Science of Water : Concepts and Applications

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Water Chemistry
Chemical testing can be divided into two types. The fi rst type measures a bulk physical property of
the sample such as volume, temperature, melting point, or mass. These measurements are normally
performed with an instrument, and one simply has to calibrate the instrument to perform the test. Most
analyses, however, are of the second type, in which a chemical property of the sample is determined
that generates information about how much of what is present.
—R.K. Smith (1993)
As mentioned previously, although no one has seen a water molecule, we have determined through
x-rays that atoms in water are elaborately meshed. Moreover, although it is true that we do not
know as much as we need to know about water—our growing knowledge of water is a work in
progress—we have determined many things about water. A large amount of our current knowledge
comes from studies of water chemistry.
Water chemistry is important because several factors about water to be treated and then
distributed or returned to the environment are determined through simple chemical analysis.
Probably the most important determination that the water practitioner makes about water is its
hardness.
Why chemistry? “I’m not a chemist,” you might say.
Simply put, when you add chlorine to water to make it safe to drink or safe to discharge into a
receiving body (usually a river or lake), you are a chemist. Chemistry is the study of substances and
the changes they undergo. Water specialists and those interested in the study of water must possess
a fundamental knowledge of chemistry.
Before beginning our discussion of water chemistry, it is important for the reader to have some
basic understanding of chemistry concepts and chemical terms. The following section presents a
review of terms, defi nitions, and concepts in chemistry.
CHEMISTRY CONCEPTS AND DEFINITIONS
Chemistry, like the other sciences, has its own language; thus, to understand chemistry, you must
understand the following concepts and key terms.
CONCEPTS
“Miscible,” “Solubility”
1. Miscible means capable of being mixed in all proportions. Simply stated, when two or more
substances disperse uniformly in all proportions when brought into contact they are said
to be completely soluble in one another, or completely miscible. The precise chemistry
defi nition is “homogenous molecular dispersion of two or more substances” (Jost, 1992).
Examples are:
• All gases are completely miscible.
• Water and alcohol are completely miscible.
•
Water and mercury (in its liquid form) are immiscible liquids.
2. Between the two extremes of miscibility, there is a range of solubility; that is, various substances
mix with one another up to a certain proportion. In many environmental situations,
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