3.3 Phase Changes
What are we learning?
 Define phase change
 Explain how temperature can be used to recognize a
phase change
 Explain what happens to the motion, arrangement and
average kinetic energy of water molecules during
phase changes
 Describe each of the 6 phase changes
 Identify phase changes as endothermic or exothermic
Characteristics of a Phase Change
 If 2 states of the same substance are present at the
same time, we describe each different state as a phase.
 Ex: an iceberg floating in the ocean- solid phase and
liquid phase
 A phase change is the reversible physical change that
occurs when a substance changes from one state of
matter to another
 Melting, freezing, vaporization, condensation,
sublimation and deposition are the 6 common phase
changes
Temperature and Phase Changes
 One way to recognize phase changes- measuring the
temperature as it is heated or cooled
 The temperature of a substance does not change
during a phase change.
Ex Naphthalene
 Naphthalene is used in
mothballs.
 This graph is of data
collected when it is heated.
 The temperature rises as it
warms up, until it hits about
80C. (the melting point)
 It will remain at 80C until it
has all melted .
Energy and Phase Changes
 During a phase change, energy is transferred between
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a substance and its surroundings.
Energy is either absorbed or released during a
phase change.
Ex: Melting- endothermic change
During an Endothermic change, the system absorbs
energy from its surroundings.
The amount of energy absorbed varies
 Ex: one gram of ice absorbs 334 joules of energy as it
melts. (this is the heat of fusion of water)

Fusion is another term for melting
Energy and Phase Changes
 When water freezes, it releases the same amount of
energy (334J) to its surroundings.
 Freezing is an exothermic change
 In exothermic change the system releases energy to its
surroundings.
Melting and Freezing
 The arrangement of molecules in water becomes less
orderly as water melts and more orderly as water
freezes. (remember the BB’s)
 Attractions between water molecules in ice keep the
molecules in fixed positions
 As ice warms up the molecules vibrate more quickly
 When ice gets near the melting point (OC), some
molecules gain enough energy to over come the
attraction with other molecules and move from their
fixed positions.
 (melting is complete when all the molecules can move).
Freezing
 When water is placed in a freezer, energy flows from
the water to the air in the freezer, and the water cools
down.
 The kinetic energy decreases and the molecules move
more slowly
 As the molecules slow down, forces of attraction have a
greater effect
 When all the molecules have been drawn into an
orderly arrangement, freezing is complete
Vaporization and Condensation
 Vaporization is the phase change where a substance
changes from a liquid to a gas
 it’s an endothermic process which means, that a
substance must absorb energy in order to change from
a liquid to a gas
 One gram of water gains 2261 joules of energy- this is
the heat of vaporization of water
 There are 2 vaporization process, boiling and
evaporation
Evaporation
 Evaporation occurs at the surface of a liquid and at
temperatures lower than the boiling point.
 Evaporation is the process that changes a substance
from a liquid to a gas at temp. below the boiling point.
 When water evaporates, some molecules near the
surface are moving fast enough to escape and become
water vapor.
 The greater the surface area, the faster it evaporates
 Vapor pressure is the pressure caused by the collisions
of water vapor and the walls of a closed container
Boiling
 As you heat up a pot of water, the
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temperature and vapor pressure of
water increases
The water boils when the vapor
pressure becomes equal to the
atmospheric pressure (the temp.
at which this happens is the
boiling point)
As the temp. increases during
boiling, the molecules move faster
When the temp gets near 100C,
some molecules below the surface
have enough kinetic energy to
overcome the attraction with
other molecules
Bubbles rise to the surface, burst
and release water vapor into the
air
The boiling point depends on
atmospheric pressure (higher
altitude = lower BP)
Condensation
 Condensation is the phase change where a substance
changes from a gas/vapor to a liquid. (morning dew!)
 Is an exothermic process (energy releasing)
Sublimation and deposition
 Sublimation the phase change where a substance goes
from a solid to a gas/vapor without changing to a
liquid first.
 When a gas /vapor changes directly to a solid without
going through the liquid phase it’s called deposition.
(exothermic)
 Causes frost to form on windows