مصادر الكيمياء موقع لتعليم الكيمياء بصورة مبسطة

A coenzyme and one of the most impor­tant compounds in the metabolism of all organisms, since it serves as a coupling agent between different enzymatic reactions. Adenosine triphosphate is adenosine diphos­phate (ADP) with an additional phosphate group attached through a pyrophosphate linkage to the terminal phosphate group (see illustration). ATP is a powerful donor of phosphate groups to suitable acceptors …

A process in which a hydrogen atom in an organic compound is re­placed by an acyl group (R-CO, where R = an organic group). The reaction in­volves substitution by a nucleophile (electron donor) at the electrophilic carbonyl group (C=O) of a carboxylic acid derivative. The substitution usually proceeds by an addition-elimination sequence. Two common reagents, with the general formula RCOX, …

A technique in which a neutron, charged particle, or gamma photon is captured by a stable nuclide to produce a different, radioactive nu­clide which is then measured. The technique is specific, highly sensitive, and applicable to almost every element in the periodic table. In neutron activation analysis (NAA), the most widely used form of activation anal­ysis, the sample to be …

A powdered, granular, or pelleted form of amorphous car­bon characterized by very large surface area per unit volume because of an enormous number of fine pores. Activated carbon is capable of collecting gases, liquids, or dis­solved substances on the surface of its pores. Adsorption on activated carbon is selective, favoring nonpolar over polar substances. Compared with other commercial adsorbents, activated …

A chemical element, Ac, atomic number 89, and atomic weight 227.0. Actinium was discovered by A. Debierne in 1899. Milligram quantities of the element are available by irradiation of radium in a nuclear reactor. Actinium-227 is a beta­emitting element whose half-life is 22 years. Six other radioisotopes with half-lives ranging from 10 days to less than 1 minute have been …

The series of elements beginning with actinium (atomic number 89) and including thorium, protactinium, uranium, and the transuranium el­ements through the element lawrencium (atomic number 103). These elements have a strong chemical resemblance to the lanthanide, or rare-earth, elements of atomic numbers 57 to 71. Their atomic numbers, names, and chemical symbols are: 89, ac­tinium (Ac), the prototype element, sometimes …

One of a large group of organic substances possessing the halocar­bonyl group, in which X stands for fluorine, chlorine, bromine, or iodine. The terms acyl and aroyl halides refer to aliphatic or aromatic derivatives, respectively. The great inherent reactivity of acid halides precludes their free existence in nature; all are made by synthetic processes. In general, acid halides have low …

An organic compound with the formula C2H2 or HC≡CH. The first member of the alkynes, acetylene is a gas with a narrow liquid range; the triple point is -81 °C (-114°F). The heat of formation (ΔHf) is +227 kilojoules/mole, and acetylene is the most endothermic compound per carbon of any hydrocarbon. The compound is thus extremely energy-rich and can decompose …

A substance that reveals, through characteristic color changes, the degree of acidity or basicity of solutions. Indicators are weak organic acids or bases which exist in more than one structural form (tautomers) of which at least one form is colored. Intense color is desirable so that very little indicator is needed; the indicator itself will thus not affect the acidity …

A chemical compound, CH3COCH3. A colorless liquid with an ethereal odor, it is the first member of the homologous series of aliphatic ketones. Its physical properties include boiling point 56.2°C (133.2°F). melting point -94.8°C (-138.6°F). and specific gravity 0.791. Acetone is used as a solvent for cellulose ethers, cellulose acetate, cellulose nitrate, and other cellulose esters. Cellulose acetate is spun …