Chapter 12 · Liquids, Solids, and Intermolecular Forces
12.5Melting, Freezing, and Sublimation
9 min · two checks
Predict
What is sublimation, and which common solid does it at room pressure?
The idea
- Define melting, freezing, sublimation, and deposition.
- Interpret a heating curve’s flat regions.
Melting turns solid into liquid at the melting point; freezing is the reverse. At the normal melting point the two are in equilibrium. Sublimation is solid to gas without a liquid stop, and deposition is the reverse. Dry ice and, more slowly, ice cubes in a freezer (freezer burn) show sublimation. Whether a substance sublimes or melts first depends on pressure and on its phase diagram.
A heating curve plots temperature against heat added. Sloped segments are temperature changes within one phase, governed by heat capacity. Flat segments are phase changes at constant temperature: the energy goes into pulling particles apart, not into making them faster. The length of the flat segment is the heat of fusion or the heat of vaporization for the sample. Water’s heat of vaporization is especially large, which is why steam burns are severe and why evaporation carries away so much heat.
Keep these
- Melting and freezing are opposites at the melting point.
- Sublimation skips the liquid. Dry ice is the classroom example.
- Flat parts of a heating curve are phase changes at constant temperature.