Energy and particles · GCSE Physics
Particle model of matter
Density, changes of state, internal energy, specific latent heat, and how gas pressure depends on particle motion.
Density is mass over volume. Changing state does not change mass or the substance — it changes the arrangement and the potential store. Latent heat is the energy for that change, without a temperature change.
The important bits
What you need to know
- 1
Density ρ = m / V. Solids are usually denser than liquids, which are denser than gases, because particles are closer together. Ice is an exception: it is less dense than water.
- 2
The particle model: solids — regular, vibrating about fixed positions; liquids — close, able to flow; gases — far apart, moving randomly at high speed.
- 3
Internal energy is the total kinetic and potential energy of the particles. Heating raises the kinetic store (temperature) or the potential store (change of state).
- 4
Specific latent heat of fusion is energy to change 1 kg from solid to liquid at constant temperature. Vaporisation is liquid to gas. E = mL.
- 5
Mass is conserved at a change of state. Temperature stays constant while the change happens, because the energy is used to break or form intermolecular forces, not to raise kinetic energy.
- 6
Gas pressure is caused by particles colliding with the walls. At constant volume, raising temperature raises pressure because particles move faster and collide more often, with more force.
- 7
The density practical: measure mass on a balance; find volume of a regular solid from lengths, or of an irregular solid from displacement of water. Repeat and average.
Go deeper
Temperature and state are different jobs for the same energy
When you heat ice at 0 °C, the temperature does not rise until it has all melted. The energy is still being transferred; it is increasing the potential store as bonds between particles loosen. That energy is mL. After that, temperature rises according to mcΔθ until 100 °C, then another plateau for boiling. A heating curve with two flat sections is this story as a graph. Students often mark the flat bits as “no energy transferred”. The opposite is true. If a question gives L and c, split the problem into chunks: heat solid, melt, heat liquid. Add the energies. Do not try to use one formula for the whole journey.
Go deeper
Gases are collisions with the container
Each bounce on a wall exerts a tiny force. Lots of bounces on an area give pressure. Squeeze the same gas into a smaller volume at constant temperature and the particles hit the walls more often, so pressure rises. Heat the gas at constant volume and they hit harder and more often. Vacuum questions work the same way: fewer particles, fewer collisions, lower pressure. You do not need the full gas laws on every specification, but you do need the particle explanation. Density questions fail when volume is in cm³ and mass in kg. Convert: 1 cm³ = 1 × 10⁻⁶ m³, or work in g/cm³ if both quantities match.
See the idea in action
A 0.12 kg block of ice at 0 °C melts. L_f for ice = 3.3 × 10⁵ J/kg. Energy to melt = mL = 0.12 × 3.3 × 10⁵ = 39 600 J. The temperature is still 0 °C when it has just become water. To raise that water to 20 °C, ΔE = mcΔθ = 0.12 × 4200 × 20 = 10 080 J. Total energy = 49 680 J, about 50 kJ. Using only mcΔθ from the start would miss the latent heat and understate the energy badly.
Exam technique
Turn knowledge into marks
State whether temperature is changing (use mcΔθ) or state is changing (use mL). Keep mass in kilograms. For density, show mass and volume measurements, then ρ = m/V with units. Explain gas pressure with collisions, not with particles “wanting more space”.
Common mistakes
Do not give these marks away
- 01
Using ΔE = mcΔθ during melting or boiling, when temperature is constant.
- 02
Mixing cm³, ml and m³ in density calculations.
- 03
Saying particles in a solid do not move, or that mass changes when ice melts.
Why does temperature stay constant while ice melts, even though it is still being heated?
ANo energy is being transferred
BEnergy is used to break intermolecular forces rather than to raise the particles’ kinetic energy
CThe mass of the ice decreases
DLatent heat only applies to gases
Show the answer
Energy is used to break intermolecular forces rather than to raise the particles’ kinetic energy. The potential store increases as the solid becomes liquid. Kinetic energy, which we measure as temperature, stays the same until melting is complete.
Quick questions
If this is the bit you searched
What is the difference between specific heat capacity and specific latent heat?
Specific heat capacity is energy to change temperature (ΔE = mcΔθ). Specific latent heat is energy to change state at constant temperature (E = mL).
How do you measure the density of an irregular object?
Measure mass on a balance. Find volume from the displacement of water in a measuring cylinder, then use ρ = m/V.