PHASE CHANGES
SECTION 3.3
CHARACTERISTICS OF PHASE
CHANGES
A. A phase change is the reversible physical
change that occurs when a substance
changes from one state of matter to
another
B. Melting, freezing, vaporization,
condensation, sublimation, and
deposition are six common phase
changes
C. Temperature and Phase Changes
1. The temperature of a substance does not
change during a phase change
2. When a solid changes to a liquid
a. At first the temperature rises as a solid warms up
b. At the melting point, the temperature stops rising
c.
and remains the same until the melting is
complete
The temperature will begin to rise again
3. When a liquid changes to a solid
a. At first the temperature of the liquid
will drop until it reaches the freezing point
b. At the freezing point, the temperature will
remain at that temperature until all of the
liquid freezes
c. After the freezing is complete, the
temperature will begin to drop again
4. The temperature at which a substance
freezes (its freezing point) is identical to the
temperature at which it melts (its melting point)
5. When a liquid changes to a gas
a. The temperature keeps rising until it
reaches the boiling point
b. The temperature then remains the same
until the boiling is complete
D. Energy and Phase Change
1. Energy is either absorbed or released during
a phase change
2. During an endothermic change, the system
absorbs energy from its surroundings
a. Melting, vaporization, and sublimation are all
b.
examples of endothermic changes
The heat of fusion is the amount of energy a
substance must absorb in order to change from a
solid to a liquid. Fusion is another term for
melting
3. One gram of water releases 334 joules
of energy to its surroundings as it freezes.
a. This is the same amount of energy
absorbed when one gram of ice melts
b. Farmers use this release of energy
to protect their crops
4. During an exothermic change, the
system releases energy to its
surroundings. Freezing, deposition, and
condensation are examples of exothermic
reactions
MELTING AND FREEZING
A. The arrangement of molecules in water
becomes less orderly as water melts and
more orderly as water freezes
B. Melting
1. In ice, the attractions between water
molecules keep the molecules in fixed
positions
2. When ice is removed form the freezer, heat
flows from the air to the ice
3. As the ice gains energy, the molecules
vibrate more quickly.
4. At the melting point of water some
molecules gain enough energy and
move from their fixed position.
5. When all of the molecules gain energy
melting is complete.
C. Freezing
1. When liquid water is placed in a
freezer, energy flows from the water to
the air in the freezer
2. This process is the reverse of the
melting process.
VAPORIZATION & CONDENSATION
A. Vaporization is the phase change in
which a substance changes from a liquid
into a gas.
B. A substance must absorb energy in order
to change from a liquid to a gas.
C. The heat of vaporization for water is
equal to 2261 joules of energy.
D. There are two vaporization processes –
boiling and evaporation.
E. Evaporation takes place at the surface of
a liquid.
1. Evaporation is the process that changes a
substance form a liquid to a gas at
temperatures below the boiling point.
2. A vapor is the gaseous phase of a substance
3. Vapor pressure is the pressure caused by the
collisions of vapor and the walls of the
container.
F. Boiling
1. When vapor pressure becomes equal
to atmospheric pressure, water boils.
2. The temperature at which this
happens is the boiling point of water.
3. As temperature increases, water
molecules move faster and faster.
4. When the temperature reaches 100°C,
some molecules below the surface have
enough kinetic energy to overcome the
attraction of neighboring molecules.
5. Because water vapor is less dense than
water, the bubbles rise to the surface.
6. The boiling point of a substance
depends on the atmospheric pressure.
a. At higher elevations the
atmospheric pressure is lower
b. This will cause the boiling point to
occur at a lower temperature.
c. Cooking will take longer at higher
elevations
G. Condensation is the phase change in
which a substance changes from a gas
or vapor to a liquid. This process is
responsible for the dew on grass and the
condensation on bathroom mirrors.
SUBLIMATION & DEPOSITION
A. Sublimation is the phase change in which
a substance changes from a solid to a
gas or vapor without changing to a liquid
first.
B. Example, at room temperature, dry ice
can directly change from a solid to a
colorless gas.
C. As dry ice sublimes, the cold carbon
dioxide vapor causes water vapor in the
air to condense and form clouds.
D. Deposition is the phase change that
occurs when a gas or vapor changes
directly into a solid without first changing
to a liquid.
E. Deposition causes frost to form on
windows