Kinetic Theory and Fluids
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Transcript Kinetic Theory and Fluids
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?