Vapor pressure – pressure exerted by evaporated
particles of a liquid on the surface of the liquid
Vapor is a gas form of a substance that is normally a
liquid (Water vapor is evaporated molecules of water in
the gas phase).
Vapor pressure
• Varies depending on the temperature of a liquid
• Higher the temperature, the higher the vapor
pressure of the liquid
• Different substances have different vapor pressure at
given temperatures
• Substances with low vapor pressure have STRONG
intermolecular forces
• Substance with high vapor pressure have WEAK
intermolecular forces
Vapor
50°C
30°C
Liquid
Lower temperature
Less vapor
Lower vapor pressure
Higher temperature
More vapor
Higher vapor pressure
A molecule of a substance enters into the vapor (gas)
phase when the intermolecular forces holding it to other
molecules are broken.
Boiling is a rapid phase change of a liquid to the vapor phase. It
occurs when the vapor pressure of a liquid is equal to the
atmospheric pressure.
Atmospheric pressure is the pressure exerted by the weight of air on
the surface of objects here on earth.
Normal atmospheric pressure = standard pressure = 101.3 kPa = 1 atm
Boiling point is the temperature at which vapor pressure of a liquid
equals the atmospheric pressure
Water boils at 100°C at normal atmospheric pressure (101.3 kPa or 1
atm). At a different atmospheric pressure, water boils at a different
temperature.
Different substances have different boiling points because strength of
intermolecular forces vary from one substance to another.
Propanone has the weakest
intermolecular forces.
Ethanoic acid has the
strongest intermolecular
forces.
normal
atmospheric pressure line
Any temperature that
intersects the curve at this
line is the normal boiling
point of the liquid.
Practice
What is the vapor pressure of propanone at 45°C?
Which liquid has the highest normal boiling point?
1) propanone
3) water
2) ethanol
4) ethanoic acid
What is the boiling point of propanone at normal
atmospheric pressure?
Pure water has the following properties:
• Boiling point at 100°C
• Freezing point at 0°C
• Vapor pressure of 101.3 kPa (at 100°C)
• No electrical conductivity
When a solute is dissolved in water to make a solution,
these physical properties of water will change.
As the number of dissolved particles in the water
INCREASES:
• Boiling point is increased or elevated
• Freezing point is decreased or depressed
• Vapor pressure is decreased or lowered
• Electrical conductivity is increased
Pure water
no dissolved particles
higher freezing point
lower boiling point
NaCl solution more dissolved particles
lower freezing point
higher boiling point
How much higher or lower are the boiling and freezing
points of a solution when compared to those of pure
water depends on:
• Number of dissolved particles the solute produced
(ionized to) in the water
• Concentration of particles in the solution
0.2 M KCl
Lowest concentration
Lowest boiling point
Highest freezing point
0.4 M KCl
0.6 M KCl
Highest concentration
Highest boiling point
Lowest freezing point
A solution of a higher concentration will produce a lower
freezing point and a higher boiling point than a solution of a
lower concentration.
To determine the concentration that produces the lowest
freezing point or highest boiling point:
LOOK for highest Molarity concentration
To determine the concentration that produces the highest
freezing point or lowest boiling point:
LOOK for lowest Molarity concentration
Practice
1) A concentration of which CaCl2 solution will have the
lowest freezing point?
a) 1 M
b) 2 M
c) 3 M
d) 4 M
2) Compared to a solution of 0.5 M LiBr, a solution of 0.1
M LiBr will have a
a) Lower freezing and lower boiling points
b) Lower freezing and higher boiling points
c) Higher freezing and lower boiling points
d) Higher freezing and higher boiling points
Cl1 M CH3OH
CH3OH
Molecular solute
(CH3OH)
1 dissolved particle
Lowest boiling point
Highest freezing point
1 M KCl
K+
Cl-
Ionic solute (KCl)
2 dissolved particles
1M CaCl2
Ca+2
Cl-
Ionic solute (CaCl2)
3 dissolved particles
Molecular substances (such as CH3OH) DO NOT dissolve into ions or break up into
elements when placed in water to dissolve. In solutions, they stay as one molecule
or one dissolved particle.
Ionic substances (such as KCl and CaCl2) dissolve or break up into ions when placed
in water. The number of dissolved particles in an ionic solution depends on the total
number of + and – ions that makes up the ionic formula.
Ionic solutes will always produce more dissolved particles
than molecular solutes. A solution of ionic solute will
always have a lower freezing point and a higher boiling
point than a solution of a molecular compound of the
same concentration.
Ionic solutes that produced three particles will have a
lower freezing point and a higher boiling point than ionic
solutes that produced two particles in solution. A solution
of CaCl2 will always have a lower freezing point and a
higher boiling point than a solution of KCl of the same
concentration.
Practice
A 2 M solution of which solute would have the highest freezing
point?
a) CaCl2
b) C6H12O6
c) KCl
d) KNO3
Which 0.1 M solution will have the highest boiling point?
a) NaNO3
b) Mg(NO3)2 c) CH3OH
d) C2H5OH
Which solution will have the lowest freezing point?
a) 1 M CH3OH
b) 1 M LiBr
c) 2 M CH3OH d) 2 M LiBr