Phase Changes

1
Phase Changes

• Learning Target
• What is involved in change of state?

2
Changes of State

• Also known as phase change
• The conversion of a substance from one physical
  state to another
• Change of state always involves a change in
  energy.

3
Potential Energy of three states
4
Vaporization Condensation

• Vaporization liquid to gas (endothermic)
• Condensation gas to liquid (exothermic)
• Vaporization Vs Evaporation
• The escape of liquid molecules from the surface
  of the liquid.
• Vaporization includes evaporation and boiling.

5
Evaporation

• Rapidly moving molecules on the surface of the
  liquid overcome force of attraction and escape.
• Slower molecules remain in a liquid state, thus
  decreasing average kinetic energy of the liquid
  which is observed by decrease in temp. (known as
  evaporative cooling)
• Evaporation ?, as temperature?.

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Liquid-Vapor Equilibrium

• Dynamic Equilibrium A state in which two
  opposing processes take place simultaneously.
• The number of molecules entering the gaseous
  state is equal to the number of molecules
  entering the liquid state.

7
Vaporization Condensation

• When does water start boiling?
• Boiling Point The temperature at which the vapor
  pressure of a liquid becomes equal to the
  atomospheric pressure.
• Heat of vaporization energy needed to convert a
  certain amount of liquid into gas.
• Condensation is exact opposite of vaporization.

8
Freezing Melting

• Freezing Point The temperature at which solid
  and liquid states of a substance exists together.
• Heat of Fusion The energy needed to convert a
  given amount of a solid into a liquid.
• Freezing is exact opposite of melting. The values
  for the both processes is same with opposite
  signs.
• Freezing (exothermic), ?H is negative
• Melting (endothermic), ?H is positive

9
Sublimation Deposition

• Sublimation solid to gas, the energy change
  associated is called Heat of sublimation.
• Deposition Gas to solid, the energy change is
  Heat of deposition.

10
Phase Diagram