<text><span class="style10">hat is Chemistry? (5 of 6)Chemical compounds</span><span class="style7">In </span><span class="style26">chemical compounds</span><span class="style7">, the atoms of more than one element come together to form either molecules or infinite structures. They are described by formulae similar to those given above for elements. For example, water has a finite structure based on one oxygen atom chemically bonded to two hydrogen atoms and is denoted by the formula H2O. Common salt (sodium chloride; NaCl) has sodium (Na) and chlorine (Cl) atoms linked together in an infinite three-dimensional lattice (similar to that formed by potassium chloride.In a pure chemical compound, all the molecules have the same ratio of different atoms and behave in an identical chemical fashion. Thus a pure sample of water, for example, behaves identically to any other pure sample, however different their origins may be. Furthermore, the same ratios of atoms are retained irrespective of whether the compound is a solid, a liquid or a gas. For example, ice, water and water vapor all have molecules with the constitution H2O. The transformation of ice into water and then into water vapor by heating is not a chemical reaction, because the identities of the molecules do not change.From the 109 chemical elements now known, more than 2 million chemical compounds have been made during the last 100 years. The chemist views chemistry as a set of molecular building blocks, constructing more and more complex and diverse molecular structures, the variety of which is limited only by his or her imagination. It is important to emphasize that the properties of a chemical compound are unique and not a sum of the properties of the individual elements from which it is made. For example, common salt does not have any properties remotely like those of metallic sodium, which catches fire on contact with water, or chlorine, which is a harmful yellow-green gas.Although all compounds are unique, they can be classified into broad families based on common chemical properties. Acids, bases, salts, and oxidizing and reducing agents are examples of such families pp.. Classifications reflecting the atoms present are also useful for cataloguing purposes: for example, hydrides, chlorides and oxides indicate compounds containing hydrogen, chlorine and oxygen respectively. Another particularly important classification is that of organic compounds, which contain carbon and are not only important for life processes but make up many modern industrial chemicals such as plastics, paints and artificial fibers.</span></text>
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<text>North Sea gas contains methane, which ΓÇö unlike coal gas ΓÇö is odorless. Tiny amounts of ethanethiol ΓÇö an evil-smelling chemical ΓÇö are therefore added to aid its detection from open taps and gas leaks.</text>
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<text>ΓÇó CHEMISTRYΓÇó OIL AND GASΓÇó IRON AND STEELΓÇó RUBBER AND PLASTICSΓÇó CHEMICALS AND BIO TECHNOLOGY</text>
