1. Phase Changes
Physical change of matter from one phase to another due to a transfer of energy.

2. Evaporation Liquid – gas at surface of a liquid.
Molecules gain KE to become a vapor. Molecules left behind lose KE.
Cooling process – molecules left behind lose KE (cooler).

3. Condensation Gas – liquid.
Gas molecules ↓KE when collide with cold surface. Condense to liquid phase.
Warming process – KE lost by condensing gas molecules warms the surface they strike.

4. Evaporation-Condensation Rates
Depends upon environment, if moist:
Condensation > evaporation (warming)
Evaporation > condensation (cooling)
Equilibrium – state of balance. Evaporation and condensation occurring at equal rates.

5. Boiling Liquid – gas beneath the surface of a liquid.
Bubbles of vapor form beneath the surface – rise – break free to the vapor phase.
↑atm. pressure - ↑boiling pt.
Cooling process – the water is being cooled relative to the ↑temp it would attain otherwise. Because of cooling, it remains 100 °C instead of getting hotter.

6. Freezing Liquid - solid.
Energy is extracted, molecules slow down and the molecular attraction overcomes the KE.
Warming process – when you make ice cubes, you put liquid water in the freezer. The freezer cools the water, taking energy out – so it must give off energy.

7. Melting
Solid – liquid.
Heat energy is added until KE is greater than the molecular attraction.
Cooling process – the source of energy is the object or material around the stuff that is melting. The source cools as energy goes to the melting object.

8. Sublimation Solid - gas.
Example: mothballs that “evaporate” without leaving a liquid, and when snow on the ground “evaporates” skipping the liquid phase altogether.

9. Phase Change Graph

10. Phase Change Diagram Temperature °C Heat Energy Heat of vaporization
C.P
Condenses
Heat of fusion
Temperature °C
100
Boiling
F.P
B.P
Freezing O
Melting
M.P
Heat Energy

11. Phase Changes Phase Changes Melting Freezing Vaporization Condensation
From what Phase-to-Phase does change occur
Absorb or Release Heat Energy?
Cooling or Warming Process
Melting
Freezing
Vaporization
Condensation
Sublimation
solid-liquid
Absorb
Cooling
liquid-solid
Release
Warming
liquid-gas
Absorb
Cooling
gas-liquid
Release
Warming
solid-gas
Absorb
Cooling

12. Heat of Fusion
Amount of heat needed to change 1g substance from solid – liquid phase.
80 calories/g for water.
Example: How many calories are needed to change 10g of ice at 0 °C to 10g of water at 0 °C?
800

13. Heat of Vaporization
Amount of heat needed to change 1g substance from liquid – gas phase.
540 calories/g for water.
Example: How many calories are needed to change 10g of water at 100 °C to entirely to water vapor?
5400

14. Sample Problem
How much heat is required to change 1g of ice at -20°C to water vapor at 130°C?
Q = mc∆t 1g · 0.5 cal/g-C · 20 °C = 10 cal.
Hf 1g · 80 cal/g = 80 cal.
Q = mc∆t 1g · 1 cal/g-C · 100 °C = 100 cal.
Hv 1g · 540 cal/g = 540 cal.
Q = mc∆t 1g. · 0.5 cal/g · 30 °C = 15 cal.
745 calories