PROPERTIES
Of Liquid's
Introduction
Intermolecular forces are attractive forces
that act between molecules or particular
in the solid or liquid states. These are
forces that form between molecules,
atoms, or ions. Generally, these attractive
forces are much weaker than bonding
forces or intramolecular forces.
Introduction
Matter is anything that occupies space and
has mass. Matter is divided into different
phases such as solid, liquid, gas, plasma,
and Bose – Einstein Condensates. The first
three phases of matter differ in various
properties such as volume/ shape, density,
compressibility, motion of their molecules,
and their molecular behavior.
Fluid is a gas or a liquid. Also, it is a
substance that can flow.
Properties
of liquids
Surface Tention
– the cohesive forces between liquid
molecules are responsible for the
phenomenon known as surface tension.
The cohesive forces between molecules
down into a liquid are shared with all
neighboring atoms. Those on the surface
have no neighboring atoms above and
exhibit stronger attractive forces upon
their nearest neighbors on the surface.
This enhancement of the intermolecular
forces at the surface is called surface
tension.
– is the measure of the elastic force in the surface of a liquid . It is
amount of energy required to stretch or increase the surface of a
liquid by a unit area. It is the energy , or work, required to increase
the surface area of a liquid due to intermolecular forces. It is
manifested as some sort of skin on the surface of a liquid or in a
drop of liquid.
– Molecules within a liquid are pulled in all directions by
intermolecular forces. Molecules at the surface area
pulled downward and sideways by other molecules, not
upward away from the surface.
– These intermolecular forces tend to
pull the molecules into the liquid and
case the surface to tighten like an
elastic film or " skin".
Liquids that have strong
intermolecular forces have higher
surface tension.
Capillary Action
– is the tendency of a liquid to rise in a narrow
tubes or be drawn into small openings such as
those between grains of a rock.
– also known as capillarity, is a result of
intermolecular attraction between the liquid and
solid materials
– Is a phenomenon wherein a liquid is able to rise
up on a narrow tube. Adhesive forces between
the tube and the liquid allow the liquid to exceed
it's weight. The narrower the tube, the higher the
liquid will reach.
Two types of forces are involved in capillary
Action:
A. Cohesion
_ is the molecular attraction between like
molecules ( the liquid molecules)
B. Adhesion
– is an attraction between unlike molecules(
such as those in water and in the particles
that make up the glass tube).
• when the cohesive forces between the liquid are
greater than the adhesive forces between the liquid
and the walls of the container, the surface of the
liquid is convex.
Example: mercury in a container
• when the cohesive forces between the liquid
molecules are lesser than the adhesive forces between
the liquid and the walls of the container, the surface
of the liquid is concave.
Example : water in a glass container
Viscosity
– it is measure of a fluids resistance to flow . In
layman's term, it is defined as the measure of
the thickness of a liquid . The greater the
viscosity, the slower the liquids flows.
– Is the ability of a fluid to resist flowing . It is related to
the movement of the molecules in the liquids and this
to the intermolecular forces present . Viscosity of a
liquid decreases with increasing temperature because
at higher temperature, the average kinetic energy
molecules that overcomes the attractive forces
between molecules is greater.
– can be measured using viscometer. It
measure the time it takes for a known volume
of a liquid to flow through the small neck.
– The larger the number of -OH groups allow glycerol
to form more H- bonds with other glycerol molecules,
making it's intermolecular stronger than those of
water, and it's resistance to flow greater.
– the size of the molecules affects the
viscosities of the hydrocarbons. The
larger the molecules, even if it non polar,
the stronger the intermolecular forces
and the greater the viscosity compared
to non polar substances made up of a
small molecules.
Vapor pressure of a liquid
– the equilibrium pressure of a vapor above it's
liquid that is , the pressure exerted by the vapor
above the surface of the liquid in a closed
container.
a. The water molecules in the liquid evaporate and go into
the vapor phase. In the open flask, some of the water
molecules in the vapor phase find their way out of the flask
are lost to the atmosphere.
b. When a liquid evaporates to a gas in a closed container,
the molecules cannot escape.
Vapor pressure and
temperature
– the image shows that as the temperature increases, the vapor
pressure of water also increases. When temperature is high, more
molecules have enough energy to escape from the liquid . At a
lower temperature, fewer molecules have sufficient energy to
escape from the liquid.
The vapor pressure for four common liquids: diethyl ether,
ethyl alcohol, water and ethylene glycerol, as the functions of
temperature . For all four liquids, the vapor pressure increases
as the temperature increases.
Vapor pressure and Strength of
intermolecular forces
– the stronger the intermolecular forces of
attraction, the lower the vapor pressure of a liquid.
Boiling point
– The boiling point of a liquid is the temperature at which
liquid converts into a gas. A liquid boils when its vapor
pressure equals the pressure acting on the surface of the
liquid . The boiling point is the temperature at which the
vapor pressure of a liquid is equal to the external pressure.
Also, it is the temperature at which liquid boils. The boiling
point of a liquid when the external pressure is 1 ATM is
called the normal boiling point.
The normal boiling point is the temperature at which
the liquid converts to a gas when the external pressure
is 1 atm. The normal boiling point of water is 100°C.
– The boiling point is directly related to molar heat of
vaporization: the higher ∆ Hvap, the higher the boiling
point.
– The boiling points of substance
often reflect the strength of the
intermolecular forces operating
among the molecules. At the
boiling point, enough energy must
be supplied to overcome the
attractive forces among molecules
before they can enter the vapor
phrase.
Molar heat of
vaporization( ∆Hvap) – It is the energy ( usually in
kilojoules) required to vaporize 1
mole of a liquid at a given
– Gas molecules move in temperature.
random directions, collide with
other gas particles and walls of
the container.
– some will strike the liquid surface and
condense back into it. In the closed flask , none
of the gas particles are able to get out of the
container, and eventually,the number of
molecules that go into gaseous state would
equal the number of molecules that condense
back.
– when the rate of the
condensation of the gas
becomes equal to the rate of
evaporation of the liquid , the
gas in the container is said to
be an equilibrium with the
liquid.
The equilibrium vapor pressure is the maximum vapor
pressure of a liquid at a given temperature and it is
constant temperature. It increases with temperature.
Molar heat of vaporization (∆ Hvap) and boiling point
Molar heat of vaporization and strength
of intermolecular forces
– the heat of vaporization may be considered
a measure of the strength of intermolecular
forces in a liquid. If the intermolecular
attraction is strong, it takes a lot of energy to
free the molecules from the liquid phase and
the heat of vaporization will be high.
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