Change of State, Melting & Solidification

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

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.

D. Link to latent heat (for calculations)

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

Example 1: Why temperature stays constant during meltingCore

Ice is heated at 0circC until it becomes water at 0circC. Explain why the temperature stays constant during melting.

Show Answer

At the melting point, energy absorbed is used to overcome forces between particles (increasing potential energy) instead of increasing the average kinetic energy.

Since average kinetic energy stays constant, the temperature stays constant until all the ice has melted.

Example 2: What happens during solidification?Core

Water at 0circC is cooled and turns into ice at 0circC. State what happens to:

1. temperature
2. energy transfer
3. particle motion/arrangement

Show Answer

1. Temperature stays constant at the freezing point during solidification.

2. Energy is released to the surroundings (latent heat is given out).

3. Particles lose energy and become arranged in fixed positions; they can only vibrate about those positions in the solid.

Example 3: Reading a heating curveCore

A heating curve shows a flat section at 0circC. What does this flat section represent? What substances are present during the flat section?

Show Answer

The flat section represents melting at the melting point.

Both solid and liquid are present together during the flat section (ice and water).

Example 4: Melting point equals freezing pointCore

For a pure substance, why is the melting point the same temperature as the freezing point?

Show Answer

Melting (solid → liquid) and freezing (liquid → solid) are the same change of state in opposite directions. For a pure substance at a given pressure, the transition between solid and liquid happens at one fixed temperature, so the melting and freezing points are the same.

Example 5: Kinetic vs potential energy during meltingCore

During melting at the melting point, what happens to:

1. the average kinetic energy of particles,
2. the potential energy between particles?

Show Answer

1. Average kinetic energy stays the same because temperature stays constant.

2. Potential energy increases because energy is used to overcome attractive forces as particles leave fixed positions and the solid structure breaks down.

7. Mind Stretchers

Mind stretcher 1: Same temperature, different internal energyExtension

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

Show Answer

No. Water at 0circC 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 0circC is heated with a constant-power heater. The temperature stays at 0circC 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.