_reaction-velocity = chemical force/chemical resistance._
Thermodynamics shows that at very low temperatures, i.e. in the
immediate vicinity of the absolute zero, there is no equilibrium, but
every chemical process advances to completion in the one or the other
direction. The chemical forces therefore act in the one direction
towards complete consumption of the reacting substance. But since the
chemical resistance is now immensely great, they can produce practically
no appreciable result.
At higher temperatures the reaction always proceeds, at least in
homogeneous systems, to a certain equilibrium, and as the chemical
resistance now has finite values this equilibrium will always finally be
reached after a longer or shorter time. Finally, at very high
temperatures the chemical resistance is in every case very small, and
the equilibrium is almost instantaneously reached; at the same time, the
affinity of the reaction, as in the case of the mutual affinity between
oxygen and hydrogen, may very strongly diminish, and we have then
chemical indifference again, not because, as at low temperatures, the
denominator of the previous expression becomes very great, but because
the numerator now assumes vanishingly small values. (W. N.)
CHEMISTRY (formerly "chymistry"; Gr. [Greek: chymeia]; for derivation
see ALCHEMY), the natural science which has for its province the study
of the composition of substances. In common with physics it includes the
determination of properties or characters which serve to distinguish one
substance from another, but while the physicist is concerned with
properties possessed by all substances and with processes in which the
molecules remain intact, the chemist is restricted to those processes in
which the molecules undergo some change. For example, the physicist
determines the density, elasticity, hardness, electrical and thermal
conductivity, thermal expansion, &c.; the chemist, on the other hand,
investigates changes in composition, such as may be effected by an
electric current, by heat, or when two or more substances are mixed. A
further differentiation of the provinces of chemistry and physics is
shown by the classifications of matter. To the physicist matter is
presented in three leading forms--solids, liquids and gases; and
although further subdivisions have been rendered necessary with the
growth of knowledge the same principle is retained, namely, a
classification based on properties having no relation to composition.
The fundamental chemical classification of matter, on the other hand,
recognizes two groups of substances, namely, _elements_, which are
substances not admitting of analysis into other substances, and
_compounds_, which do admit of analysis into simpler substances and also
of synthesis from simpler substances. Chemistry and physics, however,
meet on common ground in a well-defined branch of science, named
physical chemistry, which is primarily concerned with the correlation of
physical properties and chemical composition, and, more generally, with
the elucidation of natural phenomena on the molecular theory.
It may be convenient here to state how the whole subject of chemistry
is treated in this edition of the _Encyclopaedia Britannica_. The
present article includes the following sections:--
I. _History._--This section is confined to tracing the general trend
of the science from its infancy to the foundations of the modern
theory. The history of the alchemical period is treated in more detail
in the article ALCHEMY, and of the iatrochemical in the article
MEDICINE. The evolution of the notion of elements is treated under
ELEMENT; the molecular hypothesis of matter under MOLECULE; and the
genesis of, and deductions from, the atomic theory of Dalton receive
detailed analysis in the article ATOM.
II. _Principles._--This section treats of such subjects as
nomenclature, formulae, chemical equations, chemical change and
similar subjects. It is intended to provide an introduction,
necessarily brief, to the terminology and machinery of the chemist.
III. _Inorganic Chemistry._--Here is treated the history of
descriptive inorganic chemistry; reference should be made to the
articles on the separate elements for an account of their preparation,
properties, &c.
IV. _Organic Chemistry._--This section includes a brief history of the
subject, and proceeds to treat of the principles underlying the
structure and interrelations of organic compounds.
V. _Analytical Chemistry._--This section treats of the qualitative
detection and separation of the metals, and the commoner methods
employed in quantitative analysis. The analysis of organic compounds
is also noticed.
VI. _Physical Chemistry._--This section is restricted to an account of
the relations existing between physical properties and chemical
composition. Other branches of this subject are treated in the
articles CHEMICAL ACTION; ENERGETICS; SOLUTION; ALLOYS;
THERMOCHEMISTRY.