Properties of Liquids
Some Characteristics of Gases, Liquids and Solids
and the Microscopic Explanation for the Behavior
gas
liquid
solid
assumes the shape
and volume of its
container
particles can move
past one another
assumes the shape of
the part of the
container which it
occupies
particles can
move/slide past one
another
retains a fixed
volume and shape
rigid - particles
locked into place
compressible
lots of free space
between particles
not easily
compressible
little free space
between particles
not easily
compressible
little free space
between particles
flows easily
particles can move
past one another
flows easily
particles can
move/slide past one
another
does not flow easily
rigid - particles
cannot move/slide
past one another
Properties of Liquids and KMT
• Kinetic-Molecular Theory (moving particle)
• Fixed Volume and flexible shape
• Smaller velocity  Lower kinetic energy
– Due to intermolecular forces
– Rate of diffusion  slower
– What happens when we increase
temperature?
• Temp , Velocity , therefore, Diffusion 
Properties of Liquids
• Particles are much closer together
– Why: Attractive forces are more effective
– How: Dipole-dipole, London dispersion
(instant dipole) and hydrogen bonding
– So? Relatively high density (more mass in
a specific volume) compared to gas
– More ordered than gases due to strong
intermolecular forces
Intermolecular Forces and the
Properties of Liquids
In summary, intermolecular forces play a
large role in many of the physical properties
of liquids. These include:
•
•
•
•
vapor pressure
boiling point
surface tension
viscosity
Intermolecular Forces and the
vapor pressure
• The vapor pressure of a liquid depends
on intermolecular forces.
• When the intermolecular forces in a
liquid are strong, you expect the vapor
pressure to be low.
Intermolecular Forces and the
normal boiling point
• The normal boiling point is related to vapor
pressure and is lowest for liquids with the
weakest intermolecular forces.
• When intermolecular forces are weak, little
energy is required to overcome them.
• Consequently, boiling points are low for such
compounds.
Intermolecular Forces and the
surface tension
• Surface tension increases with increasing
intermolecular forces.
• Surface tension is the energy needed
to increase the surface area of a liquid.
• To increase surface area, it is
necessary to pull molecules apart
against the intermolecular forces of
attraction.
Surface Tension
How are surface particles different from internal
particles with respect to attractive forces?
Surface Tension
• The resistance of a liquid to an increase in
its surface area.
Surface Tension
1. Attraction of surface
molecules that cause the liquid
surface to contract and become
more spherical
2.Amount of energy required to
stretch or increase the surface
Surface Tension
– Common to ALL liquids
– A force tends to pull adjacent
parts of a liquid’s surface
together
– Directly related to the force of
attraction between particles
– Decreasing surface area to the
smallest possible size
– Liquid droplets are spherical
• Reason: Least surface area
Consequences of Surface Tension
Chemistry; The Science in Context; by Thomas R Gilbert, Rein V.
Kirss, and Geoffrey Davies, Norton Publisher, 2004, p 457
Consequences of Surface Tension
Chemistry; The Science in Context; by Thomas R Gilbert, Rein V.
Kirss, and Geoffrey Davies, Norton Publisher, 2004, p 458
http://citt.ufl.edu/Marcela/Sepulveda/html/en_tension.htm
Cohesion and Adhesion
Cohesion is the intermolecular attraction between like
molecules
Adhesion is an attraction between unlike molecules
Adhesion
Cohesion
Cohesion/Adhesion
• Why is this so?
• In mercury the cohesive forces are stronger
than the adhesive forces to glass.
Cohesion/Adhesion
• Water tends to “wet” glass because the
adhesive forces between the liquid molecules
and the glass molecules are greater than the
cohesive forces between the liquid
molecules.
• Water beads up on a wax surface because the
cohesive forces between the water molecules
are greater than the adhesive forces between
the water molecules and the wax molecules.
Viscosity
• The resistance of a liquid to flow is
called its viscosity.
• The greater a liquid’s viscosity, the
more slowly it flows.
• Viscosity is related to the ease with
which individual molecules of the liquid
can move with respect to one another.
• It depends on the attractive forces
between molecules.
Viscosity
• Viscosity decreases
with increasing
temperature.
• Because at higher
temperatures the
greater average
kinetic energy of the
molecules more
easily overcomes the
attractive forces
between molecules.
Increasing Strength of Intermolecular Force
•
•
•
•
Higher Boiling Point
Lower Vapor Pressure
Higher Surface Tension
Higher Viscosity
REFERENCES
• http://www.media.pearson.com.au/schools/cw/au_sc
h_whalley_sf1_1/int/matter.html
• http://www.footprints-science.co.uk/states.htm
• http://www.chem.purdue.edu/gchelp/atoms/states.ht
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