chemistry | engleski leksikon


/ keməstri /


ETYM From Chemist. Related to Alchemy.
Branch of science concerned with the study of the structure and composition of the different kinds of matter, the changes which matter may undergo and the phenomena which occur in the course of these changes. Organic chemistry is the branch of chemistry that deals with carbon compounds. Inorganic chemistry deals with the description, properties, reactions, and preparation of all the elements and their compounds, with the exception of carbon compounds. Physical chemistry is concerned with the quantitative explanation of chemical phenomena and reactions, and the measurement of data required for such explanations. This branch studies in particular the movement of molecules and the effects of temperature and pressure, often with regard to gases and liquids.
Molecules, atoms, and elements.
All matter can exist in three states: gas, liquid, or solid. It is composed of minute particles termed molecules, which are constantly moving, and may be further divided into atoms.
Molecules that contain atoms of one kind only are known as elements; those that contain atoms of different kinds are called compounds.
Compounds and mixtures.
Chemical compounds are produced by a chemical action that alters the arrangement of the atoms in the reacting molecules. Heat, light, vibration, catalytic action, radiation, or pressure, as well as moisture (for ionization), may be necessary to produce a chemical change. Examination and possible breakdown of compounds to determine their components is analysis, and the building up of compounds from their components is synthesis.
When substances are brought together without changing their molecular structures they are said to be mixtures.
Formulas and equations.
Symbols are used to denote the elements. The symbol is usually the first letter or letters of the English or Latin name of the element—for example, C for carbon; Ca for calcium; Fe for iron (ferrum). These symbols represent one atom of the element; molecules containing more than one atom of an element are denoted by a subscript figure—for example, water is H2O. In some substances a group of atoms acts as a single entity, and these are enclosed in parentheses in the symbol—for example (NH4)2SO4 denotes ammonium sulfate. The symbolic representation of a molecule is known as a formula.
A figure placed before a formula represents the number of molecules of a substance taking part in, or being produced by, a chemical reaction—for example, 2H2O indicates two molecules of water. Chemical reactions are expressed by means of equations as in.
NaCl + H2SO4 ® NaHSO4 + HCl.
This equation states the fact that sodium chloride (NaCl) on being treated with sulfuric acid (H2SO4) is converted into sodium bisulfate (sodium hydrogensulfate, NaHSO4) and hydrogen chloride (HCl).
Metals, nonmetals, and the periodic system.
Elements are divided into metals, which have luster and conduct heat and electricity, and nonmetals, which usually lack these properties. The periodic system, developed by John Newlands in 1863 and established by Dmitri Mendeleyev in 1869, classified elements according to their atomic weights. Those elements that resemble each other in general properties were found to bear a relation to one another by weight, and these were placed in groups or families. Certain anomalies in this system were later removed by classifying the elements according to their atomic numbers. The latter is equivalent to the positive charge on the nucleus of the atom.
Ancient civilizations were familiar with certain chemical processes—for example, extracting metals from their ores, and making alloys. The alchemists endeavored to turn base (nonprecious) metals into gold, and chemistry evolved toward the end of the 17th century from the techniques and insights developed during alchemical experiments.
Landmarks in chemistry.
Robert Boyle defined elements as the simplest substances into which matter could be resolved. The alchemical doctrine of the four elements (earth, air, fire, and water) gradually lost its hold, and the theory that all combustible bodies contain a substance called phlogiston (a weightless “fire element” generated during combustion) was discredited in the 18th century by the experimental work of Joseph Black, Antoine Lavoisier, and Joseph Priestley (who discovered the presence of oxygen in air).
Henry Cavendish discovered the composition of water, and John Dalton put forward the atomic theory, which ascribed a precise relative weight to the “simple atom” characteristic of each element. Much research then took place leading to the development of biochemistry, chemotherapy, and plastics.
1. The branch of the natural sciences dealing with the composition of substances and their properties and reactions; SYN. chemical science.
2. The way two individuals relate to each other; SYN. interpersonal chemistry.


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