Changes of state
Changes of state
| Change | Name |
|---|---|
| solid → liquid | melting |
| liquid → solid | freezing |
| liquid → gas (at the surface) | evaporation |
| liquid → gas (all through) | boiling |
| gas → liquid | condensation |
| solid → gas (directly) | sublimation |
- A pure solid melts at one fixed melting point; a pure liquid boils at one fixed boiling point.
Practice
A substance changing directly from solid to gas is called:
Sublimation is the direct solid-to-gas change (e.g. solid iodine to purple gas).
The particle explanation
- Heating a solid: particles vibrate faster; at the melting point they break free and slide — it melts.
- Heating a liquid: particles speed up; at the boiling point they escape the forces — it boils.
- During a change of state, the energy breaks the forces of attraction, not speeds the particles up — so the temperature stays constant.
Practice
Why does the temperature stay constant while a pure solid melts?
During melting the heat energy breaks the forces holding the solid together, so the temperature does not rise.
Heating and cooling curves
- A heating curve has two flat parts:
- at the melting point (solid + liquid present),
- at the boiling point (liquid + gas present).
- A cooling curve is the reverse — flat at the boiling point (condensing) and the melting point (freezing).
Practice
A heating curve has flat parts at the melting point and the boiling point.
Temperature stays level at both the melting and boiling points while forces of attraction are broken.
You've got it
Key idea
- learn the six changes: melting, freezing, evaporation, boiling, condensation, sublimation
- a change of state breaks forces of attraction, so the temperature stays constant (flat parts on the curve)
- flat parts occur at the melting point and boiling point