170 ❯ STEP 4. Review the Knowledge You Need to Score High
Capillary action is the spontaneous rising of a liquid through a narrow tube, against
the force of gravity. It is caused by competition between the intermolecular forces in the
liquid 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 attractions 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 (hydro-
gen bonding) has some very unusual properties. It will dissolve a great number of sub-
stances, 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 transform 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 solid is 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 solids lack 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
examples of amorphous solids.
Crystalline solids display 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.Unit cellFigure 12.1 The crystal lattice for a simple cubic unit cell.