Phase diagrams.doc
10 PHASE DIAGRAMS
KINETIC THEORY REVIEW
1. Volume of Particles << Volume of surrounding space
2. Gas particles are in constant rapid motion in straight lines until they collide with
another gas particle or the wall of their container.
3. Collisions are perfectly elastic.
4. The average velocity of the particles is directly related to the temperature of the
gas.
KINETIC THEORY WORKS FOR MOST CONDITIONS BUT HAS LIMITATIONS
KINETIC THEORY LIMITATIONS
When Pressure is high and Temperature is low two parts of the Kinetic Theory no longer
apply.
1. Under high pressure and low temp Volume of Particles is no longer << Volume of
surrounding space
GAS UNDER NORMAL
CONDITIONS
GAS UNDER HIGH PRESSURE
AND LOW TEMPERATURE
2. Collisions are perfectly elastic.
a. When Pressure is increased and Temperature is decreased the collisions
no longer represent elastic collisions.
b. Particles may be considered to stick together momentarily
VAPOR PRESSURE INCREASES WITH TEMPERATURE
Molecules that overcome atmospheric
pressure escape the surface – this causes
evaporation
Some water molecules from the air above collide
with the surface and stay in the liquid – this
causes condensation
LIQUID WATER
RATE OF EVAPORATION > RATE OF CONDENSATION
CONDITIONS THAT EFFECT EVAPORATION
If Pressure on the surface increases, the rate of evaporation _________
If Pressure on the surface decreases, the rate of evaporation _________
If Temperature of the liquid increases, the rate of evaporation _________
If Temperature of the liquid decreases, the rate of evaporation _________
The best conditions for evaporation:
LIQUID WATER IN A CLOSED CONTAINER

WATER BEGINS TO EVAPORATE FROM SURFACE

WATER CONTINUES TO EVAPORATE UNTIL THE PRESSURE IN
THE UPPER PORTION OF THE CONTAINER BECOMES TO
GREAT FOR ANY MORE TO EVAPORATE

EVENTUALLY PRESSURE REACHES A MAXIMUM – THIS IS
CALLED THE VAPOR PRESSURE

AT THE VAPOR PRESSURE THE RATE OF EVAPORATION =
RATE OF CONDENSATION

EQUILIBRIUM

EVAPORATION
CONDENSATION
CAUSING A LIQUID TO BOIL
 If the temperature of the liquid is increased, the pressure required to keep
evaporation from occurring increases. VAPOR PRESSURE _____
 If temperature continues to increase eventually the pressure within the liquid
will be larger than the atmosphere above it. Patomsphere = PVapor.
 Bubbles form and the liquid boils.
BOILING IS DEPENDENT UPON TEMPERATURE AND PRESSURE
If atmospheric Pressure increases, the Boiling Temp _________
If atmospheric Pressure decreases, the Boiling Temp _________
VAPOR PRESSURE INCREASES WITH TEMPERATURE – page 390
Volatility of a liquid – its tendency to vaporize (evaporate)
HIGH VOLATILITY
– evaporates easily
-- weak attractive forces between molecules
LOW VOLATILITY
– requires low Pressure and high Temp
-- strong attractive forces between molecules
PHASE DIAGRAMS RELATE TEMP, PRESSURE AND PHYSICAL
STATE
PHASE DIAGRAM FOR WATER
1 atm
LIQUID
Pressure
(atm)
SOLID
VAPOR
373 K
Temperature (K)
WATER BOILING TO STEAM CROSSES LIQUID-VAPOR LINE
VAPORIZATION
1 atm
LIQUID
Pressure
(atm)
SOLID
VAPOR
373 K
Temperature (K)
KEY POINTS ON A PHASE DIAGRAM
Critical point
The temperature above which the gas cannot be liquefied no matter how much
pressure is applied (the kinetic energy simply is too great for attractive forces to
overcome, regardless of the applied pressure)
Triple point
The "triple point" is the particular condition of temperature and pressure where all
three physical states are in equilibrium
VOCABULARY WORDS –
EVAPORATION
VAPORIZATION
CONDENSATION
FREEZING
MELTING
SUBLIMATION
CRITICAL POINT
TRIPLE POINT
NORMAL BOILING POINT
NORMAL FREEZING POINT
VAPOR PRESSURE
SATURATED VAPOR
EQUILIBRIUM
VOLATILE
1 atm
WATER
Pressure
(atm)
ICE
AN ANOMOLY OF WATER

WATER
VAPOR
Temperature (K)
373 K
The frozen state of water (ice) is actually less dense than the liquid state, thus,
the liquid state is more compact than the solid state
o Increasing pressure, which will favor compactness of the molecules, will
thus favor the liquid state
Increasing pressure will thus lower the temperature at which the solid will melt


The melting curve slopes to the left (NEGATIVE SLOPE), unlike most
compounds
At pressures below .006 atm, water will be present as either a gas or solid,
there can be no liquid phase
INTERNET REFERENCE
http://wine1.sb.fsu.edu/chm1045/notes/Forces/Phase/Forces06.htm