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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