1. Phase Change
Change of State

2. What causes a phase change?
Adding or removing thermal energy can cause a substance to change its state of matter
Adding thermal energy causes the particles to move faster (increase kinetic energy) and/or increase potential energy
Decreasing thermal energy causes the particles to move slower (decrease kinetic energy) and/or decrease potential energy

3. Solid to Liquid Melting – changing from SOLID to a LIQUID
Thermal energy is ADDED until the particles have enough energy to overcome the attractive forces between them
At its melting point, the particles of a solid substances break free from their fixed positions, and can now slide past one another

4. Liquid to Solid Freezing – changing from a LIQUID to a SOLID
Thermal energy is REMOVED until the particles do not have enough energy to overcome the attractive forces between them
At its freezing point, the particles of a liquid substance are moving so slowly that they can no longer move freely, and remain in a fixed position

5. Liquid to Gas
Vaporization (evaporation/boiling) – changing from a LIQUID to a GAS
(Evaporation – occurs only on the surface, Boiling – occurs throughout the liquid)
Thermal energy is ADDED until the particles have enough energy to overcome the attractive forces between them
During vaporization, the particles of a liquid break free from those around them and can move freely

6. Gas to Liquid Condensation – changing from a GAS to a LIQUID
Thermal energy is REMOVED until the particles slow down, move closer, and do not have enough energy to overcome the attractive forces between them.
Condensation occurs when the particles in a gas slow down and move closer together.

7. Solid to Gas –or- Gas to Solid
Sublimation – changing from a SOLID directly to a GAS
Ex/ dry ice
Deposition – changing from a GAS directly to a SOLID
Ex/ frost

10. Homework
Describe what happens to the motion and energy of the particles of an ice cube when it melts.
Describe what happens to the motion and energy of the particles of water when it freezes.