CHAMBERS'S INFORMATION FOR THE PEOPLE. 



the one which is left gets a larger share, enables 

 us to explain some apparently anomalous cases 

 of decomposition. Thus, as we have seen, in a 

 cold aqueous solution sulphuric acid decomposes 

 borate of soda, driving out boracic acid, and 

 forming sulphate of soda ; but if we mix sulphate 

 of soda and boracic acid, and heat the mixture 

 strongly in a crucible, we find that the boracic 

 acid drives out the sulphuric acid, and forms 

 borate of soda : in the first case, the boracic acid 

 is removed by becoming solid ; in the second, the 

 sulphuric acid is removed by being converted into 

 a vapour. 



The balance in such a mixture as we have been 

 considering may be disturbed, and a rearrange- 

 ment necessitated, by the removal of one of the 

 salts, as well as by the removal of one of the acids. 

 Thus, if we mix an equivalent of nitrate of lime* 

 with one equivalent of sulphuric acid, and a 

 sufficient quantity of water to retain the whole in 

 solution, the lime is divided between the nitric and 

 sulphuric acids, so that we have a mixture of sul- 

 phate of lime, nitrate of lime, sulphuric acid, and 

 nitric acid. If to this mixture we add alcohol 

 (spirit of wine), a change of conditions is produced, 

 for sulphate of lime is insoluble in dilute alcohol. 

 The result is, that the sulphate of lime is precipi- 

 tated^ and the sulphuric acid in the liquid acts 

 upon the nitrate of lime, producing more sulphate 

 of lime, which is also precipitated, and so on until 

 the whole of the nitrate of lime is decomposed, 

 and nothing is left but sulphate of lime as a pre- 

 cipitate, and nitric acid in solution. 



The action of acids upon salts is taken advan- 

 tage of in the preparation of many acids. Thus, 

 nitric acid is prepared by the action of sulphuric 

 acid on nitrate of soda ; hydrochloric or muriatic 

 acid, by the action of sulphuric acid on common 

 salt (which we may call muriate of soda) ; J tar- 

 taric acid, by the action of sulphuric acid on tar- 

 trate of lime, &c. It is also used in analysis, 

 as will be explained when we come to that branch 

 of the subject 



The action of bases upon salts will be easily 

 understood from what has just been said of the 

 action of acids upon salts. The new base may 

 first, totally decompose the salt, driving out the 

 original base ; second, partially decompose it, 

 the acid being shared between the two bases ; or, 

 third, produce no change ; and as in the case of 

 the acids, so here, a partial decomposition may be 

 made complete by removing the base as it separ- 

 ates, or by removing the newly produced salt It 

 sometimes happens that the new base, taking part 

 of the acid of the salt, leaves a basic salt ; thus, 

 when ammonia is added to nitrate of lead, nitrate 

 of ammonia is produced, and basic nitrate of lead. 

 The latter being nearly insoluble in water, is 

 precipitated. 



The action of bases upon salts is taken advantage 

 of in the preparation of bases ; thus, ammonia is 

 prepared by the action of lime upon hydrochlorate 

 of ammonia ; caustic potash and caustic soda, by 

 the action of lime upon carbonate of potash or 

 carbonate of soda ; oxide of silver, by the action 

 of caustic potash upon nitrate of silver, &c, 



* By an equivalent of a salt we mean the quantity formed by 

 the action of an equivalent of acid upon an equivalent of base. 



t When the mixture-of two liquids causes the separation of an 

 insoluble solid substance, the solid is said to be 'precipitated,' 

 and is called a ' precipitate.' 



1 This nomenclature will be more fully explained farther on. 

 310 



The action of salts upon salts. Some salts may- 

 be mixed together without any action taking place. 

 In the case of others, especially when one or both 

 are in solution, an exchange of acids and bases 

 takes place, which may be, as in the actions just 

 described, either complete or partial. Thus, if we 

 mix one equivalent of nitrate of lead and one 

 equivalent of sulphate of soda, both dissolved in. 

 water, we obtain an equivalent of sulphate of lead 

 and an equivalent of nitrate of soda. Such an 

 action is called double decomposition. In this 

 case one of the salts produced (sulphate of lead) is- 

 insoluble in water, and is therefore precipitated. 

 When all four salts namely, the two which are 

 originally mixed, and the two which are or may be 

 produced are soluble in water, it is often difficult 

 to make out whether double decomposition has 

 taken place or not, and still more difficult to deter- 

 mine the exact extent of the change. As previously 

 pointed out, a partial change may be rendered 

 complete by the removal of one of the products, 

 either as a precipitate or in the form of vapour. 

 Thus, in the example given above, the double 

 decomposition is complete, the sulphate of lead 

 being precipitated ; and when carbonate of lime 

 and muriate of ammonia are mixed, carbonate of 

 ammonia and muriate of lime are produced, and 

 the action is complete if the mixture be heated,, 

 because carbonate of ammonia is volatile, and is 

 driven off as vapour. 



Double decomposition is taken advantage of in 

 the preparation of many salts, and also in chemical 

 analysis. 



In the sketch of the characters of acids, bases,, 

 and salts, and their relations to one another just 

 given, we have omitted a very important point 

 which we must now advert to ; this is, the relation 

 of water to these classes of bodies. 



Water enters into combination with a great 

 variety of substances, and in different ways. Thus,, 

 many substances dissolve in water, and the solu- 

 tions so formed are weak compounds of water 

 and the substances dissolved ; weak compounds, 

 because the components are easily separated,, 

 moderate heat driving off the water.* Again, many 

 crystalline substances contain water as an essen- 

 tial constituent, without which they would either 

 not crystallise at all, or crystallise in a different 

 form ; hence called ' water of crystallisation.' This- 

 water also is feebly held in combination, and is 

 easily driven off. Many crystalline substances con- 

 taining water of crystallisation, lose it on exposure 

 to dry air, losing at the same time their crystalline 

 form. This change is called ' efflorescence,' and 

 crystalline substances which undergo this change 

 are said to be ' efflorescent.' Washing-soda is a 

 familiar instance. The clear, transparent crystals 

 of vashing-soda contain nearly 63 per cent, of 

 water of crystallisation. On exposure to dry air, 

 they lose water, and become white and powdery. 

 All substances containing water of crystallisation, 

 do not lose it so readily, but a temperature a few 

 degrees above the boiling-point of water is sufficient 

 to drive it off in every case. Now, there are soluble 

 acids and soluble bases, there are crystalline acids 

 and crystalline bases containing water of crystallisa- 



* Although solutions maybe said to be weak compounds because 

 easily decomposed, many soluble substances have such a strong 

 attraction for water, that they can remove it from moist air, and 

 unite with it to form a solution : such substances are said to be. 

 ' deliquescent,' and are used for the purpose of drying gases. 



