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'''Chemistry''' (from [[Egyptian language|Egyptian]] ''kēme'' (chem), meaning [[Classical element|"earth"]] '''See:''' [[Chemistry (etymology)]] for possible origins of this word. is the [[science]] concerned with  the composition, structure, and properties of [[matter]], as well as the changes it undergoes during [[chemical reaction]]s. [http://dictionary.reference.com/browse/Chemistry Chemistry]. (n.d.). Merriam-Webster's Medical Dictionary. Retrieved [[August 19]], [[2007]].  Chemistry is a [[physical science]] related to studies of various [[atom]]s, [[molecule]]s, [[crystal]]s and other  aggregates of matter whether in isolation or combination, which incorporates the concepts of [[energy]] and [[entropy]] in relation to the [[spontaneous reaction|spontaneity]] of [[chemical process]]es.

 

 

Disciplines within chemistry are traditionally grouped by the type of matter being studied or the kind of study. These include [[inorganic chemistry]], the study of [[inorganic]] matter; [[organic chemistry]], the study of [[Organic compound|organic]] matter; [[biochemistry]], the study of [[chemical substance|substances]] found in [[organisms|biological organisms]]; [[physical chemistry]], the [[energy]] related studies of chemical systems at macro, molecular and submolecular scales; [[analytical chemistry]], the analysis of material samples to gain an understanding of their [[chemical composition]] and [[Chemical structure|structure]]. Many more specialized disciplines have emerged in recent years, e.g. [[neurochemistry]] the chemical study of the mind (see [[#Subdisciplines|subdisciplines]]).   

Disciplines within chemistry are traditionally grouped by the type of matter being studied or the kind of study. These include [[inorganic chemistry]], the study of [[inorganic]] matter; [[organic chemistry]], the study of [[Organic compound|organic]] matter; [[biochemistry]], the study of [[chemical substance|substances]] found in [[organisms|biological organisms]]; [[physical chemistry]], the [[energy]] related studies of chemical systems at macro, molecular and submolecular scales; [[analytical chemistry]], the analysis of material samples to gain an understanding of their [[chemical composition]] and [[Chemical structure|structure]]. Many more specialized disciplines have emerged in recent years, e.g. [[neurochemistry]] the chemical study of the mind (see [[#Subdisciplines|subdisciplines]]).   

[[Energy]] and [[entropy]] considerations are invariably important in almost all chemical studies. Chemical substances are classified in terms of their [[structure]], phase as well as their [[chemical composition]]s. They can be analysed using the tools of [[chemical analysis]], e.g. [[spectroscopy]] and [[chromatography]].  

[[Energy]] and [[entropy]] considerations are invariably important in almost all chemical studies. Chemical substances are classified in terms of their [[structure]], phase as well as their [[chemical composition]]s. They can be analysed using the tools of [[chemical analysis]], e.g. [[spectroscopy]] and [[chromatography]].  

Chemistry is an integral part of the [[science]] curriculum both at the [[high school]] as well as the early college level. At these levels, it is often called 'general chemistry' which is an introduction to a wide variety of fundamental concepts that enable the student to acquire tools and skills useful at the advanced levels, whereby chemistry is invariably studied in any of its various sub-disciplines. [[Scientists]], engaged in  chemical [[research]] are known as [[chemists]].<ref>California Occupational Guide Number 22: Chemists[http://www.calmis.ca.gov/file/occguide/CHEMIST.HTM]</ref> Most chemists specialize in one or more sub-disciplines.

Chemistry is an integral part of the [[science]] curriculum both at the [[high school]] as well as the early college level. At these levels, it is often called 'general chemistry' which is an introduction to a wide variety of fundamental concepts that enable the student to acquire tools and skills useful at the advanced levels, whereby chemistry is invariably studied in any of its various sub-disciplines. [[Scientists]], engaged in  chemical [[research]] are known as [[chemists]].[http://www.calmis.ca.gov/file/occguide/CHEMIST.HTM] Most chemists specialize in one or more sub-disciplines.

==History==

==History==

The genesis of chemistry can be traced to the widely observed phenomenon of burning that led to [[metallurgy]]- the art and science of processing ores to get metals (e.g. [[History of metallurgy in the Indian subcontinent|metallurgy in ancient India]]). The greed for gold led to the discovery of the process for its purification, even though, the underlying principles were not well understood -- it was thought to be a transformation rather than purification. Many scholars in those days thought it reasonable to believe that there exist means for transforming cheaper (base) metals into gold. This gave way to alchemy, and the search for the [[Philosopher's Stone]], which was believed to bring about such a transformation by mere touch.[http://www.chemheritage.org/explore/ancients-time.html Alchemy Timeline]

 

The genesis of chemistry can be traced to the widely observed phenomenon of [[combustion|burning]] that led to [[metallurgy]]- the art and science of processing ores to get metals (e.g. [[History of metallurgy in the Indian subcontinent|metallurgy in ancient India]]). The greed for gold led to the discovery of the process for its purification, even though, the underlying principles were not well understood -- it was thought to be a transformation rather than purification. Many scholars in those days thought it reasonable to believe that there exist means for transforming cheaper (base) metals into gold. This gave way to alchemy, and the search for the [[Philosopher's Stone]], which was believed to bring about such a transformation by mere touch.<ref>[http://www.chemheritage.org/explore/ancients-time.html Alchemy Timeline] - Chemical Heritage Society</ref>

Some consider [[Alchemy (Islam)|medieval Muslims]] to be the earliest chemists, who introduced precise [[observation]] and controlled [[experiment]]ation into the field, and discovered numerous [[chemical substance]]s. [[Will Durant]] (1980), ''The Age of Faith (The Story of Civilization, Volume 4)'', p. 162-186, Simon & Schuster, ISBN 0671012002:

Some consider [[Alchemy (Islam)|medieval Muslims]] to be the earliest chemists, who introduced precise [[observation]] and controlled [[experiment]]ation into the field, and discovered numerous [[chemical substance]]s. [[Will Durant]] (1980), ''The Age of Faith (The Story of Civilization, Volume 4)'', p. 162-186, Simon & Schuster, ISBN 0671012002:

<br>{{quote|"Chemistry as a science was almost created by the Muslims; for in this field, where the Greeks (so far as we know) were confined to industrial experience and vague [[hypothesis]], the [[Saracen]]s introduced precise [[observation]], controlled [[experiment]], and careful records. They invented and named the [[alembic]] (al-anbiq), chemically analyzed innumerable [[Chemical substance|substance]]s, composed [[Lapidary|lapidaries]], distinguished [[alkali]]s and [[acid]]s, investigated their affinities, studied and manufactured hundreds of [[drug]]s. Alchemy, which the Muslims inherited from Egypt, contributed to chemistry by a thousand incidental discoveries, and by its method, which was the most scientific of all medieval operations."}} The most influential Muslim chemists were [[Geber]] (d. 815), [[al-Kindi]] (d. 873), [[al-Razi]] (d. 925), and [[Abū Rayhān al-Bīrūnī|al-Biruni]] (d. 1048).<ref>Dr. K. Ajram (1992), ''Miracle of Islamic Science'', Appendix B, Knowledge House Publishers, ISBN 0911119434.<br>{{quote|"[[Alexander von Humboldt|Humboldt]] regards the Muslims as the founders of chemistry."}}</ref> The works of Geber became more widely known in Europe through [[Latin]] translations by a [[pseudo-Geber]] in 14th century [[Spain]], who also wrote some of his own books under the pen name "Geber". The contribution of [[History of metallurgy in the Indian subcontinent|Indian alchemists and metallurgists]] in the development of chemistry was also quite significant. [[Will Durant]] (1935): Our Oriental Heritage: Simon & Schuster:

<br>"Chemistry as a science was almost created by the Muslims; for in this field, where the Greeks (so far as we know) were confined to industrial experience and vague [[hypothesis]], the [[Saracen]]s introduced precise [[observation]], controlled [[experiment]], and careful records. They invented and named the [[alembic]] (al-anbiq), chemically analyzed innumerable [[Chemical substance|substance]]s, composed [[Lapidary|lapidaries]], distinguished [[alkali]]s and [[acid]]s, investigated their affinities, studied and manufactured hundreds of [[drug]]s. Alchemy, which the Muslims inherited from Egypt, contributed to chemistry by a thousand incidental discoveries, and by its method, which was the most scientific of all medieval operations."}} The most influential Muslim chemists were [[Geber]] (d. 815), [[al-Kindi]] (d. 873), [[al-Razi]] (d. 925), and [[Abū Rayhān al-Bīrūnī|al-Biruni]] (d. 1048).Dr. K. Ajram (1992), ''Miracle of Islamic Science'', Appendix B, Knowledge House Publishers, ISBN 0911119434."[[Alexander von Humboldt|Humboldt]] regards the Muslims as the founders of chemistry."}}</ref> The works of Geber became more widely known in Europe through [[Latin]] translations by a [[pseudo-Geber]] in 14th century [[Spain]], who also wrote some of his own books under the pen name "Geber". The contribution of [[History of metallurgy in the Indian subcontinent|Indian alchemists and metallurgists]] in the development of chemistry was also quite significant. [[Will Durant]] (1935): Our Oriental Heritage: Simon & Schuster:

<br>{{quote|"Something has been said about the chemical excellence of cast iron in ancient India, and about the high industrial development of the Gupta times, when India was looked to, even by Imperial Rome, as the most skilled of the nations in such chemical industries as dyeing, tanning, soap-making, glass and cement... By the sixth century the Hindus were far ahead of Europe in industrial chemistry; they were masters of calcinations, distillation, sublimation, steaming, fixation, the production of light without heat, the mixing of anesthetic and soporific powders, and the preparation of metallic salts, compounds and alloys. The tempering of steel was brought in ancient India to a perfection unknown in Europe till our own times; King Porus is said to have selected, as a specially valuable gift from Alexander, not gold or silver, but thirty pounds of steel. The Moslems took much of this Hindu chemical science and industry to the Near East and Europe; the secret of manufacturing "Damascus" blades, for example, was taken by the Arabs from the Persians, and by the Persians from India.""}}

"Something has been said about the chemical excellence of cast iron in ancient India, and about the high industrial development of the Gupta times, when India was looked to, even by Imperial Rome, as the most skilled of the nations in such chemical industries as dyeing, tanning, soap-making, glass and cement... By the sixth century the Hindus were far ahead of Europe in industrial chemistry; they were masters of calcinations, distillation, sublimation, steaming, fixation, the production of light without heat, the mixing of anesthetic and soporific powders, and the preparation of metallic salts, compounds and alloys. The tempering of steel was brought in ancient India to a perfection unknown in Europe till our own times; King Porus is said to have selected, as a specially valuable gift from Alexander, not gold or silver, but thirty pounds of steel. The Moslems took much of this Hindu chemical science and industry to the Near East and Europe; the secret of manufacturing "Damascus" blades, for example, was taken by the Arabs from the Persians, and by the Persians from India.""}}

The emergence of chemistry in Europe was primarily due to the recurrent incidence of the [[Bubonic plague|plague]] and blights there during the so called [[Dark Ages]]. This gave rise to a need for medicines. It was thought that there exists a universal medicine called the [[Elixir of Life]] that can cure all diseases, but like the Philosopher's Stone, it was never found.  

The emergence of chemistry in Europe was primarily due to the recurrent incidence of the [[Bubonic plague|plague]] and blights there during the so called [[Dark Ages]]. This gave rise to a need for medicines. It was thought that there exists a universal medicine called the [[Elixir of Life]] that can cure all diseases, but like the Philosopher's Stone, it was never found.  

For some practitioners, alchemy was an intellectual pursuit, over time, they got better at it. [[Paracelsus]] (1493-1541), for example, rejected the 4-elemental theory and with only a vague understanding of his chemicals and medicines, formed a hybrid of alchemy and science in what was to be called  ''[[iatrochemistry]]''. Similarly, the influences of philosophers such as [[Sir Francis Bacon]] (1561-1626) and [[René Descartes]] (1596-1650), who demanded more rigor in mathematics and in removing bias from scientific observations, led to a [[scientific revolution]]. In chemistry, this began with [[Robert Boyle]] (1627-1691), who came up with an equations known as the [[Boyle's Law]] about the characteristics of gaseous state.<ref> [http://www.bbc.co.uk/history/historic_figures/boyle_robert.shtml BBC - History - Robert Boyle (1627 - 1691)]  Chemistry indeed came of age when [[Antoine Lavoisier]] (1743-1794), developed the theory of [[Conservation of mass]] in 1783; and the development of the [[Atomic Theory]] by [[John Dalton]] around 1800.  The Law of Conservation of Mass resulted in the reformulation of chemistry based on this law and the oxygen theory of combustion, which was largely based on the work of Lavoisier.  Lavoisier's fundamental contributions to chemistry were a result of a conscious effort to fit all experiments into the framework of a single theory. He established the consistent use of the chemical balance, used oxygen to overthrow the [[phlogiston theory]], and developed a new system of chemical nomenclature and made contribution to the modern metric system.  Lavoisier  also worked to translate the archaic and technical language of chemistry into something that could be easily understood by the largely uneducated masses, leading to an increased public interest in chemistry. All these advances in chemistry led to what is usually called the [[chemical revolution]].  The contributions of Lavoisier led to what is now called modern chemistry - the chemistry that is studied in educational institutions all over the world.  It is because of these and other contributions that [[Antoine Lavoisier]] is often celebrated as the "[[Father of Modern Chemistry]]".  The later discovery of [[Friedrich Wöhler]] that many natural substances, [[organic compound]]s, can indeed be synthesized in a chemistry [[laboratory]] also helped the modern chemistry to mature from its infancy.

For some practitioners, alchemy was an intellectual pursuit, over time, they got better at it. [[Paracelsus]] (1493-1541), for example, rejected the 4-elemental theory and with only a vague understanding of his chemicals and medicines, formed a hybrid of alchemy and science in what was to be called  ''[[iatrochemistry]]''. Similarly, the influences of philosophers such as [[Sir Francis Bacon]] (1561-1626) and [[René Descartes]] (1596-1650), who demanded more rigor in mathematics and in removing bias from scientific observations, led to a [[scientific revolution]]. In chemistry, this began with [[Robert Boyle]] (1627-1691), who came up with an equations known as the [[Boyle's Law]] about the characteristics of gaseous state. [http://www.bbc.co.uk/history/historic_figures/boyle_robert.shtml BBC - History - Robert Boyle (1627 - 1691)]  Chemistry indeed came of age when [[Antoine Lavoisier]] (1743-1794), developed the theory of [[Conservation of mass]] in 1783; and the development of the [[Atomic Theory]] by [[John Dalton]] around 1800.  The Law of Conservation of Mass resulted in the reformulation of chemistry based on this law and the oxygen theory of combustion, which was largely based on the work of Lavoisier.  Lavoisier's fundamental contributions to chemistry were a result of a conscious effort to fit all experiments into the framework of a single theory. He established the consistent use of the chemical balance, used oxygen to overthrow the [[phlogiston theory]], and developed a new system of chemical nomenclature and made contribution to the modern metric system.  Lavoisier  also worked to translate the archaic and technical language of chemistry into something that could be easily understood by the largely uneducated masses, leading to an increased public interest in chemistry. All these advances in chemistry led to what is usually called the [[chemical revolution]].  The contributions of Lavoisier led to what is now called modern chemistry - the chemistry that is studied in educational institutions all over the world.  It is because of these and other contributions that [[Antoine Lavoisier]] is often celebrated as the "[[Father of Modern Chemistry]]".  The later discovery of [[Friedrich Wöhler]] that many natural substances, [[organic compound]]s, can indeed be synthesized in a chemistry [[laboratory]] also helped the modern chemistry to mature from its infancy.

The [[discoveries of the chemical elements]] has a long history from the days of alchemy and culminating in the creation of the [[periodic table]] of the chemical elements by [[Dmitri Mendeleev]] (1834-1907) [http://chemistry.about.com/library/weekly/aa030303a.htm Timeline of Element Discovery] - About.com</ref> and later discoveries of some [[synthetic elements]].

The [[discoveries of the chemical elements]] has a long history from the days of alchemy and culminating in the creation of the [[periodic table]] of the chemical elements by [[Dmitri Mendeleev]] (1834-1907) [http://chemistry.about.com/library/weekly/aa030303a.htm Timeline of Element Discovery] and later discoveries of some [[synthetic elements]].

==Etymology==

==Etymology==

The word ''chemistry'' comes from the earlier study of alchemy, which is basically the quest to make gold from earthen starting materials.<ref>Alchemy Lab: History of Alchemy [http://www.alchemylab.com/history_of_alchemy.htm]</ref> As to the origin of the word "alchemy" the question is a debatable one; it certainly can be traced back to the Greeks, and some, following E. Wallis Budge, have also asserted [[Ancient Egypt|Egyptian]] origins. Alchemy, generally, derives from the old French ''alkemie'' from the Arabic ''al-kimia'' - "the art of transformation". The Arabs borrowed the word "kimia" from the Greeks when they conquered [[Alexandria]] in the year 642 AD. A tentative outline is as follows:

The word ''chemistry'' comes from the earlier study of alchemy, which is basically the quest to make gold from earthen starting materials.<ref>Alchemy Lab: History of Alchemy [http://www.alchemylab.com/history_of_alchemy.htm]</ref> As to the origin of the word "alchemy" the question is a debatable one; it certainly can be traced back to the Greeks, and some, following E. Wallis Budge, have also asserted [[Ancient Egypt|Egyptian]] origins. Alchemy, generally, derives from the old French ''alkemie'' from the Arabic ''al-kimia'' - "the art of transformation". The Arabs borrowed the word "kimia" from the Greeks when they conquered [[Alexandria]] in the year 642 AD. A tentative outline is as follows:

#Egyptian alchemy [5,000 BCE &ndash; 400 BCE], formulate early "element" theories such as the [[Ogdoad]].

#Egyptian alchemy [5,000 BCE - 400 BCE], formulate early "element" theories such as the [[Ogdoad]].

#Greek alchemy [332 BCE &ndash; 642 CE], the Greek king [[Alexander the Great]] conquers Egypt and founds Alexandria, having the world's largest library, where scholars and wise men gather to study.

#Greek alchemy [332 BCE - 642 CE], the Greek king [[Alexander the Great]] conquers Egypt and founds Alexandria, having the world's largest library, where scholars and wise men gather to study.

#[[Alchemy (Islam)|Arabian alchemy]] [642 CE &ndash; 1200], the Arabs take over Alexandria; [[Geber|Jabir]] is the main chemist

#[[Alchemy (Islam)|Arabian alchemy]] [642 CE - 1200], the Arabs take over Alexandria; [[Geber|Jabir]] is the main chemist

#European alchemy [1300 &ndash; present], [[Pseudo-Geber]] builds on Arabic chemistry

#European alchemy [1300 &ndash; present], [[Pseudo-Geber]] builds on Arabic chemistry

#Chemistry [1661], [[Robert Boyle|Boyle]] writes his classic chemistry text ''The Sceptical Chymist''

#Chemistry [1661], [[Robert Boyle|Boyle]] writes his classic chemistry text ''The Sceptical Chymist''

In retrospect, the definition of chemistry seems to invariably change per decade, as new discoveries and theories add to the functionality of the science. Shown below are some of the standard definitions used by various noted chemists:

In retrospect, the definition of chemistry seems to invariably change per decade, as new discoveries and theories add to the functionality of the science. Shown below are some of the standard definitions used by various noted chemists:

*'''Alchemy''' (330) – the study of the composition of waters, movement, growth, embodying and disembodying, drawing the spirits from bodies and bonding the spirits within bodies ([[Zosimos of Panopolis|Zosimos]]).<ref>Strathern, P. (2000). ''Mendeleyev’s Dream – the Quest for the Elements.'' New York: Berkley Books.</ref>

*'''Alchemy''' (330) – the study of the composition of waters, movement, growth, embodying and disembodying, drawing the spirits from bodies and bonding the spirits within bodies ([[Zosimos of Panopolis|Zosimos]]).

*'''Chymistry''' (1661) – the subject of the material principles of mixt bodies ([[Robert Boyle|Boyle]]).<ref>{{cite book| last=Boyle | first = Robert |title=The Sceptical Chymist|location=New York | publisher=Dover Publications, Inc. (reprint)|year=1661|id=ISBN 0486428257}}</ref>

*'''Chymistry''' (1661) – the subject of the material principles of mixt bodies ([[Robert Boyle|Boyle]]).ISBN 0486428257

*'''Chymistry''' (1663) – a scientific art, by which one learns to dissolve bodies, and draw from them the different substances on their composition, and how to unite them again, and exalt them to an higher perfection ([[Christopher Glaser|Glaser]]).<ref>{{cite book| last=Glaser | first = Christopher |title=Traite de la chymie|location=Paris | year=1663}} as found in: {{cite book | last = Kim | first = Mi Gyung | title = Affinity, That Elusive Dream - A Genealogy of the Chemical Revolution | publisher = The MIT Press | year = 2003 | id = ISBN 0-262-11273-6}}

*'''Chymistry''' (1663) – a scientific art, by which one learns to dissolve bodies, and draw from them the different substances on their composition, and how to unite them again, and exalt them to an higher perfection ([[Christopher Glaser|Glaser]]).ISBN 0-262-11273-6

 

*'''Chemistry''' (1730) – the art of resolving mixt, compound, or aggregate bodies into their principles; and of composing such bodies from those principles ([[Georg Ernst Stahl|Stahl]]).<ref>{{cite book| last=Stahl | first = George, E. |title=Philosophical Principles of Universal Chemistry|location=London | year=1730}}</ref>

*'''Chemistry''' (1730) – the art of resolving mixt, compound, or aggregate bodies into their principles; and of composing such bodies from those principles ([[Georg Ernst Stahl|Stahl]]).

*'''Chemistry''' (1837) – the science concerned with the laws and effects of molecular forces ([[Jean-Baptiste Dumas|Dumas]]).<ref>Dumas, J. B. (1837). 'Affinite' (lecture notes), vii, pg 4. “Statique chimique”, Paris: Academie des Sciences</ref>

*'''Chemistry''' (1837) – the science concerned with the laws and effects of molecular forces ([[Jean-Baptiste Dumas|Dumas]]).Dumas, J. B. (1837). 'Affinite' (lecture notes), vii, pg 4. “Statique chimique”, Paris: Academie des Sciences

*'''Chemistry''' (1947) – the science of substances: their structure, their properties, and the reactions that change them into other substances ([[Linus Pauling|Pauling]]).<ref>{{cite book | last = Pauling | first = Linus  | title = General Chemistry | publisher = Dover Publications, Inc. | year = 1947 | id = ISBN 0486656225}}</ref>

*'''Chemistry''' (1947) – the science of substances: their structure, their properties, and the reactions that change them into other substances ([[Linus Pauling|Pauling]]).ISBN 0486656225

*'''Chemistry''' (1998) – the study of matter and the changes it undergoes ([[Raymond Chang|Chang]]).<ref>{{cite book|author=Chang, Raymond |title=Chemistry, 6th Ed.|location=New York | publisher=McGraw Hill|year=1998|id=ISBN 0-07-115221-0}}</ref>

*'''Chemistry''' (1998) – the study of matter and the changes it undergoes ([[Raymond Chang|Chang]]). ISBN 0-07-115221-0

==Basic concepts==

==Basic concepts==

Several [[concepts]] are essential for the study of chemistry, some of them are:<ref>General Chemistry Online - Companion Notes: Matter [http://antoine.frostburg.edu/chem/senese/101/matter/]  

Several [[concepts]] are essential for the study of chemistry, some of them are:[http://antoine.frostburg.edu/chem/senese/101/matter/]  

===Atom===

===Atom===

The sequence of steps in which the reorganization of chemical bonds may be taking place in the course of a chemical reaction is called its [[Reaction mechanism|mechanism]]. A chemical reaction can be envisioned to take place in a number of steps, each of which may have a different speed. Many [[reaction intermediates]] with variable stability can thus be envisaged during the course of a reaction. Reaction mechanisms are proposed to explain the [[chemical kinetics|kinetics]] and the relative product mix of a reaction. Many [[chemists|physical chemists]] specialize in exploring and proposing the mechanisms of various chemical reactions. Several empirical rules, like the [[Woodward-Hoffmann rules]] often come handy while proposing a mechanism for a chemical reaction.

The sequence of steps in which the reorganization of chemical bonds may be taking place in the course of a chemical reaction is called its [[Reaction mechanism|mechanism]]. A chemical reaction can be envisioned to take place in a number of steps, each of which may have a different speed. Many [[reaction intermediates]] with variable stability can thus be envisaged during the course of a reaction. Reaction mechanisms are proposed to explain the [[chemical kinetics|kinetics]] and the relative product mix of a reaction. Many [[chemists|physical chemists]] specialize in exploring and proposing the mechanisms of various chemical reactions. Several empirical rules, like the [[Woodward-Hoffmann rules]] often come handy while proposing a mechanism for a chemical reaction.

A stricter definition is that "a chemical reaction is a process that results in the interconversion of chemical species".<ref>[[Gold Book]] [http://goldbook.iupac.org/C01033.html Chemical Reaction] IUPAC Goldbook</ref> Under this definition, a chemical reaction may be an [[elementary reaction]] or a [[stepwise reaction]]. An additional caveat is made, in that this definition includes cases where the [[conformer|interconversion of conformers]] is experimentally observable. Such detectable chemical reactions normally involve sets of molecular entities as indicated by this definition, but it is often conceptually convenient to use the term also for changes involving single molecular entities (i.e. 'microscopic chemical events').

A stricter definition is that "a chemical reaction is a process that results in the interconversion of chemical species".[http://goldbook.iupac.org/C01033.html] Under this definition, a chemical reaction may be an [[elementary reaction]] or a [[stepwise reaction]]. An additional caveat is made, in that this definition includes cases where the [[conformer|interconversion of conformers]] is experimentally observable. Such detectable chemical reactions normally involve sets of molecular entities as indicated by this definition, but it is often conceptually convenient to use the term also for changes involving single molecular entities (i.e. 'microscopic chemical events').

===Energy===

===Energy===

Chemical reactions are invariably not possible unless the reactants surmount an energy barrier known as the [[activation energy]]. The ''speed'' of a  chemical reaction (at given temperature T) is related to the activation energy E, by the Boltzmann's population factor <math>e^{-E/kT} </math> - that is the probability of molecule to have energy greater than or equal to E at the given temperature T. This exponential dependence of a reaction rate on temperature is known as the [[Arrhenius equation]].

Chemical reactions are invariably not possible unless the reactants surmount an energy barrier known as the [[activation energy]]. The ''speed'' of a  chemical reaction (at given temperature T) is related to the activation energy E, by the Boltzmann's population factor <math>e^{-E/kT} </math> - that is the probability of molecule to have energy greater than or equal to E at the given temperature T. This exponential dependence of a reaction rate on temperature is known as the [[Arrhenius equation]].

The activation energy necessary for a chemical reaction can be in the form of heat, light, [[electricity]] or mechanical [[force]] in the form of [[ultrasound]]. Reilly, Michael. (2007).

The activation energy necessary for a chemical reaction can be in the form of heat, light, [[electricity]] or mechanical [[force]] in the form of [[ultrasound]]. Reilly, Michael. (2007).

[http://www.newscientisttech.com/article/dn11427 Mechanical force induces chemical reaction], NewScientist.com news service, Reilly 

[http://www.newscientisttech.com/article/dn11427]

 

A related concept [[thermodynamic free energy|free energy]], which incorporates entropy considerations too, is a very useful means for predicting the feasibility of a reaction and determining the state of equilibrium of a chemical reaction, in [[chemical thermodynamics]]. A reaction is feasible only if the total change in the [[Gibbs free energy]] is negative, <math> \Delta G \le 0 \,</math>; if it is equal to zero the chemical reaction is said to be at [[chemical equilibrium|equilibrium]].

A related concept [[thermodynamic free energy|free energy]], which incorporates entropy considerations too, is a very useful means for predicting the feasibility of a reaction and determining the state of equilibrium of a chemical reaction, in [[chemical thermodynamics]]. A reaction is feasible only if the total change in the [[Gibbs free energy]] is negative, <math> \Delta G \le 0 \,</math>; if it is equal to zero the chemical reaction is said to be at [[chemical equilibrium|equilibrium]].

There are only a limited possible states of energy for electrons, atoms and molecules. These are determined by the rules of [[quantum mechanics]], which require [[quantization (physics)|quantization]] of energy of a bound system. The atoms/molecules in an higher energy state are said to be excited. The molecules/atoms of substance in an excited energy state are often much more reactive, that is amenable to chemical reactions.   

There are only a limited possible states of energy for electrons, atoms and molecules. These are determined by the rules of [[quantum mechanics]], which require [[quantization (physics)|quantization]] of energy of a bound system. The atoms/molecules in an higher energy state are said to be excited. The molecules/atoms of substance in an excited energy state are often much more reactive, that is amenable to chemical reactions.   

The phase of a substance is invariably determined by its energy and those of its surroundings. When the intermolecular forces of a substance are such that energy of the surroundings is not sufficient to overcome them, it occurs in a more ordered phase like liquid or solid as is the case with  water (H₂O), a liquid at room temperature because its molecules are bound by [[hydrogen bonds]].<ref>[http://www.chem4kids.com/files/matter_changes.html Changing States of Matter] - Chemforkids.com</ref> Whereas [[hydrogen sulfide]] (H₂S) is a gas at room temperature and standard pressure, as its molecules are bound by weaker [[Intermolecular force#Dipole-dipole interactions|dipole-dipole interactions]].  

The phase of a substance is invariably determined by its energy and those of its surroundings. When the intermolecular forces of a substance are such that energy of the surroundings is not sufficient to overcome them, it occurs in a more ordered phase like liquid or solid as is the case with  water (H₂O), a liquid at room temperature because its molecules are bound by [[hydrogen bonds]].[http://www.chem4kids.com/files/matter_changes.html] - Chemforkids.com Whereas [[hydrogen sulfide]] (H₂S) is a gas at room temperature and standard pressure, as its molecules are bound by weaker [[Intermolecular force#Dipole-dipole interactions|dipole-dipole interactions]].  

The transfer of energy from one chemical substance to other depend on the ''size'' of energy [[quanta]] emitted from one substance. However, heat energy is easily transferred from almost any substance to another mainly because the vibrational and rotational energy levels in a substance are very closely placed. Because, the electronic energy levels are not so closely spaced, ultraviolet electromagnetic radiation is not transferred with equal felicity, as is also the case with electrical energy.

The transfer of energy from one chemical substance to other depend on the ''size'' of energy [[quanta]] emitted from one substance. However, heat energy is easily transferred from almost any substance to another mainly because the vibrational and rotational energy levels in a substance are very closely placed. Because, the electronic energy levels are not so closely spaced, ultraviolet electromagnetic radiation is not transferred with equal felicity, as is also the case with electrical energy.

* [[Gay-Lussac's law]] (1809, relating pressure and temperature)

* [[Gay-Lussac's law]] (1809, relating pressure and temperature)

* [[Avogadro's law]]

* [[Avogadro's law]]

 

==Subdisciplines==

==Subdisciplines==

==Chemical industry==

==Chemical industry==

The [[chemical industry]] represents an important economic activity. The global top 50 chemical producers in 2004 had sales of 587 billion [[US dollars]] with a profit margin of 8.1% and [[research and development]] spending of 2.1% of total chemical sales.<ref>{{cite journal | title = Top 50 Chemical Producers | journal = Chemical & Engineering News | date = [[July 18]], [[2005]]  [http://pubs.acs.org/cen/coverstory/83/8329globaltop50.html}}]

The [[chemical industry]] represents an important economic activity. The global top 50 chemical producers in 2004 had sales of 587 billion [[US dollars]] with a profit margin of 8.1% and [[research and development]] spending of 2.1% of total chemical sales.[http://pubs.acs.org/cen/coverstory/83/8329globaltop50.html]

==Further reading==

==Further reading==