أكتوبر 19th, 2018

 

Arsenic

Arsenic A chemical element, symbol As, atomic number 33. Arsenic is found widely distributed in nature (approximately 5 x 10-4 of the Earth’s crust). It is one of the 22 known elements composed of only one stable nuclide, §3As; the atomic weight is 74.92158. There are 17 other radioactive arsenic nuclides known.

There are three polymorphic modifications of arsenic. The yellow cubic a-form is made by condensing the…

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Aromatic hydrocarbon

Aromatic hydrocarbon A hydrocarbon with  a chemistry similar to that of benzene. Aromatic hydrocarbons are either benzenoid or nonbenzenoid. Benzenoid aromatic hydrocarbons contain one or more benzene rings and are by far the more common and the more important commercially. Nonbenzenoid aromatic hydrocarbons have carbon rings that are either smaller or larger than the six-membered benzene ring. Their importance arises mainly from a theoretical interest in understanding those…

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Aqua regia

Aqua regia A mixture of one part by volume of concentrated nitric acid and three parts of concentrated hydrochloric acid. Aqua regia was so named by the alchemists because of its ability to dissolve platinum and gold. Either acid alone will not dissolve these noble metals.

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Antioxidant

Antioxidant A substance that, when present at a lower concentration than that of the oxidizable substrate, significantly inhibits or delays oxidative processes, while being itself oxidized. In primary antioxidants, such as polyphenols, this antioxidative activity is implemented by the donation of an electron or hydrogen atom to a radical derivative, and in secondary antioxidants by the removal of an oxidative catalyst and the consequent prevention of the initiation…

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