Untitled - Kelly Walsh High School

162 CHEMISTRY FOR THE UTTERLY CONFUSED
why we observe a meniscus with water contained in a thin tube. A meniscus is
a concave water surface due to the attraction of the water molecules adjacent
to the glass walls. No meniscus is present with mercury because of its weak
attraction to the walls.
As we have noted before, water, because of hydrogen bonding has some very
unusual properties. It will dissolve a great number of substances, both ionic and
polar covalent. This is because of its polarity and ability to form hydrogen
bonds. It has a high heat capacity (see Chapter 6). This is the heat absorbed to
cause a specific increase in temperature. Water has a high heat of vaporization.
This is the heat necessary to transform a liquid into a gas. Both of these thermal
properties are a result of the strong hydrogen bonding between the molecules.
Water has a high surface tension for the same reason. Finally, the fact that
the solid form of water (ice) is less dense than liquid water is because of hydrogen
bonds. These forces hold the water molecules in a rigid open crystalline
framework. As ice starts to melt, the crystal structure breaks, and water molecules
fill the holes in the structure. Filling the holes increases the density. The
density reaches a maximum at around 4°C and then the increasing kinetic
energy of the particles causes the density to decrease.
11-3 Solids
Don’t Forget!
At the macroscopic level, a solid is 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 become softer and softer as the temperature rises. Glass,
rubber, and charcoal are examples of amorphous solids.
Crystalline solids display a very regular ordering of the particles in a threedimensional
structure called the crystal lattice. In this crystal lattice there are
repeating units called unit cells. See your textbook for diagrams of unit cells.
In looking at crystal lattice diagrams, count all the particles in all three dimensions
that surround another particle.