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Salts. When an acid is gradually added to an alkaline substance the characteristic properties of the latter are slowly destroyed, those of the acid, also disappearing, until finally a compound is obtained which possesses neither acid nor alkaline properties. Such compounds are termed salts. As many compounds which do not show alkaline properties also neutralise acids, a more complete statement is that bases (q.v.) and acids by uniting together form salts. Snch a reaction is expressed by the following equations :-

(1) NaOH + HCl = NaCl + OH3. (2) CaO + H2SO4 = CaS04 + OH2.

It is seen that in the salts the hydrogen of the acid is replaced by the metal present in the base so that in constitution salts are compounds formed from acids by the replacement of the hydrogen by a metal. Certain groups of elements (eg. ammonium, NH4) also behave as metals in this respect, forming salts, as ammomum chloride (NH4Cl). In many acids, however, there is more than one atom of hydrogen present in the molecule. In such cases the hydrogen may or may not be completely replaced by the metal. In the former case the resulting salt is known as a normal or neutral salt - eg. normal sodium sulphate (Na2S04) If the replacement be incomplete acid salts result, as e.g. acid potassium sulphate (HKSO4). These are also known as hydrogen salts, as dihydrogen sodium phosphate (H2NaPO4). In some salts also there is present a greater amount of the base than is necessary to combine with the acid and we hence obtain basic salts. The term salt or common salt, is applied popularly to the chloride of sodium (NaCl) [SODIUM], and was the original significance, the general term being due to an after extension of the meaning. In ordinary language and in medicine the term salts also is givon to Epsom salts (q.v.), or sulphate of magnesium (MgSO4.7OH2). A number of compounds also are known under such names as salts of lemon, salts of sorre1, etc. Smelling salts consist of carbonate of ammonia (which has a strong odour of the latter), usually mixed with some perfume as lavender, etc. In organic chemistry compounds strictly analogous to salts are abundant in which certain hydrocarbon radicals play the part of the metal. They are usually known as ethereal salts or as esters.