1. A. Phase Changes Sublimation: solid  gas. Vaporization: liquid  gas.
Melting or fusion: solid  liquid.
Deposition: gas  solid.
Condensation: gas  liquid.
Freezing: liquid  solid.
Copyright 1999, PRENTICE HALL
Chapter 11

2. Energy Changes Accompanying Phase Changes
A. Phase Changes
Energy Changes Accompanying Phase Changes
Sublimation: Hsub > 0 (endothermic).
Vaporization: Hvap > 0 (endothermic).
Melting or Fusion: Hfus > 0 (endothermic).
Deposition: Hdep < 0 (exothermic).
Condensation: Hcon < 0 (exothermic).
Freezing: Hfre < 0 (exothermic).
Generally heat of fusion (enthalpy of fusion) is less than heat of vaporization:
it takes more energy to completely separate molecules, than partially separate them.
Copyright 1999, PRENTICE HALL
Chapter 11

3. Evaporation
Definition: the conversion of a liquid to a gas below its boiling point
What happens…
Molecules at the surface of the liquid break away and go into the gas or vapor state
The liquid will evaporate faster as it is heated because KE increases

4. Boiling Point
Definition: the temperature at which the vapor pressure of a liquid is just equal to the external pressure
At lower atmospheric pressures, the boiling point decreases because there is less atmospheric pressure

5. B. Heating Curves Gas - KE  Boiling - PE  Liquid - KE 
Melting - PE 
Solid - KE 

6. B. Heating Curves
Copyright 1999, PRENTICE HALL
Chapter 11

8. B. Heating Curves Temperature Change change in KE (molecular motion)
depends on heat capacity
During a phase change, adding heat causes no temperature change.

9. B. Heating Curves Phase Change change in PE (molecular arrangement)
temp remains constant
Heat of Fusion (Hfus)
energy required to melt 1 mole of a substance at its melting point

10. B. Heating Curves Heat of Vaporization (Hvap)
energy required to boil 1 mole of a substance at its b.p.
usually larger than Hfus…why?
EX: sweating, steam burns, the drinking bird

11. C. Phase Diagrams
Show the phases of a substance at different temps and pressures.

12. Given a temperature and pressure, phase diagrams tell us which phase will exist.
Features of a phase diagram:
Triple point: temperature and pressure at which all three phases are in equilibrium.
Critical point: critical temperature and pressure for the gas.