(From Grolier Multimedia Encyclopedia)


Chemistry is the physical science that deals with the composition, structure, and properties of substances and also the transformations that these substances undergo. Because the study of chemistry encompasses the entire material universe, it is central to the understanding of other sciences.

A basic chemical theory has been formulated as the result of centuries of observation and measurement of the various elements and compounds (see chemistry, history of). According to this theory, matter is composed of minute particles called atoms. The more than 100 different kinds of atoms that are known are called chemical elements. Atoms of the same element or of different elements can combine together to form molecules and chemical compounds. The atoms are held together by forces, primarily electrostatic, called chemical bonds. In a chemical reaction two or more molecules can undergo various changes to form different molecules by means of breaking and making the chemical bonds.

Branches of Chemistry

Five subdivisions traditionally are used to classify various aspects of chemistry. The study of organic chemistry originally was limited to compounds that were obtained from living organisms, but now the field deals with hydrocarbons (compounds of carbon and hydrogen) and their derivatives. The study of inorganic chemistry includes compounds derived from all of the elements except for hydrocarbons. Biochemistry is the subdivision in which the compounds and chemical reactions involved in processes of living systems are studied.

Physical chemistry deals with the structure of matter and the energy changes that occur during physical and chemical changes of matter. This field provides a theoretical basis for the chemical observations of the other subdivisions. Analytical chemistry is concerned with the identification of chemical substances, the determination of the amounts of substances present in a mixture, and the separation of mixtures into their individual components.

Special subdivisions of chemistry are now recognized that account for knowledge at the interface of chemistry and other physical sciences. For example, recent research has involved the chemical origin of life—reactions between simple molecules at low pressures to form such complex organic molecules as proteins found in living organisms.

Astrochemistry is the interdisciplinary physical science that studies the origin and interaction of the chemical constituents, especially interstellar matter, in the universe. Geochemistry is concerned with the chemical aspects of geology—for instance, the improvement of ore processing, coal utilization, shale oil recovery—and the use of chemicals to extract oil from wells that are considered dry by ordinary standards.

Nuclear chemistry deals with natural and induced transformations of the atomic nucleus. Studies in this field now center on the safe and efficient use of nuclear power and the disposal of nuclear wastes. Radiochemistry deals with radioactive isotopes of chemical elements and the utilization of those isotopes to further the understanding of chemical and biochemical systems. Environmental chemistry is a subdivision that has as its subject the impact of various elements and compounds on the ecosphere.

Tools of Chemistry

Chemistry is a precise laboratory science, and the equipment of a chemical laboratory is usually involved with measurement. Balances are used to measure mass, pipettes and burettes to measure volume, colorimeters to measure color intensities, and thermometers to measure temperature changes. Advances in electronics and computer technology have enabled the development of scientific instruments that determine the chemical properties, structure, and content of substances accurately and precisely.

Most modern chemical instrumentation has three primary components: a source of energy, a sample compartment within which a substance is subjected to the energy, and some sort of detector to determine the effect of the energy on the sample. An X-ray diffractometer, for instance, enables the chemist to determine the arrangement of atoms, ions, and molecules that constitute crystals by means of scattering X rays. Most modern laboratories contain ultraviolet, visible, and infrared spectrophotometers, which use light of various wavelengths on gaseous or liquid samples. By such a means the chemist can determine the electron configuration and the arrangement of atoms in molecules. A nuclear magnetic resonance spectrophotometer subjects a sample in a strong magnetic field to radio frequency radiation. The absorption of this energy by the sample gives the chemist information concerning the bonding within molecules. Other instruments include mass spectrometers, which use electrons as an energy source, and differential thermal analyzers, which use heat.

An entirely different class of instruments are those which use chromatography to separate complex mixtures into their components. Chemists are also using extremely short pulses of laser light to investigate the atomic and molecular processes taking place in chemical reactions at the microsecond level. These and other devices generate so much data that chemists frequently must use computers to help analyze the results.

Impact On Society

Chemistry is closely associated with four basic needs of humans: food, clothing, shelter, and medical services. The applications of chemistry usually bring to mind industries engaged in the production of chemicals. A significant portion of the chemical industry is engaged in the production of inorganic and organic chemicals, which are then used by other industries as reactants for their chemical processes. In the United States the great majority of the leading chemicals being produced are inorganic, and their manufacture is a multibillion-dollar industry.

The chemistry of polymers—large molecules made up of simple repeating units linked together by chemical bonds—includes plastics, resins, natural and synthetic rubber, synthetic fibers, and protective coatings. The growth of this segment of chemistry has been phenomenal since the late 1930s. The fabrication of natural rubber and coatings (paints, varnishes, lacquers, and enamels) derived from natural agricultural products has been a mainstay of the chemical industry for more than 150 years.

The search for new energy sources and the improvement of existing ones are in many ways chemical problems. At the heart of the petroleum industry are the processes of refining crude hydrocarbons into such products as gasoline and petrochemicals. The utilization of nuclear power depends heavily on the chemical preparation and reprocessing of fuel, the treatment and disposal of nuclear waste, and the problems of corrosion and heat transfer. The conversion and storage of solar energy as electrical energy is primarily a chemical process, and the development of fuel cells is based on either chemical or electrochemical technology.

Chemical research has been the basis of the pharmaceutical industry's production of drugs. The controlled introduction of specific chemicals into the body assists in the diagnosis, treatment, and often the cure of illness. Chemotherapy is a prime treatment in combating cancer.

Tremendous agricultural gains have been achieved since about 1940 as a result mainly of farmers' use of chemical fertilizers and pesticides. Other chemical industries include soap and detergent production; food processing; and the production of glass, paper, metals, and photographic supplies.

Specialized Uses

Outside the mainstream of what is traditionally considered chemistry is research that supports other professions. Chemistry is used by museums in art conservation and restoration, the dating of objects, and the uncovering of frauds. Forensic chemists work in crime laboratories, carrying out tests to assist law-enforcement agencies. Toxicologists study the potentially adverse effects of chemicals on biological systems, as do those involved in industrial hygiene. The chemistry involved in sanitary engineering and sewage treatment has come to be of major importance to society as populations increase and environmental concerns intensify.


Through the use of chemistry and related technology, chemical substances have been produced that either immediately or eventually are harmful to humans, animals, and the environment. Pollution is not a new problem, but the combination of a rapidly growing chemical industry and the use of sophisticated detection devices has brought the extent of pollution to public attention.

The discharge and disposal of industrial waste products into the atmosphere and water supply, for example, at Love Canal, have caused grave concern about environmental deterioration. The repeated exposure of workers to some toxic chemicals at their jobs has caused long-range health problems. In addition, the use of some pesticides and herbicides can cause long-term toxicity, the effects of which are still only partially understood. The safe storage and disposal of chemical and biological warfare agents and nuclear waste continue to be a serious problem. An advance in chemical technology almost always involves some trade-off with regard to an alteration of the environment.

Challenges and Trends

Much of the future of chemistry will lie in providing answers to such technological problems as the creation of new sources of energy and the eradication of disease, famine, and environmental pollution. The improvement of the safety of existing chemical products, for example, pesticides, is another challenge. Research into the chemical complexities of the human body may reveal new insights into a variety of diseases and dysfunctions. The improvement of industrial processes will serve to minimize the use of energy and raw materials, thereby diminishing negative environmental effects.

Norman V. Duffy

Further Reading:

American Chemical Society Staff, Chemistry in Context, 3d ed. (2000).

Brown, Theodore L., Chemistry: The Central Science, 8th ed. (1999).

Hall, Nina, ed., The New Chemistry (2001).

Hunt, Andrew, Dictionary of Chemistry (1999).

Kelter, Paul, Chemistry: A World of Choices (1999).

Sterner, Olof, Chemistry, Health, and Environment (1999).

Williams, Robert Joseph Paton, and Frusto Da Silva, J. J. R., Bringing Chemistry to Life: From Matter to Man (1999).