Science of Water : Concepts and Applications

104 The Science of Water: Concepts and Applications
COMPOUND SUBSTANCES
If we take a pure substance like calcium carbonate (limestone) and heat it, the calcium carbonate
ultimately crumbles to a white powder. However, careful examination of the heating process shows
that carbon dioxide also evolves from the calcium carbonate. Substances like calcium carbonate
that can be broken down into two or more simpler substances are called compound substances
or simply compounds. Heating is a common way of decomposing compounds, but other forms of
energy are often used as well.
Chemical elements that make up compounds such as calcium carbonate combine with each
other in defi nite proportions. When atoms of two or more elements are bonded together to form a
compound, the resulting particle is called a molecule.
√ Important Point: This law simply means that only a certain number of atoms or radicals of
one element will combine with a certain number of atoms or radicals of a different element to
form a chemical compound.
Water, H2O, is a compound. As stated, compounds are chemical substances made up of two or more
elements bonded together. Unlike elements, compounds can be separated into simpler substances
by chemical changes. Most forms of matter in nature are composed of combinations of the 100+
pure elements.
If you have a particle of a compound, for example a crystal of salt (sodium chloride), and
subdivide until you get the smallest unit of sodium chloride possible, you would have a molecule.
As stated, a molecule (or least common denominator) is the smallest particle of a compound that
still has the characteristics of that compound.
√ Important Point: Because the weights of atoms and molecules are relative and the units are
extremely small, the chemist works with units he/she identifi es as moles. A mole (symbol mol)
is defi ned as the amount of a substance that contains as many elementary entities (atoms, molecules,
and so on) as there are atoms in 12 g of the isotope carbon-12.
√ Important Point: An isotope of an element is an atom having the same structure as the
element—the same electrons orbiting the nucleus, and the same protons in the nucleus, but having
more or fewer neutrons.
One mole of an element that exists as a single atom weighs as many grams as its atomic number (so
one mole of carbon weighs 12 g), and it contains 6.022045 × 1023 atoms, which is the Avogadro’s
number.
As stated previously, symbols are used to identify elements. This is a shorthand method for
writing the names of the elements. This shorthand method is also used for writing the names of
compounds. Symbols used in this manner show kinds and numbers of different elements in the
compound. These shorthand representations of chemical compounds are called chemical formulas.
For example, the formula for table salt (sodium chloride) is NaCl. The formula shows
that one atom of sodium combines with one atom of chlorine to form sodium chloride. Let us
look at a more complex formula for the compound sodium carbonate (soda ash): Na2CO3 . The
formula shows that this compound is made up of three elements: sodium, carbon, and oxygen.
In addition, there are two atoms of sodium, one atom of carbon, and three atoms of oxygen in
each molecule.
As mentioned, when depicting chemical reactions, chemical equations are used. The following
equation shows a chemical reaction that most water/wastewater operators are familiar with: chlorine
gas added to water. It shows the formulas of the molecules that react together and the formulas
of the product molecules.
Cl 2 + H 2 O → HOCl + HCl