Melting, Boiling and EvaporationCambridge IGCSE Physics: Revision notes
Section 1
What Happens to Energy During Melting and Boiling?
Melting and boiling are changes of state that happen at a fixed temperature for a given substance. During melting or boiling:
- Energy is continuously supplied to the substance.
- The temperature does NOT rise while the change of state is happening.
- Instead, the energy is used to break the forces holding particles in their fixed arrangement (melting) or to separate particles completely so they can escape as gas (boiling).
Only once the change of state is complete does further heating cause the temperature to rise again.
A common error is to say the temperature 'keeps rising' during melting or boiling — it stays constant until the change of state is complete.
Section 2
Melting and Boiling Points of Water
At standard atmospheric pressure, water has the following fixed change-of-state temperatures:
| Change of state | Temperature |
|---|---|
| Melting point (solid to liquid) | 0 degreesC |
| Boiling point (liquid to gas) | 100 degreesC |
These values are worth memorising exactly, as they are commonly used in calculations and comparisons with other substances.
Section 3
What Happens During Condensation and Solidification?
Condensation and solidification are the reverse changes of state to boiling and melting:
- Condensation: as a gas cools, particles lose kinetic energy and move slower; when they get close enough, the forces of attraction between them pull them together into a liquid.
- Solidification (freezing): as a liquid cools further, particles lose more energy and move even slower, until the forces of attraction lock them into fixed positions, forming a solid.
In both cases, energy is released by the particles as they slow down and come closer together.
Section 4
What Is Evaporation, and How Is It Different from Boiling?
Evaporation is the escape of the most energetic particles from the surface of a liquid, and it can happen at any temperature, not just the boiling point.
Evaporation causes cooling of the remaining liquid: the particles that escape are the ones with the most kinetic energy, so the particles left behind have a lower average kinetic energy, and therefore a lower temperature.
| Feature | Evaporation | Boiling |
|---|---|---|
| Where it happens | Only at the surface of the liquid | Throughout the whole liquid |
| Temperature | Can occur at any temperature | Only at the fixed boiling point |
| Speed | Relatively slow | Rapid, with visible bubbles |
Three factors that increase the rate of evaporation:
- Higher temperature — more particles have enough energy to escape.
- Larger surface area — more particles are at the surface where they can escape.
- Air movement over the surface — moves escaped particles away, preventing them from returning to the liquid, and keeps the humidity above the surface low.
Think of a crowd where only the fastest runners manage to escape over a low fence at the edge — the ones left behind are, on average, slower (less energetic) than before.
Sweat evaporating from skin removes energy from the body, which is why sweating has a cooling effect.
Section 5
Why Does Contact with an Evaporating Liquid Cause Cooling?
When a liquid on a surface (such as skin) evaporates, the most energetic particles escape as vapour, taking their kinetic energy with them. The particles remaining in the liquid have a lower average kinetic energy, so the liquid's temperature falls. Since the liquid is in contact with the object (e.g. skin), the object also loses energy to the cooler liquid and its own temperature falls.
This is why rubbing alcohol or sweat feels cold as it evaporates from your skin — energy is being transferred away from your skin into the evaporating liquid.
Must Know
- During melting and boiling, temperature stays constant while energy input breaks the forces between particles.
- Water's melting point is 0 degreesC and its boiling point is 100 degreesC at standard atmospheric pressure.
- Condensation (gas to liquid) and solidification (liquid to solid) release energy as particles slow down and attractive forces pull them together.
- Evaporation is the escape of the most energetic particles from a liquid's surface and can happen at any temperature; it causes cooling of the remaining liquid.
- Evaporation happens only at the surface; boiling happens throughout the liquid at a fixed temperature.
- Evaporation rate increases with higher temperature, larger surface area, and greater air movement over the surface.
That's the notes covered.
Carry on to the next subtopic.