\mathrm B}\$ is Boltzmann’s constant and \$T\$ is the absolute temperature. This formula is not required for the core exam but may appear in extension questions (AO3).

3. Pressure of a Gas

Gas pressure is the macroscopic result of countless microscopic collisions of particles with the walls of their container.

• More frequent or more energetic collisions (caused by higher temperature, a larger number of particles, or a smaller volume) produce a higher pressure.
• Proportionality statement (core):

  \$p \;\propto\; \frac{N \langle E_k\rangle}{V},\$

  where \$N\$ is the number of particles, \$\langle E_k\rangle\$ the average kinetic energy and \$V\$ the volume. This links directly to the macroscopic definition \$p = F/A\$ and to the ideal‑gas law \$pV = nRT\$ (supplementary).

4. Evidence for the Particle Model – Brownian Motion

When microscopic particles (e.g. pollen grains) are suspended in a liquid they exhibit a jittery, random motion called Brownian motion. This motion is caused by invisible collisions with the much smaller, rapidly moving liquid molecules, providing direct visual evidence for the kinetic particle model.

5. States of Matter

5.1 Solids

• Arrangement: Particles are packed very close together in a regular, fixed pattern (a lattice).
• Separation: Minimal – particles are almost touching.
• Motion: Particles vibrate about fixed positions; they do not translate or rotate freely.
• Macroscopic properties: Definite shape and definite volume; very low compressibility.

5.2 Liquids

• Arrangement: Particles remain close but are arranged irregularly and are not fixed.
• Separation: Slightly larger than in a solid, allowing particles to slide past one another.
• Motion: Both vibration and translational (sliding) motion occur.
• Macroscopic properties: Indefinite shape, definite volume; low compressibility; takes the shape of its container.

5.3 Gases

• Arrangement: No regular arrangement; particles are far apart.
• Separation: Large – the volume occupied by the particles themselves is negligible compared with the container volume.
• Motion: Particles move rapidly in straight lines, colliding elastically with each other and with the container walls.
• Macroscopic properties: Indefinite shape and indefinite volume; very high compressibility; expands to fill any container.

6. Comparison of the Three States

7. Supplementary Box – Inter‑particle Forces & Ideal‑Gas Behaviour (AO3)

8. Links to Other Syllabus Sections (AO2)

The particle model underpins many other topics in the Cambridge IGCSE Physics syllabus:

• 2.2 Phase changes – energy supplied during melting or boiling is used to overcome inter‑particle forces, not to raise temperature.
• 2.3 Thermal energy transfer – conduction, convection and radiation are explained by particle collisions and vibrations.
• 2.4 Specific heat capacity – the amount of energy required to change the kinetic energy of particles.

9. Key Points to Remember (AO1)

1. The particle model links microscopic behaviour (arrangement, separation, motion) to macroscopic properties (shape, volume, compressibility, pressure).
2. Raising temperature adds kinetic energy to particles; at a phase‑change temperature the added energy is used to overcome attractive forces rather than to increase temperature.
3. Absolute zero (‑273 °C or 0 K) is the lowest possible temperature; particles have minimum kinetic energy.
4. Phase changes involve changes in particle arrangement and separation only – the particles themselves do not change.
5. Gas pressure originates from particle collisions with container walls; pressure ∝ (number of particles × average kinetic energy)/volume.
6. Brownian motion provides observable evidence that invisible particles are constantly in motion and colliding with surrounding molecules.

10. Sample Examination Question (AO2, AO3)

Question: Explain, using the particle model, why a gas can be compressed much more easily than a liquid.

Answer outline:

• In a gas the particles are far apart, so most of the container volume is empty space.
• Applying an external pressure reduces the distances between particles, decreasing the volume the gas occupies (p ∝ N⟨E_k⟩/V).
• In a liquid the particles are already close together; there is little empty space to remove, so the volume changes only slightly under pressure.
• Therefore gases are far more compressible than liquids.

11. Suggested Practical Activity – Observing Brownian Motion (AO2)

1. Place a drop of milk or a suspension of pollen grains on a microscope slide.
2. Observe under low magnification; the tiny grains will jiggle randomly.
3. Explain that the jitter is caused by invisible collisions with water molecules – a direct demonstration of the kinetic particle model.

12. Glossary of Key Terms (AO1)

• Particle model – the concept that matter consists of tiny particles in constant motion.
• Absolute zero – the temperature at which particles have minimum kinetic energy (0 K, ‑273 °C).
• Brownian motion – random jittery movement of visible particles caused by collisions with invisible molecules.
• Compressibility – the degree to which a substance’s volume decreases under pressure.
• Inter‑particle forces – attractive forces acting between neighbouring particles; strongest in solids, weaker in liquids, negligible in gases.
• Ideal‑gas law – the relationship pV = nRT, derived from the kinetic‑theory description of gases.