Supercritical Fluid
• as a liquid is heated in a sealed container, more vapor collects
causing the pressure inside the container to rise
 and the density of the vapor to increase
 and the density of the liquid to decrease
• at some temperature, the meniscus between the liquid and vapor
•
disappears and the states commingle to form a supercritical
fluid
supercritical fluid have properties of both gas and liquid states
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http://www.youtube.com/watch?v=yBRdBrnIlTQ
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The Critical Point
• the temperature required to produce a
supercritical fluid is called the critical
temperature
• the pressure at the critical temperature is called
the critical pressure
• at the critical temperature or higher
temperatures, the gas cannot be condensed to a
liquid, no matter how high the pressure gets
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Sublimation and Deposition
• molecules in the solid have thermal energy that allows
•
•
•
them to vibrate
surface molecules with sufficient energy may break
free from the surface and become a gas – this process is
called sublimation
the capturing of vapor molecules into a solid is called
deposition
the solid and vapor phases exist in dynamic equilibrium
in a closed container
 at temperatures below the melting point
 therefore, molecular solids have a vapor pressure
solid
Tro, Chemistry: A Molecular Approach
sublimation
deposition
gas
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Sublimation
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Melting = Fusion
• as a solid is heated, its temperature rises and the
molecules vibrate more vigorously
• once the temperature reaches the melting point,
the molecules have sufficient energy to
overcome some of the attractions that hold them
in position and the solid melts (or fuses)
• the opposite of melting is freezing
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Heating Curve of Water
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Phase Diagrams
• describe the different states and state changes that
occur at various temperature - pressure conditions
• areas represent states
• lines represent state changes
liquid/gas line is vapor pressure curve
both states exist simultaneously
critical point is the furthest point on the vapor pressure
curve
• triple point is the temperature/pressure condition
where all three states exist simultaneously
• for most substances, freezing point increases as
pressure increases
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Phase Diagrams
Fusion Curve
critical
point
Pressure
melting
1 atm
freezing
Liquid
Solid
normal
boiling pt.
normal
melting pt.
Sublimation
Curve
triple
point
vaporization
condensation
sublimation
Gas
deposition
Temperature
Vapor Pressure
Curve
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Pressure
Phase Diagram of Water
1 atm
critical
point
374.1°C
217.7 atm
Ice
Water
normal
melting pt.
0°C
normal
boiling pt.
100°C
triple
point
0.01°C
0.006 atm
Steam
Temperature
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Water – An Extraordinary Substance
• water is a liquid at room temperature
 most molecular substances with small molar masses are gases at room
temperature
 due to H-bonding between molecules
• water is an excellent solvent – dissolving many ionic and polar
molecular substances
 because of its large dipole moment
 even many small nonpolar molecules have solubility in water
 e.g., O2, CO2
• water has a very high specific heat for a molecular substance
 moderating effect on coastal climates
• water expands when it freezes
 at a pressure of 1 atm
 about 9%
 making ice less dense than liquid water
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