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Chemistry, science dealing with the composition of substances and the changes that occur when they react with one another. All chemical changes take place by the linking-up of atoms into molecules, a molecule being the smallest particle of a chemical compound that has that compound's characteristic properties. Chemical reactions may involve elements themselves, elements and compounds, or compounds and other compounds. Atoms themselves remain fundamentally unchanged in chemical reactions. It is the grouping of the atoms into molecules that is altered. Some chemical changes are relatively simple, while others, such as those that occur during the chemical processes of living matter, are highly complex, involving molecules that may consist of thousands of atoms. Organic chemistry deals with the chemistry of compounds of carbon (the chemical processes of life take place between substances in which carbon plays a major role). Inorganic chemistry is concerned with all the remaining elements. Apart from that broad division, there are other specialized fields. Physical chemistry deals with the physical properties and behavior of chemicals. Analytical chemistry develops laboratory methods of determining the chemical structure of materials. Chemotherapy is the treatment of diseases with chemical drugs; biochemistry is concerned with living processes; chemurgy deals with agricultural products; electrochemistry studies the electrical effects of chemical change.

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