States of Matter
Section 12.4: Phase Changes
Objectives
► Explain
how the addition and removal of
energy can cause a phase change.
► Interpret a phase diagram.
Phase Changes
► Most
substances can exist in 3 states
depending on the temperature and
pressure.
► Water exists in 3 states on Earth - ice
(solid), water (liquid), and water vapor
(gas).
► States are referred to as phases when they
coexist; for example, ice water has 2
phases, ice and liquid water.
Phase Changes & Energy
•When energy is added or
removed from a system,
one phase can change into
another.
•Melting, vaporization &
sublimation require
energy.
•In deposition,
condensation, & freezing,
energy must be removed.
Melting (or Fusion)
► Heat
(the transfer of energy
from an object at a higher
temperature to an object at a
lower one) moves from the
water into the ice. In this
case, when it is assumed the
temperature of the ice and
water are the same at 0 0C,
the energy absorbed by the
ice is NOT used to raise its
temperature.
Melting (or Fusion)
► The
energy
absorbed by the ice
goes into breaking
the hydrogen bonds
between the water
molecules in the
solid. The
molecules can then
move apart and
become liquid.
Melting (or Fusion)
► Water
has many hydrogen bonds to break, so a lot
of energy is needed to melt it (change the solid to
liquid). Different substances will require different
energies. Ionic compounds (with ionic bonds) will
require more energy.
► The
melting point of a crystalline solid is the
temperature at which the forces holding the
crystal lattice together are broken and it becomes
a liquid.
► Every
element and compound has a different
melting point.
The Heating Curve for Water
► As
the ice (in our ice
water) melts, ITS
TEMPERATURE
REMAINS CONSTANT.
► Once it is all melted,
additional energy
increases the kinetic
energy of the
molecules and the
water temperature
increases.
►Molecules
with enough energy can
escape the liquid and become a gas.
►Vaporization is the process by which a
liquid changes to a gas or vapor.
Vaporization
► As
more energy is
added, more molecules
escape to the gas
phase. As this occurs,
THE TEMPERATURE
DOES NOT CHANGE.
► When all molecules are
in the gas phase,
additional energy will
cause an increase in
temperature.
What is evaporation?
► When
vaporization
takes place only at
the surface of the
of the liquid, the
process is called
evaporation.
► This
can happen at
LOW temperatures,
especially if there is
light energy.
VAPOR PRESSURE
►The
pressure
exerted by a
vapor over a
liquid is called
vapor pressure.
►An
increase in
temperature will
increase the
vapor pressure.
Vapor Pressure Curves
►
►
A vapor pressure curve
shows how the vapor
pressure of a liquid (yaxis) changes as a function
of temperature (x-axis).
When the curve intersects
the point where the vapor
pressure equals the
atmospheric pressure (760
mm Hg), the temperature
is called the normal boiling
point.
What is boiling?
► The
temperature at
which the vapor
pressure of a liquid
equals the external
or atmospheric
pressure is called
the BOILING
POINT (or normal
boiling point).
What is boiling?
► At
the boiling point,
molecules
throughout the
liquid have enough
energy to vaporize.
Bubbles of vapor
can be seen in the
liquid.
► Bubbles then rise to
the surface .
Sublimation
► Sublimation
is when a
solid substance
changes directly to a
gas.
► Solid iodine, carbon
dioxide (dry ice),
mothballs, and solid air
fresheners sublime at
room temperature. Ice
cubes sublime in your
freezer.
Condensation
► When
water vapor
molecules lose energy
(perhaps by contact with
a cold object), their
velocity decreases. They
are then more likely to
form hydrogen bonds
when they collide with
other water molecules,
thus forming a liquid.
► The process by which a
gas or a vapor becomes
a liquid is called
condensation.
Deposition
► Deposition
is the
process by which a
substance changes
from a gas or vapor
to a solid without
first becoming a
liquid.
► Energy is released
to the surroundings
as ice crystals form
from water vapor.
Freezing (or Solidification or
Crystallization)
►
►
►
As heat flows from water to
its surroundings, the water
molecules lose kinetic energy.
Formation of hydrogen bonds
will take place which will keep
the molecules fixed into set
positions. A solid will form.
The freezing point is the
temperature at which a liquid
is converted into a crystalline
solid.
.
A New Look at the Heating Curve
► Read
this curve from
RIGHT TO LEFT - take
away heat instead of
adding it.
► Condensation occurs at
the same temperature
as vaporization.
► Freezing occurs at the
same temperature as
melting.
Phase Diagrams
►2
variables control the phase of a
substance.
 Temperature
 Pressure
►A
phase diagram is a graph of pressure vs.
temperature. It will show which phase a
substance will be in at various conditions of
temperature & pressure.
Phase Diagram
Different sections of the graph indicate different phases.
Phase Diagram
► Boundaries
between
the sections are the
temperature &
pressure conditions at
which a phase change
is occurring.
► The triple point is the
point on a phase
diagram that
represents the
temperature and
pressure at which 3
phases of a substance
can coexist.
Phase Diagram
► The
critical point
indicates the critical
pressure and critical
temperature above
which water cannot
exist as a liquid.
► At the critical
temperature, an
increase in pressure
will not change a vapor
into a liquid.
Phase Diagram
► The
phase diagram for each substance is
different because the normal boiling and
freezing points of substances are different.
Water
Carbon Dioxide