Melting, boiling and evaporation
Why sweating cools you
- When sweat dries off your skin, you feel cooler.
- A changing state — liquid turning to gas — is carrying energy away.
- Let's see how melting, boiling and evaporation work.
Melting and boiling
- Melting (solid → liquid) and boiling (liquid → gas) both need energy.
- But while the state is changing, the temperature stays the same — the energy goes into breaking the forces between particles, not into speeding them up.
- For water at normal air pressure: melting at $0\ {}^{\circ}\text{C}$, boiling at $100\ {}^{\circ}\text{C}$.
While ice is melting into water, what happens to the temperature?
During a change of state the energy goes into breaking the forces between particles, so the temperature stays constant until the change is complete.
On a heating curve, a flat (horizontal) section means:
Energy is still being added, but it is used to change the state, so the temperature does not rise — the curve is flat.
Evaporation
- Evaporation is a liquid turning to gas at its surface, and it happens below the boiling point.
- The fastest particles escape from the surface.
- That leaves slower particles behind, so the average energy drops — the liquid cools down.
- This is exactly why evaporating sweat cools your skin.
Evaporation can happen below the boiling point.
Evaporation happens at the surface at any temperature — the fastest surface particles escape. Boiling happens throughout the liquid at the boiling point.
Why does an evaporating liquid cool down?
The most energetic particles leave the surface, so the particles left behind have a lower average energy — a lower temperature.
What speeds up evaporation
- Higher temperature — more particles are moving fast enough to escape.
- Larger surface area — more particles are at the surface.
- More air movement (wind) — escaped particles are carried away, so fewer return.
- That's why washing dries fastest on a warm, breezy day.
Select all the things that make a puddle evaporate faster.
Higher temperature, larger surface area, and more airflow all speed up evaporation. A lower temperature slows it down.
You've got it
- melting and boiling need energy, but the temperature stays constant during the change
- a flat part on a heating curve = a change of state
- evaporation happens at the surface, below boiling; the fastest particles leave, so the liquid cools
- faster evaporation: higher temperature, bigger surface area, more airflow