http://www.ck12.org Chapter 13. States of Matter
According to the kinetic-molecular theory for gases, any attractive forces between the particles of a gas are so minor
that they can be ignored in most cases. For liquids, the intermolecular attractive forces are the only thing that keeps
the particles close together. Liquids and solids are referred to as the condensed states of matter. One way in which
liquids and gases are similar is that they are both fluids. Afluidis a substance that is capable of flowing from one
place to another and takes the shape of its container.
Surface Tension
Molecules within a liquid are pulled equally in all directions by intermolecular forces. However, molecules at the
surface are pulled downward and sideways by other liquid molecules, but not upward away from the surface. The
overall effect is that the surface molecules are pulled into the liquid, creating a surface that is tightened like a film (
Figure13.7 (A)). Thesurface tensionof a liquid is a measure of the elastic force in the liquid’s surface. Liquids
that have strong intermolecular forces, like the hydrogen bonding in water, exhibit the greatest surface tension.
Surface tension allows objects that are denser than water, such as the paper clip shown below (Figure13.7 (B)), to
nonetheless float on its surface. It is also responsible for the beading up of water droplets on a freshly waxed car,
because there are no attractions between the polar water molecules and the nonpolar wax.
FIGURE 13.7
(A) Molecules at the surface of a liquid are
pulled downward into the liquid, creating
a tightened surface. (B) Surface tension
allows a paper clip to float on water’s
surface.Evaporation
A puddle of water left undisturbed eventually disappears. The liquid molecules escape into the gas phase, becoming
water vapor.Vaporizationis the process in which a liquid is converted to a gas.Evaporationis the conversion of
a liquid to its vapor form below the boiling temperature of the liquid(Figure13.8). If the water is instead kept in a
closed container, the water vapor molecules do not have a chance to escape into the surroundings and so the water
level does not change. As some water molecules become vapor, an equal number of water vapor molecules condense
back into the liquid state.Condensationis the change of state from a gas to a liquid.