1. Change of Phase
Chapter 17

2. Evaporation

3. Evaporation
A change of phase from liquid to gas that occurs at the surface of a liquid.

4. The temperature of any substance is related to the average kinetic energy of its particles.
Molecules within a liquid have a variety of speeds
Molecules within a liquid also move in all directions

5. Molecules at the surface gain kinetic energy while others lose kinetic energy
Molecules gain KE at the surface by bumping into each other

6. When molecules bumped into one another
Increased KE of molecules bumped hard enough to break free from the liquid comes from the molecules remaining in the liquid.
When molecules bumped into one another
Some gain KE
Others lose KE
The average KE of the molecules remaining in the liquid is lowered
Evaporation is a cooling process

7. The fast molecules that break free from the surface are slowed as they fly away due to their attraction to the surface.
So although water is cooled by evaporation, the air above is not correspondingly warmed by the process.

8. Condensation

9. Condensation The changing of a gas to a liquid
This is the opposite of evaporation

10. Excess KE is shared with the liquid
When gas molecules near the surface of a liquid are attracted to the liquid, they strike the surface with increased KE and become apart of the liquid
Excess KE is shared with the liquid
This increases the liquids temperature
Condensation is a warming process

11. Condensation in the Atmosphere
There is always some water vapor in the air
A measure of the amount of this water vaopr is called humidity

12. Fog and Clouds Warm air rises. As it rises it expands.
As it expands it chills
As the air chills water-vapor molecules are slowed.
Lower-speed molecular collisions result in water molecules sticking together.
If there are larger and slower-moving particles or ions present, water vapor condenses upon these particles, and with sufficient buildup we have a cloud

13. Boiling

14. Evaporation can take place beneath the surface of a liquid, forming bubbles of vapor that are buoyed to the surface, where they escape.
This change of phase throughout a liquid rather than only at the surface is called boiling.

15. Bubbles in the liquid can form only when the pressure of the vapor within the bubble is great enough to resist the pressure of the surrounding liquid
So bubbles do not form until the boiling point is reached

16. Or by increasing the air pressure above the liquids surface
If the pressure is increased, the molecules in the vapor must move fast to exert enough pressure to keep the bubble form collapsing.
Extra pressure can be provided either by going deeper below the surface of the liquid
Like in geysers
Or by increasing the air pressure above the liquids surface
Which is how a pressure cooker works.

17. Melting
Evaporating
Condensing
Freezing

18. A = MELTING
B = BOILING
Temperature (F) B
Gas
Liquid A
Solid
Time (s)

19. Melting & Freezing

20. Suppose that you are holding hands with someone and both of you start jumping.
What happens as you start to jump around randomly?

21. Something like this happens to the molecules of a solid when it is heated.
As heat is absorbed, the molecules vibrate more and more violently.
If enough heat is absorbed, the attractive forces between the molecules will no longer be able to hold them together
The solid melts

22. Freezing is the opposite of this process
Energy is withdrawn from a liquid, molecular motion diminishes until finally the molecules, on the average are moving slowly enough so that the attractive forces between them are able to cause cohesion.
The molecules then vibrate about a fixed position and form a solid

23. What would happen if you were to add salt or sugar to water?
Would it freeze faster, slower, the same?
Why?

24. Regelation

25. Melting under pressure and freezing again when the pressure is reduced
The ability to do this is one of the properties of water that distinguishes if from other materials
Can you think of any examples?

26. Energy and Changes of Phase

27. If we continually add heat to a sold or liquid, the solid or liquid will eventually change phase
Energy input is required for both liquefaction of the solid and vaporization of a liquid
Energy must be extracted to reverse the process

28. A refrigerator is a heat pump that “pumps” heat from a cold environment to a warm one.
This is accomplished by a liquid of low boiling point, the refrigerant, which is pumped into the cooling unit, where it turns into a gas

29. Heat pumps of various designs are increasingly being utilized to heat and cool homes
What these heat pumps have in common with one another is that they function like a standard refrigerator.

30. An air conditioner is a heat pump in reverse.
Employing the same principles, it simply pumps heat energy from inside the home to the outside

31. Quick Check In order to become a liquid a solid must _______ energy.
In order to become a liquid a gas must ________ energy.

32. Heat of Fusion
Either the energy needed to separate molecules from the solid phase or the energy released when bonds form in a liquid that change it to the solid phase

33. Heat of Vaporization
Either the energy required to separate molecules from the liquid phase or the energy released when gas condenses to the liquid phase