Changes of state and specific latent heat — AQA GCSE Physics

Specific latent heat is the energy required to change the state of one kilogram of a substance with no change in temperature. During a change of state, energy input alters intermolecular bonds and increases internal potential energy rather than kinetic energy, keeping the temperature constant.

Changes of state and specific latent heat explore how internal energy alters when substances transition between solids, liquids, and gases. Unlike temperature changes governed by specific heat capacity, state changes occur at fixed temperatures as energy is directed into breaking or forming intermolecular bonds.

This topic connects the particle model of matter directly to quantitative thermodynamics in AQA GCSE Physics. Mastering these principles allows students to interpret heating and cooling curves accurately and solve multi-step energy conservation problems in real-world thermal systems.

• →Internal energy is the total energy stored inside a system by the particles, comprising the sum of their kinetic and potential energy stores.
• →During a change of state, mass is conserved and the change is purely physical, meaning the material recovers its original properties if the change is reversed.
• →Specific latent heat (L) is defined as the energy required to change the state of 1 kg of a substance with no change in temperature, governed by E = m x L.
• →Specific latent heat of fusion applies to transitions between solid and liquid, whereas specific latent heat of vaporisation applies to transitions between liquid and gas.
• →Heating and cooling curves feature flat horizontal plateaus during phase changes because thermal energy input or removal changes potential energy, not average kinetic energy.

• one mark for specific heat capacity involving a temperature change with no change of state
• one mark for specific latent heat involving a change of state with no temperature change
• one mark for contrasting the units, J/kg °C against J/kg
• one mark for matching each quantity to the sloping or flat section of a heating graph

• 💡Underline the verb in the question: melts and boils point to L, warms and cools point to c.
• 💡State both differences when asked to distinguish them, what changes and what stays the same, not just the units.
• 💡In a multi-stage calculation, set the work out as separate labelled stages so an examiner can award each part.
• 💡Always convert time to seconds and mass to kilograms before applying the formula E = m x L or E = P x t.
• 💡In explanation questions, explicitly distinguish between kinetic energy (linked directly to temperature) and potential energy (linked to bond breaking and intermolecular separation).
• 💡On heating curve graphs, label the sloped sections as changes in temperature (E = m x c x delta theta) and the flat horizontal plateaus as changes of state (E = m x L).

• using ΔE = m c Δθ across the plateau of a heating graph, where the temperature is constant
• inserting a temperature change into E = m L
• quoting J/kg °C as the unit of specific latent heat
• assuming a substance sitting at its melting point is receiving no energy because the thermometer reading is steady
• Believing that thermal energy input always leads to a temperature rise; during a phase change, temperature remains constant because kinetic energy does not increase.
• Treating boiling and evaporation as identical; evaporation occurs only at the surface at any temperature below boiling, whereas boiling occurs throughout the liquid at a defined boiling point.
• Assuming mass is lost during boiling or evaporation; mass is strictly conserved in all physical state changes, though gas particles may escape an open system.

1. 1Day 1: Review particle arrangements and define internal energy in terms of kinetic and potential energy stores.
2. 2Day 2: Practise sketching and annotating heating and cooling curves, marking clearly where E = mc delta theta and E = mL apply.
3. 3Day 3: Memorise the definitions and units of specific latent heat of fusion versus vaporisation, practicing formula rearrangements for m and L.
4. 4Day 4: Solve combined exam-style calculation questions involving electrical power (P = E / t) and specific latent heat (E = m x L).

• 📋Calculation questions: Rearranging and calculating values using E = m x L, often combined with electrical energy supplied (E = P x t or E = V x I x t).
• 📋Graph interpretation: Identifying states of matter, melting points, boiling points, and explaining why specific sections of a heating or cooling curve are horizontal.
• 📋Extended response (4-6 marks): Describing an experiment to measure the specific latent heat of fusion of ice or vaporisation of water, including apparatus, measurements, and sources of error.