and those attractive forces between the liquid and the tube wall. The stronger the attraction
between the liquid and the tube, the higher the level will be. Liquids that have weak attrac-
tions to the walls, like mercury in a glass tube, have a low capillary action. Liquids like water
in a glass tube have strong attractions to the walls and will have a high capillary action.
As we have noted before, water, because of its stronger intermolecular forces (hydrogen
bonding) has some very unusual properties. It will dissolve a great number of substances,
both ionic and polar covalent, because of its polarity and ability to form hydrogen bonds.
It is sometimes called the “universal solvent.” It has a high heat capacity, the heat absorbed
to cause the temperature to rise, and a high heat of vaporization, the heat needed to trans-
form the liquid into a gas. Both of these thermal properties are due to the strong hydrogen
bonding between the water molecules. Water has a high surface tension for the same reason.
The fact that the solid form of water (ice) is less dense than liquid water is because water
molecules in ice are held in a rigid, open, crystalline framework by the hydrogen bonds. As
the ice starts melting, the crystal structure breaks and water molecules fill the holes in the
structure, increasing the density. The density reaches a maximum at around 4°C; then the
increasing kinetic energy of the particles causes the density to begin to decrease.The Solid State
At the macroscopic level a solidis defined as a substance that has both a definite volume
and a definite shape. At the microscopic level, solids may be one of two types––amorphous
or crystalline. Amorphous solidslack extensive ordering of the particles. There is a lack of
regularity of the structure. There may be small regions of order separated by large areas of
disordered particles. They resemble liquids more than solids in this characteristic.
Amorphous solids have no distinct, melting point. They simply get softer and softer as the
temperature rises, leading to a decrease in viscosity. Glass, rubber, and charcoal are exam-
ples of amorphous solids.
Crystalline solidsdisplay a very regular ordering of the particles in a three-dimensional
structure called the crystal lattice. In this crystal lattice there are repeating units called unit
cells. Figure 12.1 shows the relationship of the unit cells to the crystal lattice.
Several types of unit cells are found in solids. The cubic system is the type most com-
monly appearing on the AP exam. Three types of unit cells are found in the cubic system:- Thesimple cubic unit cellhas particles located at the corners of a simple cube.
- Thebody-centered unit cellhas particles located at the corners of the cube and in the
center of the cube. - Theface-centered unit cellhas particles at the corners and one in the center of each
face of the cube, but not in the center of the cube itself.
Figure 12.2 shows three types of cubic unit cells.Five types of crystalline solid are known:- In atomic solids,individual atoms are held in place by London forces. The noble gases
are the only atomic solids known to form. - In molecular solids,lattices composed of molecules are held in place by London
forces, dipole–dipole forces, and hydrogen bonding. Solid methane and water are
examples of molecular solids. - In ionic solids,lattices composed of ions are held together by the attraction of the
opposite charges of the ions. These crystalline solids tend to be strong, with high melt-
ing points because of the strength of the intermolecular forces. NaCl and other salts are
examples of ionic solids. Figure 12.3 shows the lattice structure of NaCl. Each sodium
Solids, Liquids, and Intermolecular Forces 169