Science of Water : Concepts and Applications

Water Chemistry 101
Like solids, liquids possess a defi nite volume at a given temperature and pressure, and they tend
to maintain this volume when they are exposed to a change in either of these conditions.
Gases have no defi nite fi xed shape and their volume can be expanded or compressed to fi ll different
sizes of containers. A gas or mixture of gases like air can be put into a balloon, and will take
the shape of the balloon. Particles of gases do not stick together at all and move about freely, fi lling
containers of any shape and size.
A gas is also identifi ed by its lack of a characteristic volume. When confi ned to a container
with nonrigid, fl exible walls, for example, the volume that a confi ned gas occupies depends on its
temperature and pressure. When confi ned to a container with rigid walls, however, the volume of
the gas is forced to remain constant.
Internal linkages among its units, including between one atom and another, maintain the constant
composition associated with a given substance. These linkages are called chemical bonds.
When a particular process occurs that involves the making and breaking of these bonds, we say that
a chemical reaction or a chemical change has occurred.
Let us take a closer look at both chemical and physical changes of matter.
Chemical changes occur when new substances are formed that have entirely different properties
and characteristics. When wood burns or iron rusts, a chemical change has occurred; the
linkages—the chemical bonds—are broken.
Physical changes occur when matter changes its physical properties such as size, shape, and
density, as well as when it changes its state, i.e., from gas to liquid to solid. When ice melts or when
a glass window breaks into pieces, a physical change has occurred.
THE CONTENT OF MATTER: THE ELEMENTS
Matter is composed of pure basic substances. Earth is made up of the fundamental substances of
which all matter is composed. These substances that resist attempts to decompose them into simpler
forms of matter are called elements. To date, there are more than 100 known elements. They range
from simple, lightweight elements to very complex, heavyweight elements. Some of these elements
exist in nature in pure form; others are combined. The smallest unit of an element is the atom.
The simplest atom possible consists of a nucleus having a single proton with a single electron
traveling around it. This is an atom of hydrogen, which has an atomic weight of one because of the
single proton. The atomic weight of an element is equal to the total number of protons and neutrons
in the nucleus of an atom of an element.
To gain an understanding of the basic atomic structure and related chemical principles, it is
useful to compare the atom to our solar system. In our solar system, the sun is the center of everything.
The nucleus is the center in the atom. The sun has several planets orbiting it. The atom has
electrons orbiting the nucleus. It is interesting to note that the astrophysicist, who would likely fi nd
this analogy overly simplistic, is concerned mostly with activity within the nucleus. This is not the
case, however, with the chemist. The chemist deals principally with the activity of the planetary
electrons; chemical reactions between atoms or molecules involve only electrons, with no changes
in the nuclei.
The nucleus is made up of positive electrically charged protons and neutrons that are neutral
(no charge). The negatively charged electrons orbiting it balance the positive charge in the nucleus.
An electron has negligible mass (less than 0.02% of the mass of a proton), which makes it practical
to consider the weight of the atom as the weight of the nucleus.
Atoms are identifi ed by name, atomic number, and atomic weight. The atomic number or
proton number is the number of protons in the nucleus of an atom. It is equal to the positive charge
on the nucleus. In a neutral atom, it is also equal to the number of electrons surrounding the nucleus.
As mentioned, the atomic weight of an atom depends on the number of protons and neutrons in the
nucleus, the electrons having negligible mass. Atoms (elements) received their names and symbols
in interesting ways. The discoverer of the element usually proposes a name for it. Some elements get