P3 Particle model of matter — Section Overview

The particle model explains the properties of solids, liquids and gases in terms of the behaviour of their particles (atoms and molecules). It underpins our understanding of pressure, temperature, density and the changes of state that matter undergoes.

What this section covers

Sub-topic	Key ideas
P3.1 Changes of state	Melting, freezing, evaporation, boiling, condensation, sublimation; latent heat
P3.2 Internal energy	Total KE + PE of all particles; temperature as mean KE
P3.3 Particle motion in gases	Pressure from particle collisions; absolute zero; pV = nRT ideas
P3.4 Density	ρ = m/V; comparing densities of solids, liquids and gases

Key concepts

States of matter: Solid particles vibrate in fixed positions. Liquid particles move randomly but stay in contact. Gas particles move rapidly and are widely separated. Increasing temperature increases the average kinetic energy of particles.

Internal energy: The total energy stored in the kinetic and potential energy stores of all particles. Heating raises internal energy — this either increases temperature (particles move faster) or increases potential energy (changes of state, where temperature stays constant).

Latent heat: The energy needed to change state WITHOUT changing temperature. Specific latent heat of fusion (solid↔liquid) and vaporisation (liquid↔gas). Formula: Q = mL.

Gas pressure: Caused by particles colliding with container walls. More particles, higher temperature, or smaller volume → more collisions → higher pressure.

Absolute zero: −273 °C (0 K) — the temperature at which particles have zero kinetic energy (minimum possible internal energy). Temperature in kelvin: T(K) = T(°C) + 273.

Exam focus

• Density questions require the formula ρ = m/V; check units (kg/m³ or g/cm³).
• Latent heat questions: no temperature change during a change of state.
• For gas law questions (higher tier): show clear substitution and state the assumption (fixed mass, constant temperature etc.).