Transcript Chapter 9- The States of Matter

Vapor Pressure
Evaporation
 When
fastest particles in a sample of
liquid escape from the surface of the
liquid
 During evaporation the slower
particles are left behind
 What happens to the temperature of
a liquid as it evaporates?
Closed Container
When
first sealed the
molecules gradually escape
the surface of the liquid
The vapor above the
liquid has a pressure
As the molecules build up
above the liquid some
condense back to a liquid.
Condensation
 Slower
particles of gas feel effects of
intermolecular forces and become
liquid
 Leaves only the faster particles in the
gas
 What happens to the temperature of
a the gas during condensation?
Dynamic equilibrium
Rate of Vaporization =
Rate of Condensation
Molecules
are constantly
changing phase “Dynamic”
The total amount of liquid and
vapor remains constant
“Equilibrium”
 The
pressure of the gas at
equilibrium is called the equilibrium
vapor pressure
 As the temperature goes up, more
molecules vaporize
 The equilibrium vapor pressure
increases as temperature increases
Vapor Pressure Graph
 Vapor
pressure can be plotted vs the
temperature
 The curve obtained is the equilibrium
vapor pressure
 Since different substances all
evaporate at different rates when
heat is added, they all have different
curves
liquid
gas
Which of these substances vaporizes
most easily? Which least?
Boiling
A
liquid boils when the vapor
pressure = the external pressure
 Vapor pressure great enough to form
a bubble in the liquid
 Normal Boiling point is the
temperature a substance boils at 1
atm pressure.
 The temperature of a liquid can never
rise above it’s boiling point.
What is the boiling point of each liquid at
101.3 kPa?
Changing the Boiling Point
 Lower
the pressure (going up into
the mountains).
 Lower external pressure requires
lower vapor pressure
 Lower vapor pressure means lower
boiling point.
 Food cooks slower.
On a mountain the pressure is 90 kPa, what
is the boiling point of water there?
Changing the Boiling Point
 Raise
the external pressure (Use a
pressure cooker).
 Raises the vapor pressure needed.
 Raises the boiling point.
 Food cooks faster.