PHASES OF MATTER!
Phase Diagrams
Summarizes the effect of temperature and pressure on a substance in a closed
container.
PHASE DIAGRAM
 AB line: rate at which solid sublimes to form a gas = rate at which gas condenses to
form a solid
 The points along AB represent all combinations of temperature and pressure at
which the solid is in equilibrium with the gas.
PRESSURE TEMP DIAGRAM
BC line: rate at which liquid boils to form a gas = rate at which gas condenses to form
a liquid
PRESSURE TEMP DIAGRAM
 BD line: rate at which solid melts to form a liquid = rate at which liquid freezes to
form a solid
 The BD line is almost vertical because the melting point of a solid is not very
sensitive to changes in pressure
PRESSURE TEMP DIAGRAM
The solid-liquid phase boundary of most substances has a positive slope. This is due
to the solid phase having a higher density than the liquid, so that increasing the
pressure increases the melting temperature.
PRESSURE TEMP DIAGRAM
Changes in Pressure:
Point B is the point at which a pure substance can exist simultaneously as a
solid, a liquid, and a gas. This is called the triple point.
PRESSURE TEMP DIAGRAM
Point C is the critical point of the substance, which is the highest
temperature and pressure at which a gas and a liquid can coexist
PRESSURE TEMP DIAGRAM
The “normal” boiling point and freezing point occur at 1
atm pressure.
EXAMPLE
1. Determine the normal
melting point for the
substance graphed.
2. Determine the normal
boiling point for the
substance graphed
3. Determine the temperature
and pressure at which the
triple point occurs.
4. Is the solid more dense than
the liquid?
5. If you had this substance at
1.5atm and 200oC and
decreased the pressure to
0.5 atm, what would
happen?
TEMPERATURE VS. ENERGY GRAPH:
Melting Point and Freezing Point
Pure, crystalline solids have a characteristic
melting point, the temperature at which the
solid melts to become a liquid.
MP & FP
When the solid is turning into a liquid it remains
at a constant temperature until all of the solid
becomes a liquid. This is because the energy
being put into the solid is going into changing
the state of the solid and therefore isn’t used
to increase the temperature of the compound.
Once all of the solid has become a liquid the
temperature can increase.
MP & FP
• Liquids have a characteristic temperature at
which they turn into solids, known as their
freezing point. In theory, the melting point of
a solid should be the same as the freezing
point of the liquid.
• Melting points are often used to help identify
compounds.
BOILING POINT
When a liquid is heated, it eventually reaches a
temperature at which the vapor pressure is
large enough that bubbles form inside the
body of the liquid. This temperature is called
the boiling point. Once the liquid starts to boil,
the temperature remains constant until all of
the liquid has been converted to a gas.
BOILING POINT
Pressure and temperature both affect the freezing and boiling points. Below
is a chart that shows the freezing and boiling points of water at 1 atm.
Examples:
1. What is this substances
freezing point?
2. What is it’s melting
point?
3. At 80oC, what state of
matter is this
substance in?
4. At what temperatures is
this substance a solid?
ASSIGNMENT
(See Phase Diagram
Assign pg 18 & 19)