Particle model

Revision notes

Matter can be modelled as particles whose arrangement and motion differ in solids, liquids and gases. Density is mass divided by volume. Internal energy is the total kinetic and potential energy of the particles in a system. Heating can increase temperature or cause a change of state. Specific heat capacity links energy transfer, mass and temperature change; specific latent heat links energy transfer and change of state. Gas pressure is caused by particle collisions with container walls. At fixed volume, increasing temperature increases average particle kinetic energy and raises the pressure.

States of matter and density

The particle model explains the main differences between solids, liquids and gases. Solid particles vibrate about fixed positions, liquid particles remain close but can move past one another, and gas particles are much farther apart and move randomly.

Density is mass per unit volume, ρ = \(\dfrac{m}{V}\). A material with greater density has more mass in a given volume. Convert units before substituting into the equation.

Internal energy and heating

Internal energy is associated with the microscopic kinetic and potential energies of particles. Heating can increase temperature, change state, or both depending on the situation.

Specific heat capacity links energy transfer to mass and temperature change through E = mcΔT. During a change of state, energy can be transferred even when temperature remains constant because particle arrangement and potential energy are changing.

Gas pressure

Gas pressure is caused by particles colliding with container walls. At fixed volume, increasing temperature increases average particle kinetic energy, so collisions produce a greater pressure.

Exam technique

Use particle language in explanation questions: mention spacing, motion, collisions or energy. Do not say particles themselves expand when a substance is heated. In density questions, distinguish clearly between mass, volume and density.

Changes of state

Melting, freezing, boiling and condensing change particle arrangement and energy without changing the chemical identity of the substance. During a state change, transferred energy can alter particle potential energy while temperature remains constant.

Evaporation occurs at the surface and can happen below boiling temperature. Faster particles are more likely to escape, so the average kinetic energy of the particles left behind can decrease, producing cooling.