acid defined in 1939 yearacid - Acid (Lat. acidus, sour);
acid - Popularly, an acid is any sour substance, but chemically acids are salts of hydrogen, as belonging to a class of substances which always contain hydrogen. Other general characteristics are that they have a sour taste, possess the property of neutralising alkalis to form well-defined salts and change the colour of blue litmus paper to red. As some salts exhibit acid properties, it is necessary to add that the substances must contain no basic elements or group of elements.
It is the presence of hydrogen that gives these compounds their peculiar acid character. Lavoisier held that oxygen is the element to which acids owe their character, but Davy showed that hydrochloric acid (HCl) does not contain oxygen, and Gay-Lussac demonstrated that hydrocyanic acid (HCN) is another acid without oxygen, so the Lavoisier theory had to be abandoned. Other elements than hydrogen are present in acids. If the acid contains oxygen, e.g. sulphuric acid (H2 SO4), the abstraction of water (H2O) leaves the anhydride or acid oxide. If the acid contains no oxygen, it is indicated by the prefix hydro, e.g. hydrochloric acid (HC1), or hydrocyanic acid (HCN). The termination ic indicates that the characteristic element, or group of elements, exercises its highest valency or strength in the form of chemical energy. The termination ous means that the second highest valency obtains, while the prefix hypo, together with the termination ous, indicates some lower valency. When an acid contains a high proportion of oxygen, the prefix per is used, e.g. perchloric acid.
Some acids on treatment with the hydroxide of an alkali metal exchange all their hydrogen for metal, thus producing a normal fait, while other acids produce acid salts in addition, i.e. ail the hydrogen is not replaced. The former variety of acid is termed monobasic, and the latter class polybasic, a term including di-, tri-, and tetra-basic. Formerly all acids were thought to be monobasic, but Liebig enunciated the doctrine of polybasicity, which was further established through the researches of Graham, who showed that phosphoric acid exists in three forms, as ortho-, meta-, and pyro-phosphoric acid.
Inorganic acids or mineral acids are combinations of non-metallic elements with hydrogen only, or with an additional element generally oxygen. Organic acids are considered as derived from hydrocarbons or their alcohols by the replacement of hydrogen (H), or hydroxyl (HO), by the carboxyl group (COOH). Organic acids are further divided into aliphatic acids, i.e. those derived from open chain hydrocarbons and aromatic acids from ringed hydrocarbon nuclei. Aliphatic acids are further divided according to the parent hydrocarbon. Examples of inorganic-acids are sulphuric acid, hydrochloric acid, and nitric acid, while well-known organic acids are acetic acid, citric acid, oxalic acid, and tartaric acid.
near acid in Knolik
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