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chemistry





The science of the nature, composition, and properties of material substances, and their transformations and interconversions. In modern terms, chemistry deals with elements and compounds, with the atoms and molecules of which they are composed, and with the reactions between them. It is thus basic to natural phenomena and modern technology alike.

Chemistry may be divided into five major parts: organic chemistry, the study of carbon compounds; inorganic chemistry, dealing with all the elements except carbon, and their compounds; chemical analysis, the determination of what a sample contains and how much of each constituent is present; biochemistry, the study of the complex organic compounds in biological systems; and physical chemistry, which underlies all the other branches, encompassing the study of the physical properties of substances and the theoretical tools for investigating them. Related sciences include geochemistry and metallurgy.





History of chemistry

Practical chemistry originated with the art of the metallurgists and artisans of the ancient Middle East. Their products included not only refined and alloyed metals but also dyes and glasses, and their methods were and remain shrouded in professional secrecy. Their chemical theory, expressed in terms of the prevailing theology, involved notions such as the opposition of contraries and the mediation of a mediating third. Classical Greek science generally expressed itself in he theoretical rather than the practical, as the conflicting physical theories of Thales, Anaxagoras, Anaximenes, and Aristotle bear witness. An important concept, that matter exists as atoms – tiny individual material particles – emerged at about this time (see atomism) though it did not become dominant for another 2,000 years.

During the Hellenistic Age a new practical chemistry arose in the study of alchemy. These early alchemists sought to apply Aristotelian physical theory to their practical experiments. Alchemy was the dominant guise of chemical science throughout the medieval period. Like the other sciences, it passed through Arab hands after the collapse of the Roman world, though, unlike the case with some other sciences, great practical advances were made during this time with the discovery of alcohol distillation and methods for preparing nitric acid and sulfuric acid. Chemical theory, however, remained primitive and practitioners sought to guard their secret recipes by employing obscure and even mystical phraseology.

The 16th century saw new clarity brought to the description of metallurgical processes in the writings of Georgius Agricola and the founding by Paracelsus of the new practical science of iatrochemistry with its emphasis on chemical medicines. Jan Baptista van Helmont, the greatest of his successors , began to use quantitative experiments. In the 17th century mechanist atomism enjoyed a revival with Robert Boyle leading a campaign to banish obscurantism from chemical description. The 18th century saw the rise of the phlogiston theory of combustion, promoted by Georg Stahl and adopted by all the great chemists of the day: Joseph Black, Carl Wihelm Scheele, and Joseph Priestley (all of whom found their greatest success in the study of gases). The phlogiston theory fell before the oxygen theory of Antoine Lavoisier and his associated binomial nomenclature
, and the 19th century saw the proposal of John Dalton's atomic theory, Avogadro's hypothesis (neglected for 50 years until revived by Stanislao Cannizzaro), and the foundation of electrochemistry which in the hands of Humphry Davy, rapidly yielded two new elements, sodium and potassium.

During the 19th century chemistry gradually assumed its present form, the most notable innovations being the periodic table of Dmitri Mendeleyev, the benzene ring-structure of Kekulé, the systematic chemical thermodynamics of Josiah Gibbs, and Robert Bunsen's chemical spectroscopy.

In the opening years of the 20th century the new atomic theory revolutionized chemical theory and the interrelation of the elements was deciphered. Since then successive improvements in experimental techniques (e.g., chromatography; isotopic labeling; microchemistry) and the introduction of new instruments (infrared, nuclear magnetic resonance, and mass spectroscopes) have led to continuing advances in chemical theory. These developments have also had a considerable impact on industrial chemistry and biochemistry. One of the most significant changes in the chemist's outlook has been that his interest has moved away from the nature of chemical substance itself toward questions of molecular structure, the energetics of chemical processes, and reaction mechanism.


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