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
Cambridge IGCSE Physics 0625 – 2.1.2 Particle Model
2.1.2 Particle Model
Objective
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.
Particle Model Overview
The particle model assumes that all matter is made up of tiny particles (atoms or molecules) that are in constant motion. The state of matter (solid, liquid or gas) is explained by three key factors:
Arrangement: How the particles are positioned relative to one another.
Separation: The average distance between neighbouring particles.
Motion: The type and magnitude of movement each particle exhibits.
Solids
In a solid:
Particles are packed very close together in a regular, fixed pattern (often a lattice).
Separation between particles is minimal – they touch or are almost touching.
Particles vibrate about fixed positions but do not translate or rotate freely.
Suggested diagram: Particles in a solid shown as tightly packed circles in a regular grid, with small arrows indicating vibrational motion.
Liquids
In a liquid:
Particles are still close together but the arrangement is disordered and not fixed.
Separation is slightly greater than in a solid, allowing particles to slide past one another.
Particles have both vibrational motion and translational motion, moving past each other while remaining in contact.
Suggested diagram: Particles in a liquid shown as loosely packed circles with arrows indicating both vibration and sliding motion.
Gases
In a gas:
Particles are far apart and have no regular arrangement.
Separation is large; the volume occupied by the particles themselves is negligible compared with the volume of the container.
Particles move rapidly in straight lines, colliding elastically with each other and with the walls of the container.
Suggested diagram: Particles in a gas shown as widely spaced circles with long arrows indicating rapid, random motion.
Comparison of the Three States
Property
Solid
Liquid
Gas
Particle arrangement
Regular, fixed lattice
Irregular, close but not fixed
No fixed arrangement
Average separation
Very small
Small, slightly larger than solid
Large
Particle motion
Vibration about fixed points
Vibration + translation (sliding)
Rapid translation in all directions
Shape & volume
Definite shape & volume
Indefinite shape, definite volume
Indefinite shape & volume
Compressibility
Very low
Low
High
Key Points to Remember
The particle model explains macroscopic properties (shape, volume, compressibility) from microscopic behaviour.
Increasing temperature adds kinetic energy, causing particles to move more vigorously and, at phase‑change points, to overcome attractive forces.
Phase changes (melting, boiling) involve changes in particle arrangement and separation, not a change in the particles themselves.
Sample Examination 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 the container volume is mostly empty space.
Applying an external pressure reduces the distance between particles, decreasing the volume occupied by the gas.
In a liquid the particles are already close together; there is little empty space to remove, so the volume changes only slightly under pressure.
Mathematical Representation (Optional)
The average kinetic energy of a particle in an ideal gas is given by:
\$\$
\langle Ek \rangle = \frac{3}{2}k{\mathrm{B}}T
\$\$
where \$k_{\mathrm{B}}\$ is Boltzmann’s constant and \$T\$ is the absolute temperature.