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4.3 Energy transfer in state changes and conservation of mass

Read in conjunction with Part 4.4

Introduction to latent heat and physical changes of state

• As well as the transfer of heat energy by conduction, convection and radiation, state changes like evaporation and condensation also involves heat energy transfers and the particle model can be used to explain them.

  • ON HEATING - adding thermal kinetic energy increases the total internal energy of a substance

  • When you heat a solid, the vibrational kinetic energy of the particles is increased until they have enough KE to weaken the interparticle bonds to allow melting and the particles are free to move around in the liquid state.

  • With further heating above the melting point, the particles gain more kinetic energy and the inter-particle bonds are further weakened so that the particles at the surface with the highest KE can escape the surface (evaporate) or vapourise to the gaseous state in the bulk liquid (bubbles!) at the boiling point.

  • Sometimes a solid can change directly into a gas, that is called sublimation (the reverse can happen too e.g. hoar frost formation).

  • The graph below shows how the distribution of kinetic energy and speed of particles changes with changes in temperature - with increase in temperature, the average speed and kinetic energy of the particles increases.

    (This is an advanced level chemistry graph, but help you understand why all the particles can't suddenly evaporate!)

  • Note that it is the random movement and collisions of the particles creates a wide range of speeds/kinetic energies.

  • 

  • When the temperature is increased, more particles have a greater kinetic energy and greater speed, but only the highest speed/kinetic energy particles can escape from the surface (only the very right-hand section of the graph curves)

  • Below is a particle model of evaporation.

  • 

• ON COOLING - removing thermal kinetic energy - decreasing internal energy

• If you cool the substance, the reverse happens e.g. cool a gas so the interparticle bonds bring the particles together to condense and form a liquid.

• Further cooling reduces the KE of the liquid particles so that when the temperature is reduced to the freezing point, the interparticle forces are sufficient to 'club' the particles together to form a solid.

• All these physical state changes are reversible by adding or removing thermal energy, no new substances are formed (NOT a chemical change) and all mass is conserved. What you start with is what you finish with and all the original properties are retained.

• The only difference between the states of a substance is how the particles are arranged (as described in section 1. above).

• Note that in a closed system, mass is conserved in a system undergoing a change in state.

  • If you melt 100 g of ice, you get 100 g of water!

  • However, even with mass conservation, you can get a volume change, except for water, for the same mass, liquids occupy a slightly larger volume and gases occupy a massively greater volume than the liquid or solid form.

  • Ice is unusual that the solid ice crystals are less dense than water - which is why ice floats!

 

Read in conjunction with Part 4.4 below

INDEX of notes on Particle model theory state changes and latent heat

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Key points Thermal energy and particle models - Describing state changes with particle theory

Information sources for Doc Brown's key points: IGCSE-GCSE physics are based on textbooks and syllabus-specifications for students taking the UK AQA, Edexcel, OCR 21st Century Science, OCR Gateway science suite, WJEC, CCEA and CIE GCSE physics 9-1 level science examinations

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STATE CHANGES and PARTICLE THEORY

A comprehensive set of summary revision notes on Changes of State in Terms of Particle Theory, tailored to the GCSE/IGCSE Physics specifications for WJEC, CCEA, CIE, AQA, Edexcel, and OCR.

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Changes of State and Particle Theory

Core Principle

• All matter is made of particles that are in constant motion.
• Heating or cooling changes the energy of particles, affecting their movement and arrangement.
• State changes are physical changes — no new substance is formed, and mass is conserved.

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Describing State Changes

Process	From → To	Particle Changes	Energy Transfer	Temperature Change
Melting	Solid → Liquid	Particles gain energy, vibrate more, break free from fixed positions	Energy absorbed	Constant during change
Boiling	Liquid → Gas	Particles gain energy, move rapidly, overcome intermolecular forces	Energy absorbed	Constant during change
Evaporation	Liquid → Gas (surface only)	High-energy particles escape from surface	Energy absorbed	Gradual cooling effect
Sublimation	Solid → Gas	Particles gain enough energy to skip liquid phase	Energy absorbed	Constant during change
Freezing	Liquid → Solid	Particles lose energy, move less, form fixed structure	Energy released	Constant during change
Condensing	Gas → Liquid	Particles lose energy, move closer, form bonds	Energy released	Constant during change

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Particle Theory Summary

State	Particle Arrangement	Movement	Energy Level
Solid	Regular, tightly packed	Vibrate only	Low
Liquid	Irregular, random, close	Move past each other	Medium
Gas	Far apart, random	Rapid, random	High

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Heating and Cooling Curves

• Flat sections = state change (energy used to break/form bonds)
• Sloped sections = temperature change (kinetic energy increases)
• See Parts 4.5 and 4.6

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Required Practicals (varies by board)

• Heating ice: Record temperature versus time to observe melting and boiling points.
• Evaporation: Measure mass loss over time.
• Sublimation: Observe iodine or dry ice changing directly from solid to gas.

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Exam Tips for state changes and particle theory

• Use particle diagrams to show changes in arrangement and movement.
• Always mention energy transfer and temperature constancy during state changes.
• Know the difference between boiling and evaporation:
  • Boiling: throughout liquid, at boiling point.
  • Evaporation: surface only, below boiling point.
• Link kinetic energy to temperature and movement.

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Common Misconceptions about state changes and particle theory

• Particles change size — they don’t; only their movement and spacing change.
• Temperature rises during melting/boiling — it stays constant.
• Evaporation and boiling are the same — they differ in mechanism and conditions.
• Sublimation is rare or unnatural — it occurs in substances like iodine and CO₂.

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LATENT HEAT

A detailed and exam-board-inclusive set of summary revision notes on thermal energy changes and latent heat during changes of state, tailored for GCSE/IGCSE Physics students across WJEC, CCEA, CIE, AQA, Edexcel, and OCR.

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Thermal Energy Changes During State Change

What Happens During a Change of State?

• When a substance changes state (e.g. melting, boiling, condensing), thermal energy is transferred.
• This energy does not increase temperature - instead, it’s used to break or form intermolecular bonds.
• The internal energy increases or decreases, but the temperature remains constant during the change.

Heating and Cooling Curves (see section 4.5)

• Flat sections on a temperature-time graph = state change.
• Energy is absorbed or released without temperature change.

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Latent Heat: The Hidden Energy

Definition

• Latent heat is the energy required to change the state of a substance without changing its temperature.
• It’s called “latent” because the temperature doesn’t change - the energy is “hidden” in the bond changes.

Types of Latent Heat

Type	Description	Examples
Latent Heat of Fusion	Energy to change between solid ↔ liquid	Melting ice, freezing water
Latent Heat of Vaporisation	Energy to change between liquid ↔ gas	Boiling water, condensing steam

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Exam Tips about latent heat

• Always state that temperature remains constant during state change.
• Use correct units: J (energy), kg (mass), J/kg (latent heat).
• Know the difference between latent heat and specific heat capacity:
  • Latent heat: state change, no temperature change.
  • Specific heat capacity: temperature change, no state change.

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Common Misconceptions

• Temperature increases during melting/boiling → It does not.
• Latent heat causes temperature rise → It causes state change, not temperature change.
• Energy isn’t transferred during flat sections → It is, but it’s used to break/form bonds.
• Latent heat is the same for all substances → It varies by material and state change.

• See Parts 4.5-4.6 A heating curve - state changes on increasing the internal energy of a system AND the cooling curve - state changes on decreasing the internal energy of a system

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Keywords, phrases and learning objectives for particle models and thermal energy transfers in state changes

Be able to describe and explain the thermal energy transfers that happen in state changes.

Know that in state changes there is complete conservation of mass with no chemical change of the substance.

Be able to define, use and explain the latent heat energy changes when a substance changes physical state including the latent heat of fusion/melting, latent heat of vaporization/boiling (vaporization, vapourisation, vaporisation), latent heat of condensation, latent heat of melting/freezing

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WHAT NEXT?

TOP of page

INDEX for physics notes on particle model theory, state changes,  latent heat, heating/cooling curves

INDEX of all my THERMAL ENERGY notes

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