Revision notes · Particle model of matter

Internal energy and energy transfers

Internal energy4.3.2.1

Definition: Internal energy is the energy stored inside a system by the particles (atoms and molecules) that make it up — the total kinetic energy (from the particles vibrating or moving) and potential energy (from the forces between particles) of all the particles.

Heating a system transfers energy to its particles, increasing the internal energy. This either raises the temperature of the system, or produces a change of state — but not both at the same time (see Changes of state and specific latent heat).

Temperature changes in a system and specific heat capacity4.3.2.2

Definition: The specific heat capacity of a substance is the amount of energy required to raise the temperature of 1 kg of the substance by 1°C (or 1 K).

Equation	Units
ΔE = m × c × ΔT	ΔE (change in thermal energy) in J, m (mass) in kg, c (specific heat capacity) in J/kg°C, ΔT (temperature change) in °C

Change in thermal energy

A substance with a high specific heat capacity needs a lot of energy to change temperature — this is why it takes water a long time to heat up (and a long time to cool down) compared with many other materials.

Changes of state and specific latent heat4.3.2.3

Definition: The specific latent heat of a substance is the amount of energy needed to change the state of 1 kg of the substance, without changing its temperature. Specific latent heat of fusion is the energy needed to melt (or freeze) a substance; specific latent heat of vaporisation is the energy needed to boil/evaporate (or condense) a substance.

Equation	Units
E = m × L	E (energy) in J, m (mass) in kg, L (specific latent heat) in J/kg

Energy for a change of state

Energy is absorbed by a substance when it melts or evaporates, and released when it freezes or condenses. While a substance is actually changing state, all the energy transferred goes into breaking or forming the bonds between particles — none of it raises the temperature, so the temperature stays constant during the change of state itself.

On a graph of temperature against time for a substance being steadily heated from solid to gas, the temperature rises while it is fully solid, then stays flat while it melts (energy goes into changing state, not raising temperature), rises again while it is fully liquid, stays flat again while it boils (this flat section is usually longer, since evaporating takes more energy than melting), then rises again once it is fully gas.

Heating graph through a change of state

⚠️ Common mistake: Assuming temperature keeps rising steadily while a substance is changing state — it does not. The temperature stays constant (a flat section on a heating graph) throughout the melting or boiling process, because the energy is being used to change state, not to heat the substance further.

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