GCSE Physics Density & particles revision

The model earns its keep by explaining properties: solids keep their shape because particles cannot swap positions; liquids flow because they can slide while staying close (which also makes them nearly incompressible — the particles already touch); gases compress easily because most of a gas is empty space between particles.

The six named changes: melting (solid → liquid), freezing (liquid → solid), boiling/evaporating (liquid → gas), condensing (gas → liquid), sublimation (solid straight to gas). They are physical changes: particles stay the same substance, so mass is conserved and the change reverses. Internal energy is the total kinetic energy of the particles (their motion — set by temperature) plus their potential energy (their positions and the forces between them). Heating raises internal energy. During a change of state, the energy supplied does not raise the temperature — it goes into breaking or forming the arrangement, changing the potential part of the internal energy instead.

On a cooling curve the flat sections are exactly where the substance is freezing or condensing: particles bond back together and release latent heat, which holds the temperature steady while the surroundings take the energy. Evaporation differs from boiling — it happens at the surface at any temperature, while boiling bubbles throughout the liquid at the boiling point.

Density is mass per unit volume: ρ = m ÷ V, in kg/m³ or g/cm³, with 1 g/cm³ = 1 000 kg/m³. Densities tell stories — cork floats on water because it is less dense; gold sinks because 19 300 kg/m³ dwarfs water’s 1 000 kg/m³. A state change conserves mass but changes volume, which is why ice (about 917 kg/m³) floats on its own meltwater — freezing pushes the particles into an open arrangement that takes up more room.

1. 1Regular solid

   Measure mass on a balance; measure dimensions with a ruler (or vernier callipers) and compute volume; ρ = m ÷ V.

2. 2Irregular solid

   Measure mass on a balance. Find volume by displacement: lower the object into a eureka can filled to the spout and catch the displaced water in a measuring cylinder — the water’s volume equals the object’s volume.

3. 3Liquid

   Mass an empty measuring cylinder, add a known volume of liquid, mass again; the mass difference divided by the volume is the density.

4. 4Precision

   Use a balance to 0.1 g or better, read the bottom of the meniscus at eye level, repeat and average; the displacement method carries the biggest uncertainty (droplets left in the can).

The specific latent heat (L) of a substance is the energy needed to change the state of 1 kg of it with no change of temperature. Specific latent heat of fusion covers melting/freezing; specific latent heat of vaporisation covers boiling/condensing. The equation: E = m L — energy in joules when m is in kg and L is in J/kg. Vaporisation always has the larger L: separating particles completely takes far more energy than merely loosening the arrangement. Water’s values are huge — L(fusion) ≈ 334 000 J/kg and L(vaporisation) ≈ 2 260 000 J/kg — which is why scalding steam is so dangerous: condensing on skin releases over 2 MJ per kilogram.

Gas particles move fast and randomly, colliding with every surface of the container. Each collision delivers a tiny force; billions per second add up to a steady pressure (force per unit area). Heating the gas at fixed volume makes the particles faster, so collisions are more frequent and harder — pressure rises. Compressing the gas into a smaller volume packs the particles closer, so each particle hits the walls more often — pressure rises again. At fixed temperature the product p V = constant (for a fixed mass of gas): compress to half the volume and the pressure doubles. And compression itself heats the gas: pushing the piston in does work on the gas, transferring energy to the particles’ kinetic store — the reason a bike pump warms.