1. Topic: Phase Changes
Essential Question: How does temperature of the environment surrounding a substance affect the structure and phase change in which the substance is in?

2. In your teams discuss the following:
How does the Water Cycle work and what are the different phase changes in it (liquidsolidliquidgas)?
Why does water on the stove create bubbles?
Why does water boil at lower temperatures at higher altitudes?
What do the water molecules look like during different phase changes?

3. By the end of the lesson you should be able to…
Explain a phase change and the temperature during each change.
Describe the difference between endothermic and exothermic energy transfers during phase changes.
Explain what happens to the molecules during each phase change (melting, freezing, vaporization, condensation, sublimation & deposition)

4. MI#1: Characteristics of Phase Changes
When at least two states of a substance are present, each state is described as a phase
Phase change= a reversible physical change that occurs when a substance changes from one state of matter to another

5. MI#1: Characteristics of Phase Changes
Melting, freezing, vaporization, condensation, sublimation, and decomposition are six common phase changes.

6. MI# 2: Temperature and Phases Changes
The temperature of a substance does not change during a phase change.
Temperature remains the same until the phase change is complete.

7. MI# 2: Temperature and Phases Changes
The temperature at which a substance freezes (Freezing Point) is identical to the temperature at which it melts (Melting Point).

8. MI#3: Energy & Phases changes “Endothermic”=Melting
During a phase change energy is transferred between a substance and its surroundings.
Energy is either absorbed or released during a phase change.
Melting is an example of an endothermic change.

9. MI#3: Energy & Phases changes “Endothermic”=Melting
Endothermic change= the system absorbs energy
The amount of energy absorbed depends on the substance.
i.e. One gram of water absorbs 334 Joules of energy as it melts is its heat of fusion.

10. MI#3: Energy & Phases changes “Endothermic”=Melting
Heat of fusion= varies from substance to substance, fusion is term for “melting”
The arrangement of molecules in water becomes less orderly as water melts and more orderly as water freezes.
solid
liquid
gas

11. MI#3: Energy & Phases changes “Endothermic”=Melting
Attractions between molecules keep them in fixed positions.
Heat flows from air to ice.
Ice gains energy and molecules vibrate.
Some gain enough energy to overcome the attractions and move away from other molecules of water.
When all molecules can move melting is complete.

12. MI#4: Energy and Phase Changes “Exothermic”=Freezing
Freezing is an example of an exothermic change.
Exothermic change= the system releases energy to its surroundings

13. MI#4: Energy and Phase Changes “Exothermic”=Freezing
Energy flows from the water to the air in a freezer and the water cools down.
As kinetic energy of molecules decrease, they move more slowly.
Attractions between molecules take effect and molecules move closer together.
When all molecules have been connected into an orderly fixed arrangement freezing is complete.

14. MI#5: Vaporization
Vaporization= the phase change in which a substance changes from liquidgas
Vaporization is an endothermic process (a substance must absorb energy in order to change from a liquid to gas).

15. MI#5: Vaporization
One gram of water gains Joules of energy when it vaporizes (this is it’s heat of vaporization which varies from substance to substance).
Evaporation takes place at the surface of a liquid and occurs at temperatures below the boiling.

16. Water in a closed container
MI#5: Vaporization
Evaporation= the process that changes a substance from a liquidgas at temperatures below the substance’s boiling point
Water in a closed container
Water evaporates and vapor collects above the liquid.
Vapor pressure (caused by collisions of the vapor molecules and the walls of the container) increases as the temperature increases.

17. MI#6: Boiling: Why do bubbles form when water boils?
Heat a pot of water (temperature and vapor pressure increase).
When vapor pressure and atmospheric pressure become equal the water boils.
As the temperature increases water molecules move faster and faster.

18. MI#6: Boiling: Why do bubbles form when water boils?
When temperature reaches 100oC some molecules below the surface overcome the attraction forces and form bubbles of water vapor.
When the bubbles reach the surface, they pop and release water vapor into the air.

19. MI#7: Condensation
Condensation= the phase in which a substance changes from a gas or vaporliquid
An exothermic process that is responsible for the morning dew on grass and the “cloud” on the mirror after a shower.

20. MI#8: Sublimation and Deposition
Sublimation= endothermic phase change in which a substance changes from a solid to a gas or vapor
Example: dry ice
Goes from solid carbon dioxidegas
The gas causes water vapor to condense and forms clouds or fog of gas

21. MI#8: Sublimation and Deposition
Deposition= exothermic phase change in which a gas or vapor changes directly into a solid without first becoming a liquid (gasliquidsolid)
Opposite of sublimation
Example: frost on cold windows or glass

22. Where are you at now… Are you able to…
Explain a phase change and the temperature during each change.
Describe the difference between endothermic and exothermic energy transfers during phase changes.
Explain what happens to the molecules during each phase change (melting, freezing, vaporization, condensation, sublimation & deposition)

23. Checking For Understanding
Draw the phase change graph and label each major phase change:
Explain what happens to the temperature during each phase change.
Create two columns and explain Endothermic and Exothermic energy transfer and write which phase occurs for each.