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Some common chemistry terms:
Element - A substance that cannot be decomposed into simpler substances. In an element, all the atoms have the same number of protons or electrons although the number of neutrons may vary. There are 92 naturally occurring elements.
Compound - A substance formed by the combination of elements in fixed proportions. The formation of a compound involves a chemical reaction: i.e. there is a change in the configuration of the valence electrons of the atoms. Compounds, unlike mixtures, cannot be separated by physical means.
Mixture - A system of two or more distinct chemical substances. Homogeneous mixtures are those in which the atoms or molecules are interspersed, as in a mixture of gases or in a solution. Heterogeneous mixtures have distinguishable phases, e.g. a mixture of iron filings and sulphur. In a mixture there is no redistribution of valence electrons, and the components retain their individual chemical properties. Unlike compounds, mixtures can be separated by physical means (distillation, crystallization, etc.
Reaction - Chemistry - A change in which one or more chemical elements or compounds (the reactant) form new compounds (the products). All reactions are to some extent reversible; i.e., the products can also react to give the original reactants. However, in many cases the extent of this back reaction is negligibly small, and the reaction is regarded as irreversible. Biology - Any change in behavior of an organism in response to a stimulus.
Solution - A homogenous mixture of a liquid (the solvent) with a gas or solid (the solute). In a solution, the molecules of the solute are discrete and mixed with the molecules of solvent. There is usually some interaction between the solvent and solute molecules. Two liquids that can mix on the molecular level are said to be miscible. In this case, the solvent is the major component and the solute the minor component.
Stoichiometry - The relative proportions in which elements form compounds or in which substances react.
Titration - A method of volumetric analysis in which a volume of one reagent (the titrant) is added to a known volume of another reagent slowly from a burette until an end point is reached. The volume added before the end point is reached is noted. If one of the solutions has a known concentration, that of the other can be calculated.
End point - A known pH point of an acid and base interaction as shown by a chemical indicator change in color.
pH Scale - A logarithmic scale (from 1 (acid) to 14 (base)) for expressing the acidity or alkalinity of a solution. To a first approximation, the pH of a solution can be defined as -log 10 c, where c is the concentration of hydrogen ions in moles per cubic decimeter. A pH of below 7 indicates an acid solution; one above 7 indicates alkaline solution. More accurately, the pH depends not on the concentration of hydrogen ions but on their activity, which can not be measured experimentally. pH stands for 'potential of hydrogen'. The scale was introduced by S.P. Sorensen in 1909.
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