Chemistry (from kemà, the book of secrets of Egyptian art, hence the Arabic “al-kimiaa” “الكيمياء”) is the natural science that studies the composition, structure and properties of matter, whether in the form of elements, species, compounds, mixtures or other substances, and the changes they undergo during reactions and their relationship to chemical energy. It also studies their association through chemical bonds that produce stable molecular compounds or more or less unstable intermediates. Linus Pauling defined it as the science that studies substances, their structure (types and arrangements of atoms), their properties, and the reactions that transform them into other substances over time. Chemistry, through one of its branches known as supramolecular chemistry, deals primarily with supramolecular groups such as gases, molecules, crystals, and metals, studying their composition, statistical properties, transformations, and reactions, although general chemistry also includes the understanding, properties, and interactions of matter at the atomic scale.

Within its field, chemistry occupies an intermediate position between physics and biology.

It is often referred to as the “central science” because it connects the other natural sciences, namely physics (through physical chemistry), biology (through biochemistry), astronomy (through astrochemistry), and geology (through geochemistry),[5][6] and provides a basis for understanding basic and applied scientific disciplines at a fundamental level. For example, chemistry explains aspects of plant growth (botany), the formation of igneous rocks (geology), the formation of atmospheric ozone and the degradation of environmental pollutants (ecology), the properties of lunar soil (cosmochemistry), the effects of drugs (pharmacology), and the collection of DNA evidence at a crime scene (forensics). Most chemical processes can be studied directly in the laboratory using a variety of often well-established techniques, both for manipulating materials and for understanding the underlying processes. An alternative approach is provided by molecular modeling techniques, which draw conclusions from computational models.

Chemistry is concerned with how atoms and molecules interact through chemical bonds to form new chemical compounds. There are two kinds of chemical bonds:

  • Primary: covalent bonds, in which atoms share one or more electrons; ionic bonds, in which one atom donates one or more electrons to another atom to form ions (cations and anions); and metallic bonds.
  • Secondary: hydrogen bonds; van der Waals force bonds; ion-ion interactions; and ion-dipole interactions.

Modern chemistry evolved from alchemy, a proto-scientific practice that was esoteric in nature but also experimental, combining elements of chemistry, physics, biology, metallurgy, and medicine.

Systematization became evident with the creation of the periodic table of elements and the introduction of atomic theory, as researchers developed a fundamental understanding of states of matter, ions, chemical bonding, and chemical reactions. Since the first half of the nineteenth century, the development of chemistry has been accompanied by the emergence and expansion of a chemical industry that is relevant to today’s economy and quality of life.

The disciplines of chemistry are grouped according to the subject studied or the type of study. These include inorganic chemistry, which studies inorganic matter; organic chemistry, which studies organic matter; biochemistry, which studies substances found in biological organisms; physical chemistry, which studies the structural and energetic aspects of chemical systems at the macroscopic, molecular, and atomic scales; and analytical chemistry, which analyzes samples of matter and seeks to understand their composition and structure through various tests and reactions.

Chemistry has been of interest to countless human populations since ancient times, partly for practical reasons derived from its technological applications. From the second century B.C., alchemy, a body of knowledge about matter and its transformations linked to philosophical and esoteric beliefs, developed from Ptolemaic Egypt; modern chemistry derived from it (following the scientific revolution, and more specifically the chemical revolution of the late 18th century). Chemistry has continued to evolve as new discoveries have expanded its areas of interest and the methods used.

The main subjects of study in chemistry are

  • the properties of the constituents of matter (atoms);
  • the properties of molecular entities, such as ions or molecules, which consist of single atoms or combinations of atoms;
  • the properties of chemical species (each of which is characterized by a specific type of molecular entity and particular properties that distinguish it from other chemical species);
  • the properties of mixtures and materials composed of one or more chemical species.

This study of matter is not limited to its properties and structure at a given moment, but also concerns its transformations, called chemical reactions (which involve the breaking of bonds holding together atoms belonging to the same molecular entity and the formation of new bonds to give rise to new molecular entities). Chemical transformations should not be confused with physical transformations. The main difference between the two types of transformations lies in the magnitude of the interactions that take place between the constituents of matter: the breaking and/or creation of low-energy bonds (such as van der Waals bonds and London forces) is called a physical transformation (e.g., mixing, gas-liquid absorption, distillation, physical adsorption), whereas the breaking and/or creation of higher-energy bonds (such as covalent bonds and ionic bonds) is called a chemical transformation.

The effects of such properties and the interactions between the constituents of matter on those of the objects and matter with which we commonly deal, and the relationships between them, are also studied, which determines the broad practical importance of such studies. It is therefore a very broad field of study, the areas of which are traditionally divided according to the type of matter they deal with or the type of study.

Knowledge of the electronic structure of atoms is the basis of conventional chemistry, while knowledge of the structure of the atomic nucleus and its spontaneous and induced transformations is the basis of nuclear chemistry.

Fundamental disciplines of chemistry

  • Inorganic chemistry
  • Organic chemistry
  • Theoretical chemistry
  • Physical chemistry
  • Analytical chemistry
  • Biochemistry

Historical background

The interest in nature and the transformation of natural substances is at the origin of chemistry and has its distant roots in early artisan techniques and ancient philosophical-natural speculations. Important were metallurgy, the production of pottery and its glazing, as well as the treatment of animal and plant substances to obtain medicines, perfumes and, above all, dyes. As with agriculture and medicine, a magical-religious correspondence was established for these processes with major natural events.

In Babylonian civilization, the seven celestial bodies (Earth, Moon, and the five known planets) were developed along with the seven metals of the Earth, the seven parts of the human body, the seven colors, and the seven days of the week. Greek philosophy sought to identify the stable constituents of things and explain how they produce the observable flow of events. The four elements of Empedocles (water, earth, air, and fire), the numbers of the Pythagoreans, and the atoms of Democritus were proposed as the fundamental factors of nature. Plato attributes the four elements to four types of elementary particles of different geometric shapes composed of triangles, while for Aristotle the bodies differ in qualities and functions. A single matter can take different forms, and the four elements are produced by the four qualities, two active (hot and cold) and two passive (wet and dry).

For the Stoics, the properties of bodies are not determined by their constituents, but by a particular tension due to the divine principle of pneuma. The theories of the Greek philosophers, however, could not explain the chemical processes that were the subject of artisanal techniques. In Alexandria, especially at the beginning of the Christian era, alchemy developed, a conception of the transformation of bodies that combined craft techniques, philosophical-natural theories, and the religious motif of salvation. Of Aristotelian derivation was one of the principles adopted by alchemy, the physiological principle of organic development, according to which minerals are also born and grow in the womb of the earth, and the alchemist’s task is to reproduce this process rapidly in the artificial womb of a flask or crucible.

Another motif of Alexandrian philosophy present in alchemy was that of religious refinement and salvation achieved through the separation of the soul from the body; if even material substances have a pneuma, an active component that determines their form and quality, then it is possible to separate their essence, their “spirit,” from the bodies through combustion, distillation, sublimation, etc., accompanied by magical-religious practices. Among the spirits or principles of metals, many placed mercury and sulfur, the former conditioning their liquefaction and the latter, as the principle of flammability, their red-hot state.

The Middle Ages and the Renaissance

The alchemical tradition flourished throughout the Middle Ages in the Arab world and spread to the Christian world from the 12th century. In the late Middle Ages, many new substances of importance were produced, such as mineral nitric and sulphuric acids, alcohol (aqua vitae) used as a medicine, and above all gunpowder from China, which testifies to the importance of chemical techniques in that country.

During the Renaissance, in addition to alchemy, there appeared, often in the vernacular, independent treatises on the technique of distillation (H. Brunschwig), on the art of mining and metallurgy (V. Biringuccio and G. Agricola), in which the language of a new world of craftsmanship, more related to observation than to learned tradition, was recognized. The physician Paracelsus, who placed chemistry at the base of medicine and opened the new direction of iatrochemistry, also wanted to break with the past. He prepared new medicines of a mineral nature and admitted, besides the traditional principles (sulphur and mercury), a third principle, salt, constitutive of earthiness, of resistance to fire. They corresponded to spirit, soul and body, but also to the three states, gaseous, liquid and solid, observable in distillation. At the end of the 16th century, A. Libavius made an effective summary of the chemical processes of his time.

The seventeenth century

In the first half of the 17th century, the works of J. Béguin and J. R. Glauber showed an interest in the preparation of new products, while J. B. van Helmont brought the alchemical tradition to a fruitful conclusion. On the basis of certain experiments, he argued that water is the material basis of all bodies (except air), whose development occurs as a result of an internal forming principle (archeus). Substances, when they burn, release this principle, which organizes water into different types of gases, among which he recognized the sylvan gas (carbon dioxide) produced in the effervescence generated by the meeting of acids and alkalis.

F. Sylvius de la Böe extended the latter conception by arguing that all phenomena could be explained on the basis of the antagonism of two general principles, acid and alkali. Also widespread was J. Mayow’s theory that combustion and respiration were made possible by the presence in the air of an active ingredient which he called nitroaerial spirit, the same as is present in saltpeter and which is also responsible for some meteorological phenomena such as lightning and snow.

In the 17th century, chemistry began to fit into the new mechanistic and atomistic conception of natural phenomena (D. Sennert, J. Jungius). This work was accomplished by R. Boyle, considered the first of the great modern chemists. In his best-known work, The Skeptical Chymist, he formulated a radical critique of the traditional Aristotelian theory of the four elements and the Paracelsian theory of the three principles, arguing for a corpuscular conception of matter and asserting that the fundamental purpose of chemistry lies not in the search for principles and essences, but in the study of the composition of bodies.

The eighteenth century

However, the corpuscular theory was not sufficient to give an adequate interpretation of chemical phenomena, especially those of combustion and calcination, and this gave rise in the 18th century to the concept of the so-called phlogiston theory, formulated by E. Stahl at the beginning of the century, which performed a unifying function of the various chemical researches. Stahl postulated the existence of a principle, phlogiston, in all combustible substances, which is released during combustion and calcination. Bodies such as coal, which burn without leaving any appreciable residue, were supposed to be the richest in phlogiston, in contrast to metals, which leave a considerable residue of lime as a result of calcination.

The assertion that a constituent of the substances was lost in these reactions contrasted with the fact, established by various researchers, that the lime weighed more than the corresponding metals; this, however, was explained by admitting a negative weight of phlogiston. The decisive factor in overcoming this theory was the study of gases, or pneumatic chemistry, which was thoroughly carried out by numerous researchers during this period. At the turn of the century, S. Hales had found a way to collect gases by bubbling them through the liquid of an inverted bottle. It was the Englishman J. Black who identified carbon dioxide as a substance chemically distinct from air, which was released from calcium carbonate by heating it and could be fixed again by lime by reproducing carbonate, and was therefore called “fixed air”.

In 1766 H. Cavendish identified hydrogen, in 1772 D. Rutherford identified nitrogen, and in 1769-74 C. W. Scheele and J. Priestley identified oxygen. These and other important results of chemical analysis, often misinterpreted in terms of the phlogiston theory, found a new and revolutionary accommodation in the work of A. Lavoisier, who is considered the initiator of chemical studies on a proper scientific basis because of the innovations he introduced in research methodology and theoretical elaboration of data. He explicitly assumed the principle of conservation of mass of interacting substances and gave the exact explanation of combustion as the process of combining the combustible substance with a part of the air.

Following Priestley’s studies, he established that this part of the air consisted of oxygen, and in 1783 he decisively attacked the theory of phlogiston as a completely fictitious entity. Having clarified the nature of combustion and specified the notion of an element as a substance that could not be further separated in the laboratory with the means at his disposal, Lavoisier elaborated a general classification of chemical substances in which he gave oxygen a dominant role. In fact, he considered acids to be substances whose generic property (acidity) is due to oxygen, while the specific property is indicated by the substances with which it combines (carbon, sulfur, nitrogen, etc.). He also contributed to the creation of a new terminology that indicated the constituent elements of compounds.

The nineteenth century

Lavoisier’s Traité élémentaire de chimie (1789) formed the basis of the new chemistry of the 19th century, which was primarily concerned with determining the amount of each element present in a given substance. L. J. Proust succeeded in proving that every compound always contains a fixed and definite proportion of its constituents, and J. Dalton interpreted this law by referring to the atomistic conception. Elements are made up of atoms of varying weight and size, and every compound is the result of the union of an atom of one element with one or more atoms of another element in multiple proportions.

Dalton also developed a table of atomic weights of various elements relative to the weight of hydrogen as the unit of measurement. J. L. Gay Lussac, starting from the synthesis of water, came to the conclusion that gases combine according to simple volume ratios, and this suggested to A. Avogadro the hypothesis (1811) that, under the same physical conditions, equal volumes of different gases contain the same number of molecules. Each molecule, therefore, could be composed of several atoms of the same element, susceptible to separate and recombine during reactions. This basic hypothesis was accepted only after 1860, mainly thanks to S. Cannizzaro. In fact, at the beginning of the century, it was thought that identical atoms must repel each other, an idea that seemed to be confirmed by electrolytic decomposition, especially of alkali salts (H. Davy). The elements were distinguished into electropositive (hydrogen, alkali, etc.) and electronegative (oxygen, sulfur, etc.), and J. J. Berzelius derived the dualistic theory that chemical compounds are formed by elements of opposite electrical charge.

However, this theory proved insufficient to explain the results obtained in the field of organic chemistry. At the turn of the century, it was recognized that chemical substances of plant and animal origin (some of which had already been known and analyzed in the 18th century) were essentially composed of carbon, hydrogen, oxygen, and nitrogen. J. Liebig perfected the method of analyzing organic compounds and, with Berzelius, accepted the idea that they were the product of a life force. Towards the middle of the century, the laboratory synthesis of some organic substances (F. Wöhler, H. Kolbe, M. Berthelot) and the establishment of the principle of energy conservation led to the overcoming of the concept of vital force. It was also discovered that some organic substances of the same composition had different properties (isomers), so it was admitted that the same constituents could have a different spatial arrangement and that some of them realized stable groups (radicals) reacting as indivisible entities.

A. Laurent and J. B. Dumas, however, in contrast to the dualistic electrochemical theory, demonstrated that electropositive hydrogen could be replaced by electronegative chlorine without significantly altering the compound, and they advanced the type theory according to which the relative position of atoms in the molecule was an important factor in determining the properties of the compound. C. Gerhardt developed these ideas and distinguished a limited number of types to whose structure the structure of all others could be traced. This idea prepared the concept of valence, which, introduced by E. Frankland, was generalized in the works of F. A. Kekulé and A. Wurtz, and from 1860, with the acceptance of Avogadro’s hypothesis, allowed a remarkable clarification and development of chemistry.

Kekulé, having defined the tetravalence of carbon, paved the way for structural formulas by establishing that of a hexagonal ring for benzene. The concept of chemical structure and that of the atom were introduced and developed in the 19th century as hypotheses and schemes for interpreting the narrow field of chemical processes, but they were not based on a secure foundation in the knowledge of matter, and it was not until the rise of atomic theories in early 20th century physics that they were effectively validated.

A fundamental step in 19th century chemistry was taken by L. Meyer and D. I. Mendeleev with the classification of the elements. Mendeleev with the classification of elements, which were distributed according to their progressive atomic weight and periodic recurrence of chemical properties. Mendeleev also revealed in the periodic system some gaps corresponding to hypothetical unknown elements, which were later actually discovered with the expected properties.

In the last decades of the century, the study of the physical aspects of chemical processes (physicochemistry) had considerable theoretical and applied importance. From the study of the heat emitted and absorbed by reactions came the consideration of the mass action of the reactants involved in the equilibrium of reversible reactions (C. M. Guldberg, P. Waage). The process of catalysis (acceleration of reactions by the presence of an activating substance) was treated by W. Ostwald, while J. H. van’t Hoff established that the gas laws also apply to substances in extremely dilute solution, and S. Arrhenius specified the notion of dissociation of electrolytes.

The twentieth century

Since the last decades of the nineteenth century, great importance has come to be assumed, for pure research itself, by the development industria chimicae in the field of synthetic organics, petroleum derivatives and macromolecular compounds. From the first laboratory synthesis of textile dyes (aniline derivatives, indigo, etc.) and their industrial preparation with coal derivatives, the synthetic production of a large number of organic compounds (pharmaceuticals, gasoline, rubber, fibers, etc.) was achieved in a few decades. The isolation and synthesis of inorganic substances was also extended to an industrial level with the metallurgical production (aluminum) and that of fertilizers and explosives (atmospheric nitrogen fixation).

In the twentieth century, the extremely fruitful development of biological chemistry made it possible to analyze in detail the most subtle processes of living organisms and to begin the synthesis of some of their basic compounds (vitamins, hormones, proteins). In fact, in this century, the methodological and descriptive barrier that had previously distinguished inorganic chemistry, or the chemistry of inanimate objects, organic chemistry, or the chemistry of compounds found in living organisms, and biological chemistry, understood as the science that studies the complex of chemical phenomena that make a cell capable of reproducing itself and thus of living, was overcome.

The development of chemistry has helped to overcome the view of life and biological mechanisms as a mystery, suggesting instead as the central point of vital capacity a refined organization of matter worthy of study and a source of scientific observations and practical applications. The new conception of the structure of the atom and the possibility of harnessing the enormous amounts of energy it contains also represents a fundamental achievement of chemistry in the 20th century. In fact, the transformation of atomic mass into energy, by reproducing the phenomenon present in stars, makes it possible to make large amounts of energy available to processes that would not be possible without large energy inputs. Just think of the energy consumption of a large city and its inhabitants.

The development of studies for the preparation of new chemical compounds has made it possible to obtain substances used for the construction of inexpensive objects. For example, certain reactions that produce simple chemicals, called monomers, are at the origin of the processes of forming chemical bonds between many monomer molecules and can give rise to polymers that, when conveniently processed, provide plastics. These have revolutionized the market for objects.

On the other hand, the development of techniques for the preparation of chemical compounds has also made it possible to add to drugs of plant or animal origin a large number of synthetic drugs, which are now often essential for the treatment of widespread diseases for which there were no effective remedies until recently, such as cardiovascular diseases (now treated with synthetic blood pressure regulators) or cancers (some of which can be treated with synthetic anti-cancer drugs). Nor should we forget the new technology of metals, which has made it possible to obtain metals with particularly useful technical properties, such as those of semiconductors, necessary for the construction of computers. Chemistry has thus contributed greatly to transforming nature in ways that have improved the quality of life, at least in the industrialized world.

The question today is how to avoid the negative side effects of the thousands of new chemical compounds that are being produced and the materials in which they are incorporated. Chemical pollution, consumption of unnecessary drugs, radioactive waste, production of chemicals for war purposes are phenomena of today. Thus, a new chemistry is born, one that studies ways to improve the quality of life without harming the environment or human beings.

Applications of chemistry

Chemistry and industry.

Industrial chemistry is concerned with the large-scale synthesis of chemicals for various applications, optimizing the cost-benefit ratio of the entire chemical production cycle. Specifically, from appropriate raw materials, semi-finished or finished products are obtained through a series of processes within a chemical plant that meet the specifications and technical requirements for their practical use. Some of the best known industrial chemical processes include the Haber-Bosch process for the synthesis of ammonia and the Ostwald process for the synthesis of nitric acid. The petrochemical and synthetic polymer industries are another large and very active field.

Chemistry and medicine

Pharmaceutical chemistry is the field of research for the synthesis and therapeutic application of new drugs. It is based on the theoretical study of the chemical and physical properties of molecules and models of pharmacological interaction with the organism. A suitable synthesis strategy is then formulated, also taking advantage of the combinatorial chemistry approach, and the new drug obtained may enter the testing phase, which, if successful, may allow it to be placed on the market. In addition to these pharmacological aspects, chemistry is also proving to be a useful tool in diagnostic medicine through the possibility of carrying out specific clinical laboratory chemistry tests. Radioactive isotopes are used in nuclear medicine.

Chemistry and the environment

The growing awareness of low environmental impact and the need to apply sustainable development policies has led to the emergence of so-called green chemistry. This discipline aims to reduce the impact of chemical processes by putting into practice concepts such as using raw materials from renewable sources, reducing waste and rejects, and using bio-sustainable and environmentally friendly compounds. On the other hand, environmental chemistry focuses on the study of the chemistry and biochemistry of the environment: it is interested in freshwater and marine chemistry, soil chemistry, and atmospheric chemistry. It not only understands the basics of chemistry, but also extends its field of study and research to phenomena related to pollution and the effects of toxic substances released into the environment, proposing remedies.

Chemistry and cultural heritage

Chemistry applied to cultural heritage deals with the materials used in art and the analytical techniques, invasive and non-invasive, used for the instrumental study of works of art[14]. It is also concerned with the dating of artifacts and methods of restoration and conservation. He studies the mechanisms and factors that contribute to the deterioration of art artifacts and tries to remedy their effects.

Other disciplines

There are numerous specializations and disciplines of chemistry that can be considered part of the core disciplines and often also part of other related scientific disciplines, such as pharmaceutical chemistry, industrial chemistry, polymer and macromolecular chemistry, food chemistry, solid-state and surface chemistry, astrochemistry, cosmochemistry, electrochemistry, geochemistry, theoretical chemistry, cytochemistry, histochemistry, clinical chemistry, nuclear chemistry, radiochemistry, radiation chemistry, organometallic chemistry, stereochemistry, environmental chemistry, green chemistry, photochemistry, sonochemistry, soil chemistry, atmospheric chemistry, radiopharmaceutical chemistry, aerothermochemistry, restoration chemistry, cultural heritage chemistry, chemical structure chemistry, magnetochemistry, quantum chemistry, femtochemistry, colloid chemistry, interfacial chemistry, combinatorial chemistry, computational chemistry, mathematical chemistry, cheminformatics, chemometrics, materials chemistry, cement chemistry, soft chemistry, supramolecular chemistry, nanochemistry.

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