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Change of State, Melting & Solidification

Key idea: Understand melting and freezing using the particle model, why temperature stays constant during change of state, and how to interpret heating curves (O Level).

  • SEC G3 Physics 2027
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Learning objectives

  • Compare physical properties of solids, liquids and gases
  • Explain state properties using particle arrangement, motion, forces and separation
  • Infer random molecular motion from a Brownian-motion experiment
  • Relate temperature rise to increased average kinetic energy of particles
  • Explain gas pressure using particle collisions with container walls
  • Explain heating from higher to lower temperature until thermal equilibrium
  • Describe conduction in solids through particle vibration and mobile electrons
  • Describe convection in fluids through density changes and bulk motion
  • Explain that energy transfer by electromagnetic radiation needs no material medium
  • Explain how surface colour, texture, temperature and area affect radiation transfer rate
  • Apply conduction, convection and radiation in everyday systems
  • describe internal energy as an energy store that is made up of the total kinetic energy associated with the random motion of the particles and the total potential energy between the particles in the system
  • Define heat capacity and specific heat capacity
  • Apply energy transferred = mass × specific heat capacity × temperature change
  • describe melting/solidification and boiling/condensation as processes of energy transfer without a change in temperature
  • Explain the difference between boiling and evaporation
  • Define latent heat and specific latent heat
  • Apply energy transferred for a change of state = mass × specific latent heat
  • Explain latent heat using particle behaviour
  • Sketch and interpret a cooling curve

1. Definition

A. Change of state

A change of state happens when a substance changes between solid, liquid, and gas.

During a change of state of a pure substance, the temperature stays constant even though energy is being transferred.

B. Melting and solidification

Melting is the change of state from solid → liquid (at the melting point).

Solidification (freezing) is the change of state from liquid → solid (at the freezing point).

2. Key Ideas

  • For a pure substance:
    • melting happens at a fixed temperature (melting point)
    • solidification happens at a fixed temperature (freezing point)
    • melting point = freezing point (same temperature)
  • During melting/solidification:
    • temperature stays constant
    • energy is transferred:
      • absorbed during melting
      • released during solidification
  • Particle model explanation:
    • energy transfer changes the particles’ arrangement and potential energy as attractive forces are overcome or re-form,
    • average kinetic energy stays constant, so temperature stays constant.

3. Detailed Explanations

A. What a heating curve shows

Heating curve for a pure substanceA temperature-time graph rises through solid, liquid, and gas regions, with constant-temperature plateaus during melting and boiling.TimeTemperaturesolid warmsmelting: solid + liquidliquid warmsboiling: liquid + gasgas warmsenergy transferred to substance →
Schematic heating curve for a pure substance at fixed pressure. Slopes and plateau lengths are not to scale; a plateau shows two states present at constant temperature.

When a solid is heated:

  1. temperature rises (particles gain kinetic energy on average)
  2. at the melting point, temperature stays constant while melting happens
  3. after all solid has melted, temperature rises again as the liquid is heated

B. Melting (solid → liquid)

At the melting point:

  • particles have enough energy to start breaking out of fixed positions
  • energy transferred to the substance overcomes some attractive forces and changes the particle arrangement
  • the particles can move past each other (liquid state)

So during melting:

  • potential energy increases
  • average kinetic energy stays constant → temperature stays constant

C. Solidification (liquid → solid)

During solidification:

  • particles lose energy
  • forces between particles pull them into fixed positions (solid structure)
  • energy is released to the surroundings

Temperature stays constant at the freezing point until all the liquid has solidified.

The energy transferred during a change of state is called latent heat.

For calculations, see: Latent Heat & Specific Latent Heat.

4. Common Mistakes

  • Saying the temperature increases during melting/solidification (it stays constant during the change of state).
  • Forgetting to state “for a pure substance” when describing a fixed melting/freezing point.
  • Saying “heat is stored” (energy is stored as internal energy; heat is energy in transfer).
  • Saying the particles “stop moving” in a solid (they still vibrate).
  • Assuming a longer plateau always means a larger specific latent heat. Plateau duration also depends on mass, power, and energy losses.

5. Exam Tips

  • Use the key mark points:
    • “temperature stays constant”
    • “energy absorbed/released”
    • “energy changes potential energy between particles”
    • “average kinetic energy stays constant”
  • If a graph has a flat section:
    • state “change of state”
    • identify “melting point / freezing point”
    • state “solid and liquid coexist” during melting.

6. Worked Examples

Modelled example 1

Why temperature stays constant during melting

Core

Problem

Ice at 0°C is heated until it becomes water at 0°C. Explain why the temperature remains constant during melting.
Study the worked solution

  1. Track the transferred energy

    Method

    Use the energy to overcome some attractive forces and change particle arrangement.

    Reason

    Particles leave fixed positions as the solid structure breaks down.

    Working

    Particle potential energy increases during melting.

  2. Link kinetic energy to temperature

    Method

    Keep average particle kinetic energy constant during the change.

    Reason

    Temperature depends on average kinetic energy, not the potential-energy change.

    Working

    Average kinetic energy constant → temperature constant.

Guided practice 2

What happens during solidification?

About 5 min

Problem

Water at 0°C solidifies into ice at 0°C. Describe the temperature, energy transfer and particle arrangement during the change.

Reverse the melting process consistently

Energy transfer

Hints

Hint 1: use the fixed freezing point
Temperature stays constant while both states coexist.
Hint 2: reverse the arrangement change
Particles become fixed in a solid structure and vibrate about fixed positions.
View solution step by step

  1. State temperature and energy transfer

    Method

    Keep temperature at the freezing point and release energy to the surroundings.

    Reason

    Attractive forces re-form while the phase change proceeds.

    Working

    Temperature: 0°C; energy transfer: outward.

  2. Describe the particles

    Method

    Arrange particles into fixed positions where they continue to vibrate.

    Reason

    Solidification produces a solid structure; it does not stop particle motion.

    Working

    Liquid mobility → vibration about fixed positions.

Common misconception 3

Kinetic vs potential energy during melting

Find and correct the mistake

Learner response

A student says: “Heating during melting makes particles move faster, so both average kinetic energy and temperature rise.” Diagnose the first error and state what happens to average kinetic and potential energy.

Assign the absorbed energy correctly

Correct energy changes

View solution step by step

  1. Use the temperature plateau

    Method

    Keep average kinetic energy unchanged.

    Reason

    Temperature remains constant throughout melting.

    Working

    Average kinetic energy: constant.

  2. Locate the energy increase

    Method

    Increase particle potential energy.

    Reason

    Absorbed energy overcomes attractive forces as particles leave fixed positions.

    Working

    Potential energy: increases.

Examiner practice 4

Reading a heating curve

2 marks

Examination question

A pure substance’s heating curve has a flat section at 0°C. State the process and the states present during the flat section. [2 marks]

Identify process and coexistence

View solution step by step

  1. Identify the process

    1 mark

    Method

    State that melting occurs at the melting point.

    Reason

    A flat section during heating marks a constant-temperature change of state.

    Working

    Process: melting.

  2. State the phases

    1 mark

    Method

    State that solid and liquid coexist.

    Reason

    Some substance has melted while the remainder is still solid.

    Working

    States present: ice and water.

Challenge 5

Melting point equals freezing point

Minimal support

Reverse-process transfer

For a pure substance at fixed pressure, explain why its melting point and freezing point are the same temperature.

Relate the forward and reverse transitions

Hints

Hint 1: identify the same two states
Both processes are transitions between the solid and liquid states.
Hint 2: reverse the direction
Melting is solid to liquid; freezing is liquid to solid.
View solution step by step

  1. Pair the reverse processes

    Method

    Describe melting and freezing as the same solid–liquid transition in opposite directions.

    Reason

    They involve the same two phases under the same pressure condition.

    Working

    Solid ⇄ liquid.

  2. Apply the pure-substance condition

    Method

    State that the transition occurs at one fixed temperature.

    Reason

    A pure substance at fixed pressure has a definite solid–liquid transition temperature.

    Working

    Melting point = freezing point.

7. Mind Stretchers

Mind stretcher 1: Same temperature, different internal energyExtension

Ice at 0°C and water at 0°C are both at the same temperature. Do they have the same internal energy? Explain.

Show Answer

No. Water at 0°C has higher internal energy because energy was absorbed during melting to overcome forces between particles.

The temperature is the same (average kinetic energy is the same), but the potential energy between particles is higher in the liquid.

Mind stretcher 2: Why does melting take time even at constant temperature?Extension

Ice at 0°C is heated with a constant-power heater. The temperature stays at 0°C for several minutes. Suggest why.

Show Answer

The heater’s energy is being used to melt the ice (overcome forces between particles) rather than raise temperature.

Melting requires a large amount of energy, so it takes time even though temperature is constant.

8. Practice and next step

Annotate the melting plateau in the Thermal Physics Explorer with state, temperature, kinetic energy and potential energy. Next, compare liquid–gas changes in boiling, condensation and evaporation.

Continue with the next resource in this course.

Course and syllabus information
Course
SEC G3 Physics
Edition
SEC G3 Physics 2027