The particle model and density

Three states of matter

The particle (kinetic) model treats matter as tiny particles in constant motion. Their arrangement, spacing and movement determine the bulk physical properties.

A solid keeps its own shape and volume; a liquid has fixed volume but fills the container shape; a gas fills the container completely.

Changes of state

solid ⇌ liquid (melting / freezing) ⇌ gas (boiling / condensing). Mass is conserved — no particles are created or destroyed.

Internal energy and temperature

Heating raises the kinetic energy of particles → they move faster → temperature rises. At a melting or boiling point added energy goes into breaking particle-particle forces (potential energy) instead of raising T, giving a flat plateau on a heating curve.

Density (P1 calculation)

ρ = m / V (kg/m³ or g/cm³)

Solids have the highest density (particles closest); gases the lowest. A regular solid: measure mass on a balance, calculate V = l × w × h. An irregular solid: use a displacement (eureka) can — V = volume of water displaced.

OCR PAG P1 — measuring density

Pupil practical: determine the density of a regular and irregular object. Common errors include not zeroing the balance, parallax on the measuring cylinder, and using mm where the formula expects cm.

OCR exam tip

When asked to "explain" why a property arises, link arrangement → forces → behaviour. e.g. "particles in a solid are in fixed positions held by strong forces, so a solid keeps its shape" — three half-marks pulled together.