Melting, boiling and evaporation
Introduced| English | Meaning |
|---|---|
| melting/ˈmeltɪŋ/ | The change of a substance from solid to liquid. |
| boiling/ˈbɔɪlɪŋ/ | Vaporisation throughout a liquid when its vapour pressure matches the surrounding pressure. |
| evaporation/ɪˌvæpəˈreɪʃn/ | The escape of particles from the surface of a liquid into its vapour without the whole liquid boiling. |
| boiling point/ˈbɔɪlɪŋ pɔɪnt/ | The temperature at which a liquid's vapour pressure equals the surrounding pressure. |
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 熔化, boiling 沸腾 and evaporation 蒸发 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 $100\ {}^{\circ}\text{C}$.
The heating curve — watch the temperature pause
While the substance is melting or boiling the temperature stays flat, even though heat is still going in — the energy breaks bonds instead of warming it.
While ice is melting into water, what happens to the temperature?
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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:
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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 沸点.
- 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.
Temperature stays flat while the substance melts and while it boils
Evaporation can happen below the boiling point.
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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?
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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.
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Higher temperature, larger surface area, and more airflow all speed up evaporation. A lower temperature slows it down.
Worked examples
1. Ice is heated steadily. The thermometer reads $0\ °\text{C}$ for $5$ minutes although the heater is on. The energy is breaking the forces between the particles, so the ice melts without getting hotter. Only when all the ice has melted does the temperature rise.
2. Wet towels dry fastest on a warm, windy day, spread out on a line. Warm: more particles are fast enough to escape. Windy: escaped particles are carried away. Spread out: a bigger surface area.
Evaporation is not boiling. Water evaporates at room temperature, only from the surface. Boiling happens at $100\ °\text{C}$, with bubbles forming all through the liquid.
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
Write from a melting temperature that stays at 60 degrees
- A substance melts at 60 degrees Celsius. Energy is still being supplied.
- State the temperature change during melting. Evaporation at 25 degrees can still cool the liquid.
A substance melts at 60 degrees Celsius while energy is supplied. What happens to its temperature during melting, and can evaporation at 25 degrees Celsius cool the liquid?
A flat part of the heating curve is a change of state. The fastest particles leave during evaporation.
What happens to the temperature while this substance melts?
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The energy goes into breaking forces between particles, so the thermometer stays at 60 degrees Celsius. Other options — It rises steadily (because energy is being supplied); It falls to 0 degrees Celsius (the melting point of water).