Chemical substance

A chemical substance is a form of matter having constant chemical composition and characteristic properties.[1][2] It cannot be separated into components by physical separation methods, i.e., without breaking chemical bonds.[3] Chemical substances can be simple substances[4], chemical compounds, or alloys. Chemical elements may or may not be included in the definition, depending on expert viewpoint.[4]

Chemical substances are often called 'pure' to set them apart from mixtures. A common example of a chemical substance is pure water; it has the same properties and the same ratio of hydrogen to oxygen whether it is isolated from a river or made in a laboratory. Other chemical substances commonly encountered in pure form are diamond (carbon), gold, table salt (sodium chloride) and refined sugar (sucrose). However, in practice, no substance is entirely pure, and chemical purity is specified according to the intended use of the chemical.

Chemical substances exist as solids, liquids, gases, or plasma, and may change between these phases of matter with changes in temperature or pressure. Chemical substances may be combined or converted to others by means of chemical reactions.

Forms of energy, such as light and heat, are not matter, and are thus not "substances" in this regard.

Kochendes wasser02
Steam and liquid water are two different forms of the same chemical substance, water.

Definition

Nile red 01
Colors of a single chemical (Nile red) in different solvents, under visible and UV light, showing how the chemical interacts dynamically with its solvent environment.

A chemical substance may well be defined as "any material with a definite chemical composition" in an introductory general chemistry textbook.[5] According to this definition a chemical substance can either be a pure chemical element or a pure chemical compound. But, there are exceptions to this definition; a pure substance can also be defined as a form of matter that has both definite composition and distinct properties.[6] The chemical substance index published by CAS also includes several alloys of uncertain composition.[7] Non-stoichiometric compounds are a special case (in inorganic chemistry) that violates the law of constant composition, and for them, it is sometimes difficult to draw the line between a mixture and a compound, as in the case of palladium hydride. Broader definitions of chemicals or chemical substances can be found, for example: "the term 'chemical substance' means any organic or inorganic substance of a particular molecular identity, including – (i) any combination of such substances occurring in whole or in part as a result of a chemical reaction or occurring in nature".[8]

In geology, substances of uniform composition are called minerals, while physical mixtures (aggregates) of several minerals (different substances) are defined as rocks. Many minerals, however, mutually dissolve into solid solutions, such that a single rock is a uniform substance despite being a mixture in stoichiometric terms. Feldspars are a common example: anorthoclase is an alkali aluminum silicate, where the alkali metal is interchangeably either sodium or potassium.

In law, "chemical substances" may include both pure substances and mixtures with a defined composition or manufacturing process. For example, the EU regulation REACH defines "monoconstituent substances", "multiconstituent substances" and "substances of unknown or variable composition". The latter two consist of multiple chemical substances; however, their identity can be established either by direct chemical analysis or reference to a single manufacturing process. For example, charcoal is an extremely complex, partially polymeric mixture that can be defined by its manufacturing process. Therefore, although the exact chemical identity is unknown, identification can be made to a sufficient accuracy. The CAS index also includes mixtures.

Polymers almost always appear as mixtures of molecules of multiple molar masses, each of which could be considered a separate chemical substance. However, the polymer may be defined by a known precursor or reaction(s) and the molar mass distribution. For example, polyethylene is a mixture of very long chains of -CH2- repeating units, and is generally sold in several molar mass distributions, LDPE, MDPE, HDPE and UHMWPE.

History

The concept of a "chemical substance" became firmly established in the late eighteenth century after work by the chemist Joseph Proust on the composition of some pure chemical compounds such as basic copper carbonate.[9] He deduced that, "All samples of a compound have the same composition; that is, all samples have the same proportions, by mass, of the elements present in the compound." This is now known as the law of constant composition.[10] Later with the advancement of methods for chemical synthesis particularly in the realm of organic chemistry; the discovery of many more chemical elements and new techniques in the realm of analytical chemistry used for isolation and purification of elements and compounds from chemicals that led to the establishment of modern chemistry, the concept was defined as is found in most chemistry textbooks. However, there are some controversies regarding this definition mainly because the large number of chemical substances reported in chemistry literature need to be indexed.

Isomerism caused much consternation to early researchers, since isomers have exactly the same composition, but differ in configuration (arrangement) of the atoms. For example, there was much speculation for the chemical identity of benzene, until the correct structure was described by Friedrich August Kekulé. Likewise, the idea of stereoisomerism – that atoms have rigid three-dimensional structure and can thus form isomers that differ only in their three-dimensional arrangement – was another crucial step in understanding the concept of distinct chemical substances. For example, tartaric acid has three distinct isomers, a pair of diastereomers with one diastereomer forming two enantiomers.

Chemical elements

Schwefel 01
Native sulfur crystals. Sulfur occurs naturally as elemental sulfur, in sulfide and sulfate minerals and in hydrogen sulfide.

An element is a chemical substance made up of a particular kind of atom and hence cannot be broken down or transformed by a chemical reaction into a different element, though it can be transmuted into another element through a nuclear reaction. This is so, because all of the atoms in a sample of an element have the same number of protons, though they may be different isotopes, with differing numbers of neutrons.

As of 2012, there are 118 known elements, about 80 of which are stable – that is, they do not change by radioactive decay into other elements. Some elements can occur as more than a single chemical substance (allotropes). For instance, oxygen exists as both diatomic oxygen (O2) and ozone (O3). The majority of elements are classified as metals. These are elements with a characteristic lustre such as iron, copper, and gold. Metals typically conduct electricity and heat well, and they are malleable and ductile.[11] Around a dozen elements,[12] such as carbon, nitrogen, and oxygen, are classified as non-metals. Non-metals lack the metallic properties described above, they also have a high electronegativity and a tendency to form negative ions. Certain elements such as silicon sometimes resemble metals and sometimes resemble non-metals, and are known as metalloids.

Chemical compounds

Potassium-ferricyanide-sample
Potassium ferricyanide is a compound of potassium, iron, carbon and nitrogen; although it contains cyanide anions, it does not release them and is nontoxic.

A pure chemical compound is a chemical substance that is composed of a particular set of molecules or ions. Two or more elements combined into one substance through a chemical reaction form a chemical compound. All compounds are substances, but not all substances are compounds.

A chemical compound can be either atoms bonded together in molecules or crystals in which atoms, molecules or ions form a crystalline lattice. Compounds based primarily on carbon and hydrogen atoms are called organic compounds, and all others are called inorganic compounds. Compounds containing bonds between carbon and a metal are called organometallic compounds.

Compounds in which components share electrons are known as covalent compounds. Compounds consisting of oppositely charged ions are known as ionic compounds, or salts.

In organic chemistry, there can be more than one chemical compound with the same composition and molecular weight. Generally, these are called isomers. Isomers usually have substantially different chemical properties, and often may be isolated without spontaneously interconverting. A common example is glucose vs. fructose. The former is an aldehyde, the latter is a ketone. Their interconversion requires either enzymatic or acid-base catalysis.

However, tautomers are an exception: the isomerization occurs spontaneously in ordinary conditions, such that a pure substance cannot be isolated into its tautomers, even if these can be identified spectroscopically or even isolated in special conditions. A common example is glucose, which has open-chain and ring forms. One cannot manufacture pure open-chain glucose because glucose spontaneously cyclizes to the hemiacetal form.

Substances versus mixtures

Vintage cranberry glass
Cranberry glass, while it looks homogeneous, is a mixture consisting of glass and gold colloidal particles of ca. 40 nm diameter, which give it a red color.

All matter consists of various elements and chemical compounds, but these are often intimately mixed together. Mixtures contain more than one chemical substance, and they do not have a fixed composition. In principle, they can be separated into the component substances by purely mechanical processes. Butter, soil and wood are common examples of mixtures.

Grey iron metal and yellow sulfur are both chemical elements, and they can be mixed together in any ratio to form a yellow-grey mixture. No chemical process occurs, and the material can be identified as a mixture by the fact that the sulfur and the iron can be separated by a mechanical process, such as using a magnet to attract the iron away from the sulfur.

In contrast, if iron and sulfur are heated together in a certain ratio (1 atom of iron for each atom of sulfur, or by weight, 56 grams (1 mol) of iron to 32 grams (1 mol) of sulfur), a chemical reaction takes place and a new substance is formed, the compound iron(II) sulfide, with chemical formula FeS. The resulting compound has all the properties of a chemical substance and is not a mixture. Iron(II) sulfide has its own distinct properties such as melting point and solubility, and the two elements cannot be separated using normal mechanical processes; a magnet will be unable to recover the iron, since there is no metallic iron present in the compound.

Chemicals versus chemical substances

While the term chemical substance is a precise technical term that is synonymous with chemical for chemists, the word chemical is used in general usage in the English speaking world to refer to both (pure) chemical substances and mixtures (often called compounds),[13] and especially when produced or purified in a laboratory or an industrial process.[14][15][16] In other words, the chemical substances of which fruits and vegetables, for example, are naturally composed even when growing wild are not called "chemicals" in general usage. In countries that require a list of ingredients in products, the "chemicals" listed are industrially produced "chemical substances". The word "chemical" is also often used to refer to addictive, narcotic, or mind-altering drugs.[14][15]

Within the chemical industry, manufactured "chemicals" are chemical substances, which can be classified by production volume into bulk chemicals, fine chemicals and chemicals found in research only:

  • Bulk chemicals are produced in very large quantities, usually with highly optimized continuous processes and to a relatively low price.
  • Fine chemicals are produced at a high cost in small quantities for special low-volume applications such as biocides, pharmaceuticals and speciality chemicals for technical applications.
  • Research chemicals are produced individually for research, such as when searching for synthetic routes or screening substances for pharmaceutical activity. In effect, their price per gram is very high, although they are not sold.

The cause of the difference in production volume is the complexity of the molecular structure of the chemical. Bulk chemicals are usually much less complex. While fine chemicals may be more complex, many of them are simple enough to be sold as "building blocks" in the synthesis of more complex molecules targeted for single use, as named above. The production of a chemical includes not only its synthesis but also its purification to eliminate by-products and impurities involved in the synthesis. The last step in production should be the analysis of batch lots of chemicals in order to identify and quantify the percentages of impurities for the buyer of the chemicals. The required purity and analysis depends on the application, but higher tolerance of impurities is usually expected in the production of bulk chemicals. Thus, the user of the chemical in the US might choose between the bulk or "technical grade" with higher amounts of impurities or a much purer "pharmaceutical grade" (labeled "USP", United States Pharmacopeia). "Chemicals" in the commercial and legal sense may also include mixtures of highly variable composition, as they are products made to a technical specification instead of particular chemical substances. For example, gasoline is not a single chemical compound or even a particular mixture: different gasolines can have very different chemical compositions, as "gasoline" is primarily defined through source, properties and octane rating.

Naming and indexing

Every chemical substance has one or more systematic names, usually named according to the IUPAC rules for naming. An alternative system is used by the Chemical Abstracts Service (CAS).

Many compounds are also known by their more common, simpler names, many of which predate the systematic name. For example, the long-known sugar glucose is now systematically named 6-(hydroxymethyl)oxane-2,3,4,5-tetrol. Natural products and pharmaceuticals are also given simpler names, for example the mild pain-killer Naproxen is the more common name for the chemical compound (S)-6-methoxy-α-methyl-2-naphthaleneacetic acid.

Chemists frequently refer to chemical compounds using chemical formulae or molecular structure of the compound. There has been a phenomenal growth in the number of chemical compounds being synthesized (or isolated), and then reported in the scientific literature by professional chemists around the world.[17] An enormous number of chemical compounds are possible through the chemical combination of the known chemical elements. As of May 2011, about sixty million chemical compounds are known.[18] The names of many of these compounds are often nontrivial and hence not very easy to remember or cite accurately. Also it is difficult to keep the track of them in the literature. Several international organizations like IUPAC and CAS have initiated steps to make such tasks easier. CAS provides the abstracting services of the chemical literature, and provides a numerical identifier, known as CAS registry number to each chemical substance that has been reported in the chemical literature (such as chemistry journals and patents). This information is compiled as a database and is popularly known as the Chemical substances index. Other computer-friendly systems that have been developed for substance information, are: SMILES and the International Chemical Identifier or InChI.

Identification of a typical chemical substance
Common name Systematic name Chemical formula Chemical structure CAS registry number InChI
Alcohol, or
ethyl alcohol
Ethanol C2H5OH
Ethanol-2D-skeletal
[64-17-5] 1/C2H6O/c1-2-3/h3H,2H2,1H3

Isolation, purification, characterization, and identification

Often a pure substance needs to be isolated from a mixture, for example from a natural source (where a sample often contains numerous chemical substances) or after a chemical reaction (which often give mixtures of chemical substances).

See also

Notes and references

  1. ^ Hale, Bob (2013-09-19). Necessary Beings: An Essay on Ontology, Modality, and the Relations Between Them. OUP Oxford. ISBN 9780191648342. Archived from the original on 2018-01-13.
  2. ^ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version:  (2006–) "Chemical Substance". doi:10.1351/goldbook.C01039
  3. ^ Hunter, Lawrence E. (2012-01-13). The Processes of Life: An Introduction to Molecular Biology. MIT Press. ISBN 9780262299947. Archived from the original on 2018-01-13.
  4. ^ a b Scerri, Eric (2005). "Simples and Compounds". www.iupac.org. Retrieved 15 May 2018.
  5. ^ Hill, J. W.; Petrucci, R. H.; McCreary, T. W.; Perry, S. S. General Chemistry, 4th ed., p5, Pearson Prentice Hall, Upper Saddle River, New Jersey, 2005
  6. ^ "Pure Substance – DiracDelta Science & Engineering Encyclopedia". Diracdelta.co.uk. Archived from the original on 2013-05-11. Retrieved 2013-06-06.
  7. ^ Appendix IV: Chemical Substance Index Names Archived 2007-12-03 at the Wayback Machine
  8. ^ "What is the TSCA Chemical Substance Inventory?". US Environmental Protection Agency. Archived from the original on 2009-06-05. Retrieved 2009-10-19.
  9. ^ Hill, J. W.; Petrucci, R. H.; McCreary, T. W.; Perry, S. S. General Chemistry, 4th ed., p37, Pearson Prentice Hall, Upper Saddle River, New Jersey, 2005.
  10. ^ Law of Definite Proportions Archived November 18, 2007, at the Wayback Machine
  11. ^ Hill, J. W.; Petrucci, R. H.; McCreary, T. W.; Perry, S. S. General Chemistry, 4th ed., pp 45–46, Pearson Prentice Hall, Upper Saddle River, New Jersey, 2005.
  12. ^ The boundary between metalloids and non-metals is imprecise, as explained in the previous reference.
  13. ^ compound Archived 2017-11-07 at the Wayback Machine in Oxford Online Dictionaries
  14. ^ a b chemical Archived 2017-11-07 at the Wayback Machine in Oxford Online Dictionaries
  15. ^ a b Random House Unabridged Dictionary Archived 2017-11-07 at the Wayback Machine, 1997
  16. ^ "What is a chemical". Nicnas.gov.au. 2005-06-01. Archived from the original on 2013-06-16. Retrieved 2013-06-06.
  17. ^ Joachim Schummer. "Coping with the Growth of Chemical Knowledge: Challenges for Chemistry Documentation, Education, and Working Chemists". Rz.uni-karlsruhe.de. Archived from the original on 2013-09-17. Retrieved 2013-06-06.
  18. ^ "Chemical Abstracts substance count". Cas.org. Archived from the original on 2012-05-11. Retrieved 2013-06-06.

External links

Biogeochemical cycle

In ecology and Earth science, a biogeochemical cycle or substance turnover or cycling of substances is a pathway by which a chemical substance moves through biotic (biosphere) and abiotic (lithosphere, atmosphere, and hydrosphere) compartments of Earth. There are biogeochemical cycles for the chemical elements calcium, carbon, hydrogen, mercury, nitrogen, oxygen, phosphorus, selenium, and sulfur; molecular cycles for water and silica; macroscopic cycles such as the rock cycle; as well as human-induced cycles for synthetic compounds such as polychlorinated biphenyl (PCB). In some cycles there are reservoirs where a substance remains for a long period of time (such as an ocean or lake for water).

CAS Registry Number

A CAS Registry Number, also referred to as CASRN or CAS Number, is a unique numerical identifier assigned by the Chemical Abstracts Service (CAS) to every chemical substance described in the open scientific literature (currently including all substances described from 1957 through the present, plus some substances from the early or mid 1900s), including organic and inorganic compounds, minerals, isotopes, alloys and nonstructurable materials (UVCBs, of unknown, variable composition, or biological origin).The registry maintained by CAS is an authoritative collection of disclosed chemical substance information. It currently identifies more than 144 million unique organic and inorganic substances and 67 million protein and DNA sequences, plus additional information about each substance. It is updated with around 15,000 additional new substances daily.

Chemical compound

A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) composed of atoms from more than one element held together by chemical bonds. A chemical element bonded to an identical chemical element is not a chemical compound since only one element, not two different elements, is involved.

There are four types of compounds, depending on how the constituent atoms are held together:

molecules held together by covalent bonds

ionic compounds held together by ionic bonds

intermetallic compounds held together by metallic bonds

certain complexes held together by coordinate covalent bonds.A chemical formula is a way of expressing information about the proportions of atoms that constitute a particular chemical compound, using the standard abbreviations for the chemical elements, and subscripts to indicate the number of atoms involved. For example, water is composed of two hydrogen atoms bonded to one oxygen atom: the chemical formula is H2O. Many chemical compounds have a unique numerical identifier assigned by the Chemical Abstracts Service (CAS): its CAS number.

A compound can be converted to a different chemical composition by interaction with a second chemical compound via a chemical reaction. In this process, bonds between atoms are broken in both of the interacting compounds, and then bonds are reformed so that new associations are made between atoms.

Chemical energy

Chemical energy is the potential of a chemical substance to undergo a transformation through a chemical reaction to transform other chemical substances. Examples include batteries, food, gasoline, and etc. Breaking or making of chemical bonds involves energy, which may be either absorbed or evolved from a chemical system.

Energy that can be released or absorbed because of a reaction between a set of chemical substances is equal to the difference between the energy content of the products and the reactants, if the initial and final temperatures are the same. This change in energy can be estimated from the bond energies of the various chemical bonds in the reactants and products. It can also be calculated from , the internal energy of formation of the reactant molecules, and the internal energy of formation of the product molecules. The internal energy change of a chemical process is equal to the heat exchanged if it is measured under conditions of constant volume and equal initial and final temperature, as in a closed container such as a bomb calorimeter. However, under conditions of constant pressure, as in reactions in vessels open to the atmosphere, the measured heat change is not always equal to the internal energy change, because pressure-volume work also releases or absorbs energy. (The heat change at constant pressure is called the enthalpy change; in this case the enthalpy of reaction, if initial and final temperatures are equal).

Another useful term is the heat of combustion, which is the energy mostly of the weak double bonds of molecular oxygen released due to a combustion reaction and often applied in the study of fuels. Food is similar to hydrocarbon and carbohydrate fuels, and when it is oxidized to carbon dioxide and water, the energy released is analogous to the heat of combustion (though not assessed in the same way as a hydrocarbon fuel — see food energy).

Chemical potential energy is a form of potential energy related to the structural arrangement of atoms or molecules. This arrangement may be the result of chemical bonds within a molecule or otherwise. Chemical energy of a chemical substance can be transformed to other forms of energy by a chemical reaction. As an example, when a fuel is burned the chemical energy of molecular oxygen is converted to heat, and the same is the case with digestion of food metabolized in a biological organism. Green plants transform solar energy to chemical energy (mostly of oxygen) through the process known as photosynthesis, and electrical energy can be converted to chemical energy and vice versa through electrochemical reactions.

The similar term chemical potential is used to indicate the potential of a substance to undergo a change of configuration, be it in the form of a chemical reaction, spatial transport, particle exchange with a reservoir, etc. It is not a form of potential energy itself, but is more closely related to free energy. The confusion in terminology arises from the fact that in other areas of physics not dominated by entropy, all potential energy is available to do useful work and drives the system to spontaneously undergo changes of configuration, and thus there is no distinction between "free" and "non-free" potential energy (hence the one word "potential"). However, in systems of large entropy such as chemical systems, the total amount of energy present (and conserved by the first law of thermodynamics) of which this Chemical Potential Energy is a part, is separated from the amount of that energy—Thermodynamic Free Energy (which Chemical potential is derived from)—which (appears to) drive the system forward spontaneously as its entropy increases (in accordance with the second law).

Chemical species

A chemical species is a chemical substance or ensemble composed of chemically identical molecular entities that can explore the same set of molecular energy levels on a characteristic or delineated time scale. The term is applied equally to a set of chemically identical atomic or molecular structural units in a solid array.In supramolecular chemistry, chemical species are those supramolecular structures whose interactions and associations are brought about via intermolecular bonding and debonding actions, and function to form the basis of this branch of chemistry.

Clathrate compound

A clathrate is a chemical substance consisting of a lattice that traps or contains molecules. The word clathrate is derived from the Latin clatratus meaning with bars or a lattice. Traditionally, clathrate compounds are polymeric and completely envelop the guest molecule, but in modern usage clathrates also include host–guest complexes and inclusion compounds. According to IUPAC, clathrates are "Inclusion compounds in which the guest molecule is in a cage formed by the host molecule or by a lattice of host molecules."

Drug

A drug is any substance (other than food that provides nutritional support) that, when inhaled, injected, smoked, consumed, absorbed via a patch on the skin, or dissolved under the tongue causes a physiological (and often psychological) change in the body.In pharmacology, a drug is a chemical substance of known structure, other than a nutrient of an essential dietary ingredient, which, when administered to a living organism, produces a biological effect. A pharmaceutical drug, also called a medication or medicine, is a chemical substance used to treat, cure, prevent, or diagnose a disease or to promote well-being. Traditionally drugs were obtained through extraction from medicinal plants, but more recently also by organic synthesis. Pharmaceutical drugs may be used for a limited duration, or on a regular basis for chronic disorders.Pharmaceutical drugs are often classified into drug classes—groups of related drugs that have similar chemical structures, the same mechanism of action (binding to the same biological target), a related mode of action, and that are used to treat the same disease. The Anatomical Therapeutic Chemical Classification System (ATC), the most widely used drug classification system, assigns drugs a unique ATC code, which is an alphanumeric code that assigns it to specific drug classes within the ATC system. Another major classification system is the Biopharmaceutics Classification System. This classifies drugs according to their solubility and permeability or absorption properties.Psychoactive drugs are chemical substances that affect the function of the central nervous system, altering perception, mood or consciousness. They include alcohol, a depressant (and a stimulant in small quantities), and the stimulants nicotine and caffeine. These three are the most widely consumed psychoactive drugs worldwide and are also considered recreational drugs since they are used for pleasure rather than medicinal purposes. Other recreational drugs include hallucinogens, opiates and amphetamines and some of these are also used in spiritual or religious settings. Some drugs can cause addiction and all drugs can have side effects. Excessive use of stimulants can promote stimulant psychosis. Many recreational drugs are illicit and international treaties such as the Single Convention on Narcotic Drugs exist for the purpose of their prohibition.

Iodobenzamide

Iodobenzamide (IBZM or iolopride) is a chemical substance. Pharmaceutically it is a dopamine antagonist and it can be used by nuclear medicine physicians as a radioactive tracer for SPECT where the radioactive isotope is iodine-123 or iodine-125. The main purpose of a brain study with IBZM is the differentiation of Parkinson's disease from other neurodegenerative diseases such as Lewy Body dementia and multiple system atrophy.

Material

A material is a chemical substance or mixture of substances that constitute an object. Materials can be pure or impure, a singular composite or a complex mix, living or non-living matter, whether natural or man-made, either concrete or abstract. Materials can be classified based on different properties such as physical and chemical properties (see List of materials properties), geological, biological, choreographical, or philosophical properties. In the physical sense, materials are studied in the field of materials science.

In industry, materials are inputs to production or manufacturing processes. They may either be raw material, that is, unprocessed, or processed before being used in more advanced production processes, either by distillation or synthesis (synthetic materials).

Types of materials include:

Biomaterial, of biological origin

Composite material, composed of multiple materials with differing physical properties

Textiles, sometimes referred to as "material"

Genetic material

Permissible exposure limit

The permissible exposure limit (PEL or OSHA PEL) is a legal limit in the United States for exposure of an employee to a chemical substance or physical agent such as high level noise. Permissible exposure limits are established by the Occupational Safety and Health Administration (OSHA). Most of OSHA’s PELs were issued shortly after adoption of the Occupational Safety and Health (OSH) Act in 1970.For chemicals, the chemical regulation is usually expressed in parts per million (ppm), or sometimes in milligrams per cubic meter (mg/m3). Units of measure for physical agents such as noise are specific to the agent.

A PEL is usually given as a time-weighted average (TWA), although some are short-term exposure limits (STEL) or ceiling limits. A TWA is the average exposure over a specified period, usually a nominal eight hours. This means that, for limited periods, a worker may be exposed to concentration excursions higher than the PEL, so long as the TWA is not exceeded and any applicable excursion limit is not exceeded. An excursion limit typically means that "...worker exposure levels may exceed 3 times the PEL-TWA for no more than a total of 30 minutes during a workday, and under no circumstances should they exceed 5 times the PEL-TWA, provided that

the PEL-TWA is not exceeded." Excursion limits are enforced in some states (for example Oregon) and on the federal level for certain contaminants such as asbestos.

A short-term exposure limit is one that addresses the average exposure over a 15-30 minute period of maximum exposure during a single work shift. A ceiling limit is one that may not be exceeded for any time, and is applied to irritants and other materials that have immediate effects.

Physical change

Physical changes are changes affecting the form of a chemical substance, but not its chemical composition. Physical changes are used to separate mixtures into their component compounds, but can not usually be used to separate compounds into chemical elements or simpler compounds.Physical changes occur when objects or substances undergo a change that does not change their chemical composition. This contrasts with the concept of chemical change in which the composition of a substance changes or one or more substances combine or break up to form new substances. In general a physical change is reversible using physical means. For example, salt dissolved in water can be recovered by allowing the water to evaporate.

A physical change involves a change in physical properties. Examples of physical properties include melting, transition to a gas, change of strength, change of durability, changes to crystal form, textural change, shape, size, color, volume and density.

An example of a physical change is the process of tempering steel to form a knife blade. A steel blank is repeatedly heated and hammered which changes the hardness of the steel, its flexibility and its ability to maintain a sharp edge.

Many physical changes also involve the rearrangement of atoms most noticeably in the formation of crystals. Many chemical changes are irreversible, and many physical changes are reversible, but reversibility is not a certain criterion for classification. Although chemical changes may be recognized by an indication such as odor, color change, or production of a gas, every one of these indicators can result from physical change.

Piscicide

A piscicide is a chemical substance which is poisonous to fish. The primary use for piscicides is to eliminate a dominant species of fish in a body of water, as the first step in attempting to populate the body of water with a different fish. They are also used to combat parasitic and invasive species of fish.

Examples of piscicides include rotenone, saponins, TFM, niclosamide and Antimycin A (Fintrol).

Preferred IUPAC name

In chemical nomenclature, a preferred IUPAC name (PIN) is a unique name, assigned to a chemical substance and preferred among the possible names generated by IUPAC nomenclature. The "preferred IUPAC nomenclature" provides a set of rules for choosing between multiple possibilities in situations where it is important to decide on a unique name. It is intended for use in legal and regulatory situations.Currently, preferred IUPAC names are written only for part of the organic compounds (see below). Rules for the remaining organic and inorganic compounds are still under development.

The "Preferred names in the nomenclature of organic compounds" (Draft 7 October 2004) replace two former publications: the "Nomenclature of Organic Chemistry", 1979 (the Blue Book) and "A Guide to IUPAC Nomenclature of Organic Compounds, Recommendations 1993". As of May 2010, these draft recommendations have yet to gain formal approval.

Protonation

In chemistry, protonation is the addition of a proton (H+) to an atom, molecule, or ion, forming the conjugate acid. Some examples include

the protonation of water by sulfuric acid:

H2SO4 + H2O ⇌ H3O+ + HSO−4

the protonation of isobutene in the formation of a carbocation:

(CH3)2C=CH2 + HBF4 ⇌ (CH3)3C+ + BF−4

the protonation of ammonia in the formation of ammonium chloride from ammonia and hydrogen chloride:

NH3(g) + HCl(g) → NH4Cl(s)Protonation is a fundamental chemical reaction and is a step in many stoichiometric and catalytic processes. Some ions and molecules can undergo more than one protonation and are labeled polybasic, which is true of many biological macromolecules. Protonation and deprotonation (removal of a proton) occur in most acid-base reactions; they are the core of most acid-base reaction theories. A Brønsted–Lowry acid is defined as a chemical substance that protonates another substance. Upon protonating a substrate, the mass and the charge of the species each increase by one unit, making it an essential step in certain analytical procedures such as electrospray mass spectrometry. Protonating or deprotonating a molecule or ion can change many other chemical properties, not just the charge and mass, for example hydrophilicity, reduction potential, and optical properties can change.

Reactivity

Reactivity may refer to:

Reactivity (chemistry), the rate at which a chemical substance tends to undergo a chemical reaction

Reactive programming, a property of an execution model whereby changes are automatically propagated through a dataflow network

Reactivity (psychology)

Reactivity (electronics)

Reactivity (chemistry)

In chemistry, reactivity is the impetus for which a chemical substance undergoes a chemical reaction, either by itself or with other materials, with an overall release of energy.

Reactivity refers to:

the chemical reactions of a single substance,

the chemical reactions of two or more substances that interact with each other,

the systematic study of sets of reactions of these two kinds,

methodology that applies to the study of reactivity of chemicals of all kinds,

experimental methods that are used to observe these processes

theories to predict and to account for these processes.The chemical reactivity of a single substance (reactant) covers its behavior in which it:

Decomposes

Forms new substances by addition of atoms from another reactant or reactants

Interacts with two or more other reactants to form two or more productsThe chemical reactivity of a substance can refer to the variety of circumstances (conditions that include temperature, pressure, presence of catalysts) in which it reacts, in combination with the:

Variety of substances with which it reacts

Equilibrium point of the reaction (i.e., the extent to which all of it reacts)

Rate of the reactionThe term reactivity is related to the concepts of chemical stability and chemical compatibility.

Silicon monohydride

Silicon monohydride is a chemical substance occurring as a molecule found in stars and probably existing in interstellar space, or as a monolayer on the surface of solid silicon. The SiH molecule is a radical, and can be made experimentally by striking an electric arc to silicon on a low pressure hydrogen gas.

Substance

Substance may refer to:

Chemical substance, a material with a definite chemical composition

Importance or depth

Matter, anything that has mass and takes up space

Substance theory, an ontological theory positing that a substance is distinct from its properties

Systematic name

A systematic name is a name given in a systematic way to one unique group, organism, object or chemical substance, out of a specific population or collection. Systematic names are usually part of a nomenclature.

A semisystematic name or semitrivial name is a name that has at least one systematic part and at least one trivial part.

Creating systematic names can be as simple as assigning a prefix or a number to each object (in which case they are a type of numbering scheme), or as complex as encoding the complete structure of the object in the name. Many systems combine some information about the named object with an extra sequence number to make it into a unique identifier.

Systematic names often co-exist with earlier common names assigned before the creation of any systematic naming system. For example, many common chemicals are still referred to by their common or trivial names, even by chemists.

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