Particle model

Revision notes

The particle model describes matter as particles with spaces and interactions between them. Solids have particles vibrating about fixed positions, liquids have particles that remain close but move past one another, and gases have widely separated particles moving randomly. Density is ρ = \(\dfrac{m}{V}\).

Density and the particle model

Density is mass per unit volume, ρ = \(\dfrac{m}{V}\). A dense material has more mass packed into each unit volume; this does not necessarily mean each individual particle is larger. Solids usually have particles closely packed in fixed relative positions, liquids have closely packed particles that can move past each other, and gases have particles much farther apart moving randomly.

For density calculations, convert units before substituting. Common errors come from mixing grams with kilograms or cubic centimetres with cubic metres. For an irregular solid, volume can be found by displacement if the object can be safely submerged.

Gas pressure

Gas pressure results from particles colliding with container walls and changing momentum. Increasing temperature at fixed volume increases average kinetic energy, so particles move faster and produce more frequent and/or harder collisions; pressure rises. Reducing volume at constant temperature gives particles less distance to travel between wall collisions, so collision frequency rises and pressure increases.

For qualitative gas questions, connect the chain explicitly: temperature or volume changes → particle motion/collision rate changes → force on walls changes → pressure changes.

Changes of state

Melting, freezing, boiling, condensing and sublimation are physical changes. The material's particles remain the same substance; their arrangement and energy change. Evaporation can occur below boiling point because higher-energy particles at the surface can escape. Evaporation cools the remaining liquid because the average kinetic energy of the particles left behind decreases.

Exam technique

Microscopic explanations earn marks when they mention particles, movement, spacing, collisions or energy—not when they simply restate the observation. If temperature stays constant during melting or boiling, explain where the transferred energy goes rather than saying 'no energy is needed'.

Gas pressure, volume and temperature

The pV relationship and work done on a gas are separate-physics content.

Worked example: a gas at 100 kPa is compressed at constant temperature from 3.0 × 10⁻³ m³ to 1.5 × 10⁻³ m³. p₂ = p₁V₁ ÷ V₂ = 100 × 3.0 ÷ 1.5 = 200 kPa.