13.3. Solids http://www.ck12.org
As a solid is heated, the average kinetic energy of its particles still increases, but due to their relatively fixed positions,
this manifests itself as stronger and more rapid vibrations. Eventually, the organization of the particles within the
solid structure begins to break down and the solid starts to melt. Themelting pointis the temperature at which a
solid changes into a liquid. At its melting point, the disruptive vibrations of the particles in the solid overcome the
attractive forces operating within the solid. As with boiling points, the melting point of a solid is dependent on the
strength of those attractive forces. Sodium chloride (NaCl) is an ionic compound, so it consists of a multitude of
strong ionic bonds. Sodium chloride melts at 801°C. Ice (solid H 2 O) is a molecular compound whose molecules are
held together by hydrogen bonds. Though hydrogen bonds are the strongest of the intermolecular forces, they are
still much weaker than ionic bonds. The melting point of ice is 0°C.
The melting point of a solid is the same as the freezing point of the corresponding liquid. At that temperature, the
solid and liquid states of the substance are in equilibrium. For water, this equilibrium occurs at 0°C.
H 2 O(s)⇀↽H 2 O(l)You can view a cartoon with a simple explanation of the properties of solids at http://www.abpischools.org.uk/pa
ge/modules/solids-liquids-gases/slg2.cfm?age=Age%20range%207-11&subject=Science. Page 2 is specifically
about the properties of solids.
Vapor Pressure of a Solid
In the last lesson, you learned about the vapor pressure of a liquid and its dependence upon temperature. Solids also
have a vapor pressure, though it is generally much less than that of a liquid. A snow bank will gradually disappear
even if the temperature stays below 0°C. The snow does not melt but, instead, passes directly from the solid state to
the vapor state.Sublimationis the change of state from a solid to a gas without passing through the liquid state.
Iodine is an example of a substance for which sublimation can be readily observed at room temperature, as seen
below (Figure13.14). Although the vapor pressure of solid iodine at room temperature is actually quite low (<1
mmHg), its vapor is a distinctive purple color and has a very strong scent, making it easy to detect.
FIGURE 13.14
Solid iodine sublimes readily, forming a purple vapor.The following video shows the sublimation of iodine in a beaker. The beaker is covered with an evaporating dish
filled with ice water, and the dark iodine crystals are deposited on the outside of that dish.Depositionis the change
of state from a gas to a solid.