Kinetic Molecular
Theory and the the
Nature of Fluids
A model for liquids/Evaporation
Vapor Pressure
Boiling Point
Kinetic Molecular Theory
 States that that all substances are made
of tiny particles (atoms and molecules)
that are in constant motion
 The kinetic energy and motion of these
particles can help us to understand the
nature of fluids and phase changes
A model for Liquids and
gases- Kinetic Theory
 Liquids and Gases have kinetic energy---allows
flow
 No attraction between gas particles
 Intermolecular attraction between liquid
particles keeps them together
 Interplay between disruptive motions of
particles in a liquid and attractions among
particles determines physical properties of
the liquid.
Phase Changes using
Kinetic Theory
Solid 
Low KE
IM Forces
greater than
KE so
substance
stays
together
Liquid
Medium KE
 Gas
Highest KE
KE Forces
overcome
the IM forces
to escape
Density and Pressure
 Liquids much more dense than gas due
to intermolecular attraction
 Increased pressure has little effect on
liquids and solids volume
 Solids and Liquids are then called
condensed states of matter
Evaporation
 Conversion of liquid to gas = vaporization
 Most molecules don’t have enough kinetic
energy to break free
 When vaporization is at the surface without
boiling = evaporation
 During evaporation only those molecules
with a certain minimum kinetic energy can
escape from the surface of the liquid
Further Evaporation
 Some escaping particle rebound back in
off of air particles
 Heating increases kinetic energy which
increases evaporation
 Removal of these higher energized
particles leaves a lower average energy
 THEREFORE: evaporation is a cooling
process
Evaporation example
 Sweating uses evaporation as a cooling
process
 Liquid on your skin takes heat energy
 Liquid evaporates taking that energy with
it
 Leaves you with a lower temperature
Vapor Pressure
 Vapor Pressure – measure of the force
exerted by the gas leaving the surface of
a liquid
 Over time they increase and particles
condense ---eventually return to liquid
state
Vapor Pressure
 In a system of constant vapor pressure, a
dynamic equilibrium exists between
vapor and liquid
 Equilibrium because
 rate of evaporation = rate of condesation
Vapor Pressure
 As temperature increases, vapor pressure
increases
 Particles in liquid have increased kinetic
energy
 More KE, able to escape surface
 Collide with “walls”
 Vapor pressure measured with a manometer
 Vapor increases, difference between levels
increases
Boiling Point
 Rate of evaporation increases as a liquid
is heated
 KE increases as temperature increases
 When a liquid is heated to a
temperature at which particles in the
liquid have enough KE to vaporize, the
liquid begins to boil.
Boiling Point
 Boiling point- temperature at which the
vapor pressure of the liquid is equal to
the external pressure on the liquid
 Not all liquids have same boiling point
 Changes in altitude also affect boiling
point
Boiling Point
 Boiling similar to evaporation
 Particles with most KE rise to surface and
break free
 Temperature of boiling liquid never rises
above boiling point
 Escaping particles take growing energy with
them
Boiling Point
 Interesting fact:
 Burn from steam more severe than a burn
from water
 Steam particles can carry more energy than
water particles
 Collides with skin harder
Boiling Point
 NORMAL BOILING POINT table
ASSESSMENT
 What factors determine the physical
properties of liquids?
ASSESSMENT
 Explain how evaporation lowers the
temperature of a liquid.
ASSESSMENT
 What is vaporization?
ASSESSMENT
 Define vapor pressure.
ASSESSMENT
 What is a normal boiling point?