Particle model: density and internal energy
Physics · GCSE · The School
Density is mass packed into a volume
Density = mass ÷ volume, in kg/m³. Particles in a solid are packed close together, so a solid is usually denser than the same substance as a gas, where particles are far apart. Ice is the well-known exception: its particles lock into a more open structure than liquid water, so ice is LESS dense than water and floats.
Three states, one particle model
A solid holds particles in fixed positions, vibrating on the spot. A liquid keeps them close but free to move past each other. A gas spreads them far apart, moving randomly and fast. Changing state means giving particles enough energy to overcome the forces holding them in the tighter arrangement, not creating new particles or changing what they are.
Heating raises internal energy
Internal energy is the total kinetic and potential energy of a substance’s particles. Heating usually raises kinetic energy, so particles move faster and temperature rises. During a change of state, energy instead raises potential energy, breaking bonds between particles — and while that is happening, temperature does not rise at all, even though heating continues.
Specific heat capacity: a worked calculation
Change in thermal energy = mass × specific heat capacity × change in temperature, or ΔE = mcΔT. Water’s specific heat capacity is 4,200 J/kg°C. To heat 2 kg of water from 20°C to 80°C: ΔT = 60°C, so ΔE = 2 × 4,200 × 60 = 504,000 J. The same method finds the energy released when something cools, just with ΔT taken as negative.
Gas pressure comes from particle collisions
Gas particles constantly collide with the walls of their container, and pressure is the total force of those collisions spread over the area. Heating a gas at constant volume speeds the particles up: they hit the walls harder and more often, so pressure rises. Nothing about the particles themselves changes — only how fast and how often they collide.
Two mistakes this topic invites
Particles do not get bigger when heated — they move faster and spread further apart, which is why substances expand. And boiling is not the same as evaporation: evaporation happens at any temperature, only at the surface, from whichever particles happen to have enough energy to escape; boiling happens throughout the liquid at one fixed temperature, its boiling point.