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Phase Changes. Physical Science Mr. Moss RHS. Phases. When 2 states are present at the same time, we describe each as a phase. Here, we see 2 phases of water: Solid Phase Liquid Phase. Phase Change Definition.

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phase changes

Phase Changes

Physical Science

Mr. Moss


  • When 2 states are present at the same time, we describe each as a phase.
  • Here, we see 2 phases of water:
    • Solid Phase
    • Liquid Phase
phase change definition
Phase Change Definition
  • A 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
    • Melting
    • Freezing
    • Vaporization
    • Condensation
    • Sublimation
    • Deposition







phase change
Phase Change
  • NOTE:
    • The temperature of the substance DOES NOT change during a phase change!!!!
  • All phase changes are related to energy and temperature.
energy during phase changes
Energy during Phase Changes
  • Law of Conservation of Energy
    • Neither created or destroyed.
    • Transferred or Transformed
  • In phase changes, energy is either absorbed or released.
    • Endothermic – energy is absorbed.
      • Ice melting
    • Exothermic – energy is released.
      • Water freezing
heat of fusion
Heat of Fusion
  • 1g of ice absorbs 334 joules (J) of energy as it melts.
  • This amount of energy is the Heat of Fusion for water.
  • This is another term for the melting process.
triple and critical point
Triple and Critical Point
  • The Triple Point is where the substance exists in all 3 states.
  • The Critical Point is where, under extreme high temps and pressure, the liquid and gaseous states are indistinguishable.
melting freezing
Melting & Freezing
  • Water is a molecule made up of 2 atoms of Hydrogen and 1 atom of Oxygen.
  • The arrangement of molecules becomes less orderly as water melts.
  • The arrangement of molecules becomes more orderly as water freezes.
  • Endothermic process
  • Heat energy is transformed into increased kinetic energy.
  • Causes molecules to vibrate more rapidly.
    • Some gain enough to overcome the attractive forces and move from the fixed locations.
    • This is the melting point.
  • Any energy gained after the phase change increases the average kinetic energy and the temperature rises.
  • Exothermic
  • Average kinetic energy (Ke) decreases causing molecules to slow down.
  • At the freezing point, the attractive forces begin to draw the molecules into an orderly arrangement.
    • Continues until all have been fixed.
  • Any energy removed after the phase change is complete decreased the average kinetic energy of the molecules and the temperature decreases.
  • Vaporization happens when a substance changes from a liquid into a gas.
    • Endothermic
    • Heat of vaporization
      • Water gains 2258 J of energy when it vaporizes at 100° C.
    • 2 vaporization processes
      • Boiling
      • Evaporation
        • Takes place at the surface of a liquid
        • Occurs at temps below the boiling point
  • Process that changes a substance from a liquid to a gas at temps below the boiling point.
  • In a closed container:
    • Water vapor collects above the liquid
    • Pressure caused by molecules colliding with container is called vapor pressure.
      • Pressure increases as temp increases.
  • As you apply heat energy, temperature and vapor pressure increase.
  • When vapor pressure = atmospheric pressure, the water boils.
  • Called the boiling point.
  • Depends on Atmospheric Pressure
    • Higher elevations have less atmospheric pressure
      • Takes longer to cook food.
  • The phase change in which a substance changes from a gas to a liquid.
  • The water vapor transferred heat to the glass and condensed into a liquid.
sublimation deposition
Sublimation & Deposition
  • Sublimation is the phase change from a solid to a gas without going through the liquid phase.
    • Endothermic
    • Dry Ice
  • Deposition is the phase change from a gas to a solid without going through the liquid phase.
    • Exothermic
    • Frost on windows