مصادر الكيمياء موقع لتعليم الكيمياء بصورة مبسطة

An enzyme that has two structurally distinct forms, one of which is active and the other inactive. In the active form, the quaternary structure of the enzyme is such that a substrate can interact with the enzyme at the active site. The conformation of the substrate-binding site becomes al­tered in the inactive form and inter­action with the substrate is not …

Compounds that contain the group >C=C=C<, in which three carbon atoms are linked by two adja­cent double bonds. The outer carbon atoms are each linked to two other atoms or groups by single bonds. The simplest example is 1,2-propadiene, CH2CCH2 . Allenes are dienes with typical reactions of alkenes. Under basic conditions. they often convert to alkynes. In an allene, …

A type of polyester resin used in paints and other sur­face coating. The original alkyd resins were made by copolymerizing phthalic anhydride with glycerol, to give a brittle cross-linked polymer. The properties of such resins can be modified by adding monobasic acids or alcohols during the polymeriza­tion.

A complex poly­saccharide occurring in the cell walls of the brown algae (Phaeophyta). Algin strongly absorbs water to form a viscous gel. It is produced commer­cially from a variety of species of Laminaria and from Macrocystis pyrifera in the form of alginates, which are used mainly as a stabilizer and tex­turing agent in the food industry.

Compounds formed by reaction between hydroxylamine and an aldehyde RCOH + H2NOH → RCH:NOH + H2O  If R is an aliphatic group, the al­doxime is generally a liquid or low ­melting solid. If R is aromatic, the aldoxime is a crystalline solid. Al­doximes have a planar structure and can exist in two isomeric forms. In the syn-form, the OH group …

A hormone produced by the adrenal glands that controls excretion of sodium by the kidneys and thereby maintains the balance of salt and water in the body fluids.

affinity chromatography A bio­chemical technique for purifying nat­ural polymers, especially proteins. It functions by attaching a specific lig­and by covalent bonding to an insolu­ble inert support. The ligand has to have a specific affinity for the poly­mer, so that when a solution contain­ing the ligand is passed down a column of the material it is specifi­cally retarded and thus separated …

A low-density porous trans­parent material that consists of more than 90% air. Usually based on metal oxides or silica, aerogels are used as drying agents and insulators.

adsorption isotherm An equation that describes how the amount of a substance adsorbed onto a surface de­pends on its pressure (if a gas) or its concentration (if in a solution), at a constant temperature. Several ad­sorption isotherms are used in sur­face chemistry including the BET isotherm and the Langmuir ad­sorption isotherm. The different isotherms correspond to different as­sumptions about the …

adsorption indicator (absorption indicator) A type of indicator used in reactions that involve precipita­tion. The yellow dye fluorescein is a common example, used for the reac­tion NaCl(aq) + AgNO3(aq) → AgCl(s) + NaNO3(aq) As silver nitrate solution is added to the sodium chloride, silver chlo­ride precipitates. As long as Cl” ions are in excess, they adsorb on the precipitate particles. …