Section 3.3

Phase Changes

Phase Change- is the reversible physical change that occurs when a substance changes from one state of matter to another.There are 6 common phase changes:1. Melting2. Freezing3. Vaporization4. Condensation5. Sublimation6. Deposition

Phase Change Diagram

Gas

LiquidSolid

• The temperature of a substance does not change during a phase change.

• Energy is either absorbed or released during a phase change.

• Endothermic change- is where the system absorbs energy from the surroundings.

• Ex: Melting (also called fusion)- need heat• The amount of energy a substance must

absorb to change from a solid to a liquid is called the heat of fusion.

• Exothermic change- the system releases energy to its surroundings.

• Ex: Freezing- release heat to freezeMelting and Freezing• The arrangement of molecules in water

become less orderly as water melts and more orderly as water freezes.

• Substances do not have to be cold to freeze. Means becoming a solid.

• Ex: Cookies baking

Vaporization and Condensation

• Vaporization is an endothermic process.• Changing from a liquid to a gas.• Heat of vaporization is the amount of energy a

substance must absorb to change from a liquid to a gas.

• Evaporation• The process that changes a substance from a

liquid to a gas temperature below the substance’s boiling point.

• It takes place at the surface of a liquid.• Vapor pressure- the pressure caused by collisions

of this vapor and the walls of the closed container.• Boiling- (for water) this is where the vapor

pressure becomes equal to the atmospheric pressure.

• Condensation-the phase change in which a substance changes from a gas to a liquid.

• This is an exothermic process.• Water vapors transfer heat to an object and cools

into a liquid. Ex: Dew on grass

Sublimation and Deposition• Sublimation- the phase change in which a

substance changes from a solid to a gas or vapor.

• This is an endothermic process.• Ex: Dry ice (solid carbon dioxide)• Deposition- the phase change in which a

substance changes from a gas or vapor directly to a solid.

• This is an exothermic process.• Ex: Frost