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Intermolecular Forces, Solids and Liquids 161
induces a dipole in another molecule. These weak dipoles lead to an attraction.
Although this is an extremely weak interaction, it is strong enough to allow us
to liquefy nonpolar gases such as hydrogen, H 2. If there were no intermolecular
forces attracting these molecules, it would be impossible to liquefy hydrogen.
The more electrons present, the greater the London force.
11-2 Properties of Liquids
At the microscopic level, the liquid particles are in constant motion. The particles
may exhibit short-range areas of order, but these usually do not last very long.
Clumps of particles may form and then break apart. At the macroscopic level, a
liquid has a specific volume but no fixed shape. Three additional macroscopic
properties deserve discussion: surface tension, viscosity, and capillary action.
In the body of a liquid, intermolecular forces pull the molecules in all directions.
At the surface of the liquid, the molecules pull down into the body of the
liquid and from the sides. There are no molecules above the surface to pull in
that direction. The effect of this unequal attraction is that the liquid tries to
minimize its surface area. The minimum surface area for a given quantity of
matter is a sphere. In a large pool of liquid, where sphere formation is not possible,
the surface behaves as if it had a thin stretched elastic membrane or
“skin” over it. The surface tension is the resistance of a liquid to an increase in
its surface area. It requires force to break the attractive forces at the surface.
The greater the intermolecular force, the greater the surface tension. Polar liquids,
especially those that utilize hydrogen bonding, have a much higher surface
tension than nonpolar liquids.
Viscosity is the resistance to flow. Important factors influencing the viscosity of
a liquid are the intermolecular forces and the temperature. The stronger the
intermolecular force, the greater the viscosity. As the temperature increases,
the kinetic energy of the particles increases. The higher kinetic energy will overcome
the intermolecular attractive forces. This causes a lower viscosity. In some
cases, another factor is the size of the molecule. Large and complex molecules
will have difficulty moving past one another. If they cannot easily move past
each other, the viscosity will be high.
Capillary action is the spontaneous rising of a liquid through a narrow tube
against the force of gravity. It is due to competition of intermolecular forces
within the liquid and attractive forces between the liquid and the tube wall. The
stronger the attraction between the liquid and the wall, the higher the level
rises. Liquids that have weak attractions to the walls, like mercury in a glass
tube, have low capillary action. Liquids like water in a glass tube have strong
attractions to the walls and will have high capillary action. This also explains