DECOMPOSITION 11 



substance in solution is treated with hydrochloric acid, by which mercury, silver, and 

 lead are removed. The mercury will only be perfectly removed if it exists entirely 

 in the state of a subsalt. Lead is only partially precipitated, and will be subsequently 

 found in the next group. The precipitate by hydrochloric acid is to be boiled with 

 water, which will remove the chloride of lead, and leave the chlorides of mercury and 

 silver. The latter may be separated by means of ammonia, which will dissolve the 

 chloride of silver and convert the mercury into a black powder, in which the metal 

 can be detected by special tests. The fluid filtered from the precipitate by hydro- 

 chloric acid is to have a stream of hydrosulphuric acid gas (sulphuretted hydrogen) 

 passed through it for a considerable time, or until no more precipitation occurs. By 

 this means antimony, arsenic, tin, cadmium, gold, mercury, silver, lead, bismuth, and 

 copper are thrown down, and must be separated from each other by special processes. 

 The filtrate from the precipitate by hydrosulphuric acid is to have ammonia added in 

 slight excess, and then a solution of sulphide of ammonium as long as any precipita- 

 tion takes place. By this means nickel, cobalt, iron, manganese, zinc, alumina, and 

 chromium, are thrown down ; also baryta, strontia, and lime, if they happen to be in 

 combination with phosphoric, oxalic, or boracic acids, or if united to fluorine. ^ From 

 the filtrate, carbonate of ammonia mixed with chloride of ammonium, precipitates 

 baryta, strontia, and lime. The filtrate from the last precipitate can only contain 

 magnesia, or the alkalis. The above brief description of the mode of dividing the 

 above metals into groups will be sufficient to give an idea of the processes employed 

 for decomposing complex mixtures into simple ones. 



Inorganic acids are usually removed from metals by converting the latter into an 

 insoluble compound, while the acid remais in solution either in the free state or 

 combined with a body of such a nature as not to mask the reactions of the acid with 

 reagents. This is often done in the laboratory by boiling the metallic salt with an 

 alkaline carbonate. The metals are, consequently, either converted into oxides or 

 carbonates insoluble in water, while the acid unites with the alkali to form a soluble 

 salt capable of being obtained by filtration in such a condition as to permit the nature 

 of the acid to be made known by means of appropriate tests. It is usually necessary 

 to neutralise the solution carefully before testing for the acid. 



It is seldom necessary in researches to reduce inorganic alkalis to their elements, 

 their constitution being usually ascertained by converting their constituents into 

 new forms capable of being weighed or measured with accuracy. If, for instance, 

 it was necessary to ascertain the constitution of sulphuric acid, it would be sufficient 

 to determine the quantity of baryta contained in the sulphate. On the other hand, 

 acids susceptible of assuming, when pure, the gaseous condition may have their con- 

 stitution determined by decomposing a known volume with a substance capable 

 of combining with one ingredient and liberating the other in the gaseous state. 

 Thus hydrosulphuric acid may be analysed by heating it with potassium, which will 

 remove the sulphur and liberate the hydrogen. 



In decomposing inorganic alkalis with the view of separating the metals contained 

 in them, we usually have to avail ourselves of very powerful affinities. This arises 

 from the fact, that the substances in question are, generally, produced by the union 

 of a metal with oxygen, the metal having so strong a tendency to combine with that 

 element, that mere exposure to the air is sufficient to determine their union into a 

 compound of great stability. In order, therefore, to decompose the alkalis of this 

 class, it is necessary to find some substance having a powerful tendency to combine 

 with oxygen under certain conditions. Now it has been found that carbon, if raised 

 to an exceedingly high temperature, and employed in great excess, is capable of 

 removing the oxygen, even from such bodies as potassium and sodium, the affinity 

 of \vhich for oxygen is very great. 



Inorganic neutral bodies are generally decomposed either by the ordinary pro- 

 cesses of analysis, or, where the neutrality arises from the substance under examina- 

 tion being a compound of an acid and a base, by separating the two by treatment 

 with a reagent capable of combining with one to the exclusion of the other. This is 

 a process frequently available in quantitative analysis. As an illustration, we may 

 take the decomposition of the carbonates by a mineral acid in an apparatus which 

 permits the carbonic acid set free to be accurately estimated by weighing. See 

 CARBONATES. 



Another instance of the decomposition of a neutral body, by treating it with a 

 substance capable of combining with one of the constituents and separating the other 

 in a free state, is the decomposition of sulphate of potash by baryta. If a solution of 

 the salt be boiled with excess of solution of baryta, sulphate of baryta is produced, 

 and caustic potash set free. The excess of baryta is removed by boiling in the air 

 until the whole of the latter base is converted into the insoluble carbonate. A pre- 

 cisely analogous process is the ordinary mode of preparing caustic potash by boiling 

 its carbonate with quicklime. 



