1. Phase Changes (Section 3.3)

2. Characteristics of Phase Changes
Phase change is a reversible physical change that occurs when a substance changes from one state of matter to another
Melting, freezing, vaporization, condensation, sublimation, and deposition are six common phase changes

3. Most substances can exist in 3 states, depending on temperature and pressure
If energy is added or removed, the states can change

4. Temperature and Phase Changes
The temperature of a substance does not change during a phase change
Temperature remains the same until the phase change is complete
The temperature at which a substance freezes (freezing point) is identical to the temperature at which it melts (melting point)

5. Phase Changes that REQUIRE Energy (Endothermic)

6. Melting
Melting Point: the temperature where the forces holding together the crystal lattice of a solid are broken, turning it into a liquid
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
When all can move melting is complete

7. Vaporization Process of a liquid changing into a gas or vapor
Vaporization - the phase change in which a substance changes from a liquid to a gas
Vaporization is an endothermic process
A substance must absorb energy in order to change from a liquid to a gas
One gram of water gains 2261 joules of energy when it vaporizes
This is its heat of vaporization
Heat of vaporization varies from substance to substance

8. Evaporation: when vaporization occurs only at the surface of a liquid
Boiling Point: the temperature where the pressure of the liquid equals the external or atmospheric pressure
Vaporization
Evaporation takes place at the surface of a liquid and occurs at temperatures below the boiling point
Evaporation - the process that changes a substance from a liquid to a 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 and the walls of the container
The vapor pressure increases as the temperature increases
Boiling
Heat a pot of water
Temperature and vapor pressure increase
When vapor pressure and atmospheric pressure become equal the water boils
As temperature increases water molecules move faster and faster
When it reaches 100 degrees Celsius 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
Takes place throughout a liquid

9. Sublimation
Process where a solid changes directly into a gas, without becoming a liquid
Sublimation - phase change in which a substance changes form a solid to a gas or vapor
Endothermic
Example is dry ice
Goes from solid carbon dioxide to a gas
The gas causes water vapor to condense and that’s how it forms clouds or the fog

10. Phase Changes that RELEASE Energy (Exothermic)

11. Freezing
When enough energy is removed, the bonds between molecules become frozen into set positions
Freezing Point: the temperature at which a liquid turns into a crystalline solid
Energy flows from the water to the air in a freezer and the water cools down
As kinetic energy of molecules decrease, the move more slowly
Attractions between molecule take effect
When all molecules have been drawn into orderly arrangement freezing is complete

12. Condensation Process where a gas or vapor becomes a liquid
Energy is released when new bonds are formed
Condensation - the phase change in which a substance changes from a gas or vapor to a liquid
An exothermic process
Responsible for the morning dew on grass and the “cloud” on the mirror after you take a shower.

13. Deposition Process of a gas or vapor changing directly into a solid
Energy is released as crystals are formed
Deposition - phase change in which a gas or vapor changes directly into a solid without first becoming a liquid
Exothermic
Opposite of sublimation
Causes frost on cold window glass