Physical Science
Mr. Moss
RHS
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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
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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
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Melting
Freezing
Vaporization
Condensation
Sublimation
Deposition
Melting
Freezing
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NOTE:
◦ The temperature of the substance DOES NOT
change during a phase change!!!!
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All phase changes are related to energy and
temperature.
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Law of Conservation of Energy
◦ Neither created or destroyed.
◦ Transferred or Transformed
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In phase changes, energy is either absorbed
or released.
◦ Endothermic – energy is absorbed.
 Ice melting
◦ Exothermic – energy is released.
 Water freezing
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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.
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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.
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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.
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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.
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Any energy gained after the phase change
increases the average kinetic energy and the
temperature rises.
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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.
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Any energy removed after the phase change
is complete decreased the average kinetic
energy of the molecules and the temperature
decreases.
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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
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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.
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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.
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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.
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Sublimation is the phase change from a solid
to a gas without going through the liquid
phase.
◦ Endothermic
◦ Dry Ice
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Deposition is the phase change from a gas to
a solid without going through the liquid
phase.
◦ Exothermic
◦ Frost on windows