Melt an ice cube and it doesn’t warm up as it turns to water — its temperature just sits still. Understanding why is the key to this whole section.
📘 What you need to know
A change of state (or phase change) happens whenever matter shifts between solid, liquid and gas
Thermal energy is transferred to or from a substance during a phase change
This transfer does not change the substance’s temperature — only its potential energy is affected
The four main phase changes are melting, freezing, vaporisation and condensation
Every substance has its own characteristic melting/freezing point and boiling point
The Four Phase Changes
Melting — solid → liquid, as the substance absorbs thermal energy
Freezing — liquid → solid, as the substance releases thermal energy
Vaporisation (boiling) — liquid → gas, as the substance absorbs thermal energy
Condensation — gas → liquid, as the substance releases thermal energy
Teal arrows absorb thermal energy; blue arrows release it — the same pair of temperatures governs each side
Why Temperature Doesn’t Change During a Phase Change
During a phase change, the thermal energy being transferred doesn’t affect the kinetic energy of the particles — only their potential energy. Since temperature is a measure of average kinetic energy alone, a substance’s temperature stays fixed throughout the whole process, right up until the change of state is complete.
Melting and freezing both happen at a substance’s melting/freezing point; vaporisation and condensation both happen at its boiling point. For a given substance, these two temperatures are fixed and won’t shift no matter how much energy is supplied or removed.
Quick recap: Phase changes transfer thermal energy without changing temperature, because only particle potential energy is affected, not kinetic energy.
WE 1
Identify the phase change taking place in each scenario: (a) dew forming on grass overnight, (b) a chocolate bar softening and turning liquid in the sun, (c) molten wax hardening as a candle burns.
(a)
Water vapour in the air turns into liquid droplets — this is condensation.
(b)
Solid chocolate turns into liquid as it absorbs thermal energy — this is melting.
(c)
Liquid wax turns back into a solid as it releases thermal energy — this is freezing.
WE 2
A pan of water is boiling steadily at 100 °C on a stove. Explain why its temperature stays at 100 °C even though the burner continues to supply thermal energy underneath it.
Reasoning
The thermal energy supplied is being used to increase the potential energy of the water’s particles, fully overcoming the intermolecular forces holding them together as it turns into steam.
Why the temperature doesn’t rise
Because none of this energy is increasing the average kinetic energy of the particles, the temperature remains fixed at the boiling point until all the water has vaporised.
Energy goes into vaporising the water, not into heating it further
💡 Top tips
Melting and freezing occur at the same temperature for a given substance — it’s the direction of energy flow that differs
The same is true for vaporisation and condensation at the boiling point
If a question describes a flat section on a temperature-time graph, it’s almost always describing a phase change in progress
Whether energy is absorbed or released tells you the direction of the phase change — solid → liquid → gas always absorbs, the reverse always releases
⚠ Common mistakes
Assuming a substance’s temperature keeps rising throughout heating, without a plateau during any phase change
Mixing up a substance’s melting point with its boiling point
Describing freezing as a process that absorbs energy, when it actually releases it
Forgetting that thermal energy is still being transferred during a phase change, even though the temperature isn’t changing
Up next: Specific Heat Capacity — where we put a number on exactly how much energy it takes to heat something up.
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