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A chemical substance is a material with a definite chemical composition. It is a 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. 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 . A pure chemical substance cannot be separated into other substances by a process that does not involve any Chemical Reaction and is rarely found in nature. Some typical chemical substances can be Diamond , Gold , Salt ( Sodium Chloride ) and Sugar ( Sucrose ). Generally, chemical substances exist as '' Solid '', '' Liquid '', or '' Gas '', and may change between these '' Phases Of Matter '' with changes in Temperature or Pressure . DEFINING Chemical substances (also sometimes referred to as a pure substances) are often defined as ''any material with a definite chemical composition'' in most introductory general chemistry textbooks. 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 According to this definition a chemical substance can either be a pure chemical element or a pure chemical compound. However, there are exceptions to this definition, a pure substance can also be defined as a form of also includes several Alloys of uncertain composition. http://www.cas.org/ASSETS/58D34DD3892142D18F5C3B0A004D3A0C/indexguideapp.pdf ELEMENTS See Also: Chemical element An Element is a chemical substance that is made up of a particular kind of atoms and hence cannot be broken down or transformed by a chemical reaction into a different element, though it can be transmutated 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 '' Isotope s'', with differing numbers of neutrons. There are about 120 known elements, about 80 of which are stable, that is, they do not change by '' such as Iron , Copper , and Gold . Metals typically conduct electricity and heat well, and they are Malleable and Ductile . Hill, J. W.; Petrucci, R. H.; McCreary, T. W.; Perry, S. S. ''General Chemistry'', 4th ed., pp45-46, Pearson Prentice Hall, Upper Saddle River, New Jersey, 2005. Around a dozen elementsThe boundary between metalloids and non-metals is imprecise, as explained in the previous reference. such as Carbon , Nitrogen , and Oxygen are classified as '' Non-metal s''. Non-metals lack the metallic properties described above, they also have a high Electronegativity and a tendency to form Negative Ions Called Anions . Certain elements such as Silicon sometimes resemble metals and sometimes resemble non-metals, and are known as '' Metalloid s''. See Also: List of elements by name CHEMICAL COMPOUNDS See Also: Chemical compound A pure chemical compound is a chemical substance that is composed of a particular Set of Molecule s or Ion s. Two or more elements combined into one substance, through a Chemical Reaction , form what is called a chemical compound. A chemical compound can be either atoms '' Bonded '' together in '' Molecule s'' or Crystal s in which atoms, molecules or ions form a crystalline Lattice . Compounds based primarily on carbon and hydrogen atoms are called '' Organic Compound s'', and all others are called '' Inorganic Compound s''. Compounds containing bonds between carbon and a Metal are called Organometallic Compound s. Compounds in which components share electrons are known as Covalent compounds. Compounds consisting of oppositely charged Ion s are known as '' Ionic compounds'', or Salt s. In organic chemistry, there can be more than one chemical compound with the same composition and molecular weight. Generally, these are called Isomer s. Isomers usually have substantially different chemical properties, may be isolated and do not spontaneously convert to each other. 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, there are also Tautomer s, where isomerization occurs spontaneously, such that a pure substance cannot be isolated into its tautomers. 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. See Also: list of organic compounds See Also: list of inorganic compounds SUBSTANCES VERSUS MIXTURES See Also: Mixture 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 (56 Gram s (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 somewhat technical term used most often by professional chemists, the word chemical What is a chemical is more widely used in the . NAMING AND INDEXING Every chemical substance has one or more Systematic Name s, 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 Pharmaceutical s 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. and the International Chemical Identifier or InChI. ISOLATION, PURIFICATION, CHARACTERISATION 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). This is usually done using physical processes such as Distillation , Filtration , Liquid-liquid Extraction , and Evaporation . These same techniques, along with others such as Recrystallisation , may also be used to purify the substance. Once the pure material has been prepared, its chemical and physical properties may be examined in order to characterise the substance. Finally, Chemical Analysis may be used to identify the Chemical Composition and to Assay the purity. REFERENCES AND NOTES |
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