Melting, Boiling and Evaporation
Physics· 2.2.3· 12 min read
1. Core: Changes of State & Melting/Freezing★★☆☆☆⏱ 3 min
Change of State
A reversible physical change where a substance changes between solid, liquid and gas states, with no change to its chemical identity. Temperature stays constant during the change.
Melting occurs when a solid absorbs enough thermal energy to break the rigid interparticle bonds holding it in a fixed structure, turning into a liquid. Freezing is the reverse process: liquid particles lose energy, form strong bonds, and turn into a solid. Both processes happen at the same fixed temperature (melting/freezing point) for a pure substance at constant pressure.
A student heats a block of pure ice at -5°C until it becomes water at 10°C. They notice the temperature stays at 0°C for 4 minutes. Explain this observation.
- 1
- Thermal energy is continuously supplied to the ice from the heat source.
- 2
- At 0°C (the melting point of pure ice), all supplied energy is used to break the strong bonds between ice particles, rather than increasing their average kinetic energy.
- 3
- Temperature is a measure of average particle kinetic energy, so it stays constant until all ice has melted into liquid water.
Exam tip:
Always link constant temperature during state changes to energy used for bond breaking/making, not changes to particle kinetic energy, to secure full marks.
2. Core: Boiling & Condensation★★☆☆☆⏱ 3 min
Boiling Point
The fixed temperature at which a pure liquid boils (turns to gas throughout the bulk of the liquid) at a given atmospheric pressure.
Boiling is a bulk process: gas bubbles form throughout the entire liquid, rise to the surface, and escape. Condensation is the reverse process, where gas particles lose energy, form bonds, and turn into liquid, also occurring at the boiling point temperature. Boiling point changes with atmospheric pressure: lower pressure = lower boiling point, e.g. water boils at ~70°C on the peak of Mount Everest.
A camper boiling water at sea level notices the temperature stays at 100°C even when they turn the heat up higher. Explain why the temperature does not rise.
- 1
- 100°C is the boiling point of pure water at standard sea level atmospheric pressure.
- 2
- Extra thermal energy supplied by the higher heat setting is used to overcome attractive forces between water particles to turn them into water vapour, not to increase their average kinetic energy.
- 3
- Since temperature measures average kinetic energy, it stays constant until all the water has boiled away.
3. Core: Evaporation & Cooling★★☆☆☆⏱ 3 min
Evaporation
A change of state in which the more energetic particles escape from the surface of a liquid to become gas. Evaporation can happen at any temperature below the boiling point, and it causes the remaining liquid to cool.
Evaporation only happens at the surface of a liquid, and it can occur at any temperature below the boiling point. At any moment the particles in a liquid have a range of kinetic energies. The fastest, most energetic particles near the surface can overcome the attractive forces of the particles around them and escape into the air as gas. Because the particles that leave carry away more than the average amount of energy, the average kinetic energy of the particles left behind falls. Temperature is a measure of the average kinetic energy of the particles, so the liquid that remains cools down. This is why evaporation always causes cooling of a liquid.
A shallow dish of water is left in a warm room. No heat is taken away from the water by the room, yet the water that remains is measured to be slightly cooler than the room. Explain, in terms of particles, why the remaining water cools.
- 1
- The particles in the water have a range of kinetic energies. Only the most energetic particles at the surface have enough energy to escape into the air by evaporation.
- 2
- As these high-energy particles leave, the average kinetic energy of the particles remaining in the dish decreases.
- 3
- Temperature is a measure of the average kinetic energy of the particles, so the temperature of the remaining water falls. Evaporation therefore cools the liquid.
Exam tip:
For Core, remember two facts about evaporation: it is the escape of the more energetic particles from the surface of a liquid, and it causes the remaining liquid to cool.
4. Extended Only: Boiling vs Evaporation & Rate of Evaporation★★★☆☆Extended only⏱ 4 min
Three factors change the rate of evaporation: 1) a higher temperature of the liquid, 2) a larger surface area of liquid exposed to the air, and 3) greater air movement (a draught or wind) over the surface. A drier (less humid) surrounding also speeds evaporation up. Higher temperature gives more particles enough energy to escape; a larger surface area means more particles are at the surface where escape happens; and moving air removes vapour particles from just above the surface so fewer return to the liquid.
Feature | Boiling | Evaporation |
|---|---|---|
Temperature of occurrence | Fixed boiling point only | Any temperature < boiling point |
Location in liquid | Throughout entire liquid (bulk process) | Only at liquid surface |
Bubbles | Bubbles form inside liquid | No bubbles form |
Temperature of remaining liquid | Stays constant at boiling point | Decreases (cooling effect) |
Extended candidates should also be able to explain the cooling of a body that is in contact with an evaporating liquid. When a liquid evaporates from the surface of an object, the most energetic particles leave the liquid. The liquid, and the object it is touching, are left with lower-energy particles, so both cool down. This is why sweat evaporating from skin cools the body, and why a person feels cold when they step out of a swimming pool into moving air.
Explain why wet washing hung on a line dries faster on a hot, windy day than on a cold, still, humid day.
- 1
- Higher temperature increases the average kinetic energy of water particles in the washing, so more particles have enough energy to escape the liquid surface.
- 2
- Wind blows away water vapour particles that have collected near the surface of the washing, maintaining a concentration gradient so more particles can escape.
- 3
- Low humidity on dry days means there are fewer water vapour particles in the surrounding air, so more particles can escape from the liquid surface. All three factors increase the rate of evaporation, so washing dries faster.
Exam tip:
When asked to compare boiling and evaporation, refer to all four features in the table above; when asked about cooling, explain that the most energetic particles leave, lowering the average kinetic energy of what remains.
5. Core: Heating & Cooling Graph Interpretation★★☆☆☆⏱ 2 min
Heating/cooling graphs plot temperature of a substance against time as it is heated or cooled at a constant rate. Sloped regions show the temperature of a single state (solid, liquid, gas) increasing or decreasing. Flat horizontal regions show state changes, where temperature stays constant even though heat is being added or removed. The lower flat region on a heating graph is melting, the higher flat region is boiling.
A heating curve for a pure substance has flat regions at 18°C and 234°C. State the melting point and boiling point of the substance.
- 1
- The lower flat region corresponds to melting (solid to liquid), so the melting point is 18°C.
- 2
- The higher flat region corresponds to boiling (liquid to gas), so the boiling point is 234°C.
6. Common Pitfalls
Wrong move:
Stating temperature rises during melting because heat is added
Why:
Thermal energy is used to break interparticle bonds during state changes, not to increase particle kinetic energy, so temperature stays constant
Correct move:
Explicitly link constant temperature during state changes to energy used for bond breaking/making, not kinetic energy changes
Wrong move:
Confusing evaporation and boiling as identical processes
Why:
Boiling is a bulk process at fixed temperature, while evaporation is a surface process that occurs at any temperature below boiling point
Correct move:
Refer to the 4 key differences (temperature, location, bubbles, cooling effect) when distinguishing the two processes
Wrong move:
Claiming evaporation only happens at high temperatures
Why:
Evaporation occurs at any temperature above the melting point of the liquid, e.g. puddles dry up on cold winter days
Correct move:
Clarify that only boiling requires the liquid to be at its boiling point
Wrong move:
Forgetting boiling point changes with atmospheric pressure
Why:
Boiling occurs when liquid vapour pressure equals external atmospheric pressure, so lower external pressure lowers boiling point
Correct move:
Link boiling point variation to changes in surrounding air pressure when asked
Wrong move:
Labelling flat regions on heating graphs as 'heating phases'
Why:
Flat regions correspond to state changes, not periods of temperature increase for a single state
Correct move:
Label flat regions as melting/boiling (heating) or freezing/condensation (cooling) as appropriate
7. Quick Reference Cheatsheet
Process | State Change | Key Feature |
|---|---|---|
Melting | Solid → Liquid | Constant temperature at melting point, heat absorbed |
Freezing | Liquid → Solid | Constant temperature equal to melting point, heat released |
Boiling | Liquid → Gas | Bulk process, constant temperature at boiling point, heat absorbed |
Condensation | Gas → Liquid | Constant temperature equal to boiling point, heat released |
Evaporation (Core) | Liquid → Gas | Surface escape of more energetic particles, any temp < boiling point, cools the liquid |
8. Frequently Asked
Why does temperature not rise when ice is melting?
Heat energy supplied during melting is used to break the strong interparticle bonds in solid ice, instead of increasing the average kinetic energy of particles. Since temperature measures average particle kinetic energy, it stays constant until all ice has melted.
What is the main difference between boiling and evaporation?
Boiling is a bulk process that occurs only at a fixed boiling point, while evaporation is a surface process that happens at any temperature below the boiling point of the liquid.
Why does boiling point decrease at high altitudes?
Boiling occurs when the vapour pressure of the liquid equals external atmospheric pressure. Atmospheric pressure is lower at high altitudes, so less energy is needed for vapour pressure to match external pressure, lowering the boiling point.
Going deeper
What's Next
Now that you have mastered melting, boiling and evaporation for CIE IGCSE Physics 0625, you are ready to progress to more advanced thermal physics topics. Next, explore thermal capacity, which explains how much thermal energy is required to raise the temperature of different substances, followed by the three mechanisms of thermal energy transfer: conduction, convection and radiation. Make sure to practise past paper questions on change of state heating and cooling curves, as these are high-frequency questions in both core and extended exam papers. If you are sitting the extended paper, practise comparison questions between boiling and evaporation to secure full marks on this subtopic.