What is Chemistry?
At one time it was easy to define chemistry.
The traditional definition goes something like this: Chemistry is the study of
the nature, properties, and composition of matter, and how these undergo
changes. That served as a perfectly adequate definition as late as the 1930s,
when natural science (the systematic knowledge of nature) seemed quite clearly
divisible into the physical and biological sciences, with the former being
comprised of physics, chemistry, geology and astronomy and the latter consisting
of botany and zoology. This classification is still used, but the emergence of
important important fields to study such as oceanography, paleobotany,
meteorology, pharmacy and biochemistry, for example, have made it increasingly
clear that the dividing lines between the sciences are no longer at all sharp.
Chemistry, for instance, now overlaps so much with geology (thus we have
geochemistry), astronomy (astrochemistry), and physics (physical and analytical
chemistry) that it is probably impossible to devise a really good modern
definition of chemistry, except, perhaps, to fall back on the operational
definition: Chemistry is what chemists do!
Chemistry plays an important part in all of the other
natural sciences, basic and applied. Plant growth and metabolism, the formation
of igneous rocks, the role played by ozone in the atmosphere, the degradation of
environmental pollutants, the properties of lunar soil, the medical action of
drugs, establishment of forensic evidence: none of these can be understood
without the knowledge and perspective provided by chemistry. Indeed, many people
study chemistry so that they can apply it to their own particular field of
interest. Of course, chemistry itself is the field of interest for many people,
too. Many study chemistry not to apply it to another field, but simply to learn
more about the physical world and the behaviour of matter from a chemical
viewpoint. Some simply like "what chemists do" and so decide to
"do it" themselves.
Chemistry is a way of studying matter. What is
matter? As is true with many of those words which are really basic to science, matter
is hard to define. It is often said that matter is anything which has mass and
occupies space. But then what are "mass" and "space"?
Although we can define these, the process yields very little insight into what
matter is. So let us just say that matter is anything which has real physical
existence; in a word matter is just stuff. Iron, air, wool, gold, milk,
aspirin, monkeys, rubber, and pizza - these are all matter. Some things which
are not matter are heat, cold, colours, dreams hopes, ideas, sunlight, beauty,
fear, and x-rays. None of these is "stuff"; none is matter.
A sample of matter can be either a pure substance
or a mixture. A pure substance has a fixed, characteristic composition an a
fixed, definite set of properties. Pure substances are for example copper, salt,
diamond, water, table sugar, oxygen, mercury, vitamin C, and ozone. A pure
substance may be a single element, such as copper or oxygen, or a compound
of two or more elements in a fixed ratio, such as salt (39.34 % sodium and 60.66
% chlorine) or table sugar (42.11 % carbon, 6.48 % hydrogen, and 51.41 %
oxygen).
A mixture is a collection of pure substances
simply mixed together. Its composition is variable, as are its properties.
Examples of mixtures are milk, wood, concrete, saltwater, air, granite, motor
oil, chocolate, and elephants.
A pure substance can be a solid, a liquid,
or a gas; these are the three states of matter A solid maintains
its volume and shape; a liquid, its volume only; and a gas, neither. Solids tend
to be hard and unyielding; liquids maintain their volumes and flow to adopt the
shapes of their containers. The ability to flow is called fluidity, and
so gases and liquids are called fluids.
One of the goals of chemistry is to be able to
describe the properties of matter in terms of its internal structure, the
arrangement and interrelationship of its parts. This word, structure, sometimes
refers to the physical arrangement of particles, such as atoms or molecules
in space. At other times it is used to indicate some other arrangement, such as
the arrangement of energy levels of an electron in an atom. The structure
of matter determines its properties. Properties can be classed as either
physical or chemical. A physical property of a substance can be characterized
without specific reference to any other substance and usually describes the
response of the substance to some external influence, such as heat, light,
force, electricity, etc. Physical properties include boiling point, melting
point, thermal (heat) conductivity, colour, refractive index, viscosity,
reflectivity, hardness, tensile strength, and electrical conductivity.
A chemical property, on the other hand, describes a chemical
change: the interaction of one substance with another , or the change of one
substance into another. Iron rusts in a moist environment, unrefrigerated milk
turns sour, wood burns in air, photographs bleach when exposed to sunlight for a
long time, dynamite explodes - each of these is a chemical property because each
involves chemical change. During chemical changes, substances are actually
changed into other substances. The simultaneous disappearance of some
substances (called the reactants) and appearance of others (the
products) is characteristic in chemical change (chemical reaction). Chemical
changes are generally characterized by pronounced internal structural
rearrangements.
Physical changes are not characterized by the
transformation of one substance into another, but rather by the change of the
form of a given substance. The bending of a piece of copper wire fails to change
the property of copper into another substance; crushing a block of ice leaves
only crushed ice; melting an iron nail yields a substance still called iron:
These are all usually accepted as physical changes.
Properties of matter may also be categorized as
either macroscopic or microscopic. A macroscopic property describes
characteristics or behaviour of a sample which is large enough to see, handle,
manipulate, weigh, etc. A microscopic property describes the behaviour of a much
smaller sample of matter, an atom or molecule for instance.. Macroscopic and
microscopic properties are often different. A banana is yellow, but we do not
use colour to describe an atom. Some properties, on the other hand, can be either
microscopic or macroscopic; mass is one of these.
Another word that is often used is system. A
system is a portion of the universe which we wish to observe or consider. The
size of the portion is usually small and a system may be a real one (in a test
tube or flask, for example), or an imaginary one which this text is just
referring to.
Viewed from an historical point of view, it
is clear that scientific knowledge has been obtained and that therefore
science has "advanced" in a series of fairly logical steps. On
the other hand, counterparts to these steps are difficult to identify in the
day-to-day professional activities of a scientist. The way in which science and
in particular chemistry advances can be describes in terms of a series of cycles
(see diagram below). Observations and data (and laws) lead to the proposal of
theories that, in turn, suggest predictions which can be tested by designing new
experiments, and the whole process starts all over again.
John B. Russell, General
Chemistry, McGraw-Hill International Book Company, 1980
Last Update: 02-04-2001