http://www.ck12.org Chapter 13. States of Matter
FIGURE 13.3
In this image, you see small pools of ele-
mental mercury. The Latin name for mer-
cury is hydrargyros, a compound word
meaning "water-silver" (hydr- = water, -
argyros = silver), since it is liquid like
water but shiny like silver. Mercury is the
only pure metal that exists as a liquid at
room temperature.spread out over the bottom of the barrel.
Intermolecular Forces in Liquids
Molecules in the liquid state are held in close proximity to one another by intermolecular forces. Any molecule con-
taining an –OH group (such as water and the various alcohols) will form hydrogen bonds with adjacent molecules,
causing a group of molecules to take on the liquid state. A polar molecule like CH 2 Cl 2 cannot form hydrogen bonds,
but molecules are held together in the liquid state by dipole-dipole interactions. Br 2 is not polar, but interactions with
other Br 2 molecules through dispersion forces cause Br 2 to be a liquid at room temperature as well. Intermolecular
forces are also responsible for various properties of liquids, such as viscosity, surface tension, and vapor pressure.
Viscosity
Viscositymeasures a fluid’s resistance to flow –the higher the viscosity, the slower the flow of the material. One
factor that affects viscosity is the strength of the intermolecular forces in the material. Molecules that exhibit higher
intermolecular forces tend to have higher viscosities. Temperature also influences viscosity. A higher temperature
results in a decrease in viscosity, since molecules are moving faster and the intermolecular forces are more easily
disrupted. Most cooking oils are more viscous than water, but when heated, the viscosity decreases and the oil
spreads more easily. Motor oils are ranked by viscosity. Lower viscosity oils provide less drag on the engine, but
they are also used up faster than a higher-viscosity material.
Surface Tension
Surface tensionis a physical property of liquids that is caused by the attraction of liquid molecules due to various
intermolecular forces. This property is allows the surface of a liquid to resist an external force, and is a measure of
the amount of energy required to increase the surface area of a liquid. Surface tension is basically a measure of the
“pull” of surface molecules on one another.
Intermolecular forces on a molecule in the interior of a liquid are exerted in all directions. The forces on a surface
molecule are more directional, causing the molecule to be pulled toward other surface molecules and down toward
the interior of the liquid. This property has consequences both for the behavior of the liquid in contact with solids
and the influence of temperature on the liquid.