HYDROCHLORIC ACID.] 



CHEMISTRY. 



317 



state, and having a specific gravity of 1'21, it is composed 

 of the gas united with six equivalents of water ; its 

 symbol is H, Cl, 6HO ; the equivalent being 90 '5. 



Hydrochloric acid gas is readily procured from common 

 salt, by adding dilute sulphuric acid to that substance in 

 a retort. A gentle heat is to be applied, and the gas 

 will be given off in abundance. Owing to its great 

 attraction for water, it seizes the aqueous vapour present 

 in the air, producing white fumes. It may be collected 

 in bottles, in a similar manner to chlorine. On pouring 

 cold water, or putting a piece of ice into a jar of the gas, 

 it is rapidly absorbed, and considerable heat is evolved. 

 Its presence may readily be detected by means of a rod 

 dipped in liquid ammonia, when dense white fumes of 

 sal-ammoniac are afforded. Hence, if a bottle of the gas 

 be taken into a stable, the presence of ammonia will be 

 instantly detected by the production of the fumes named. 



Hydrochloric acid is produced on a large scale during 

 the manufacture of soda. The common salt is decomposed 

 by means of sulphuric acid, and the gas given off is con- 

 densed by water. 



For laboratory use, the acid may be prepared as 

 follows : 



Experiment 24. Introd uce into a glass or earthen retort 

 (trine common salt and water, and add to this nearly as 

 much sulphuric acid. Apply heat by means of a spirit- 

 lamp or furnace, and receive the gas in a Florence flask 

 or globular receiver containing cold water, the stein of 

 the retort just touching the surface of the water. Keep 

 the outside of the receiver as cold as possible, by covering 

 it with a cloth on which water is kept dropping. The 

 gas will lie rapidly absorbed, and the aqueous acid pro- 

 duced. The acid will be sufficiently strong for all 

 laboratory purposes. The reason the stem of the retort 

 is not placed below the surface of the water in the 

 receiver, is, that the absorption of the gas takes place so 

 rapidly that the liquid would rush into the retort, and 

 perhaps break it. The following cut shows a convenient 

 mode of carrying out this plan. 



Fig. 40. 



A different arrangement is used when a large quantity 

 of the gas is prepared. Bottles with two or three necks 

 and connecting pipes are used. These are made of glass 

 or earthenware, and are of large size. The gas is pro- 

 duced, in globular vessels of iron lined with lead, from 

 Fig. 41. 



lit and sulphuric acid ; and the specific gravity of the 

 cid to prepared, will be greater than that produced by 



the process we last described, because the condensation 

 of the gas is more perfect, a larger portion being dis- 

 solved in the water placed in the receiving vessels. The 

 preceding engraving illustrates this process. 



The chemical changes produced during the manu- 

 facture of hydrochloric acid may be thus represented : 



Materials used. 



Chloride of Sodium- 

 Chlorine M-5 



Sodium 23-0 



Sulphuric Acid ...40'U 



Water 



Ojygen 8-0 



Hydrogen I'O 



Water ... 



Products. 



56-5 Hydrochloric Acid. 



1-0 Sulphate of Soda. 

 Water. 



107-5 



1075 



Tn the above diagram, the proportion of water form- 

 ing crystallised sulphate of soda is omitted, so as to 

 avoid confusion of figures to tho uninitiated student. We 

 have only, therefore, represented absolutely dry sul- 

 phate of soda as the product. We therefore notice, 

 that 58^ parts of common salt should afford 36* parts 

 of hydrochloric acid gas, which, during absorption, com- 

 bines with at least 54 parts of water, to form Uie strong 

 aqueous acid. 



We have hitherto simply spoken of acids in a very 

 general manner ; and as hydrochloric acid is the first to 

 which we have drawn special attention, we shall avail our- 

 selves of the opportunity of more particularly referring 

 to the general characteristics of these substances. 



Generally speaking, the test for tho presence of a free 

 acid, is a sour taste when it is applied to the palate ; 

 the power of converting vegetable colours of blue tint to 

 a reddish hue, and of forming salts or salt-like bodies. 

 In some cases, the acid, or sour taste, is scarcely dis- 

 tinguishable : the prussic acid is an instance of the 

 kind. Carbonic acid has but feeble and transitory 

 effects on vegetable blues ; but, in every case, an acid 

 hag the power of forming a salt when combined with a 

 base ; or, what is equivalent thereto, a compound is 

 formed, which answers in its analogies to a salt. The 

 following experiment with hydrochloric acid, will serve 

 the double purpose of illustrating the general properties 

 of acids, and the special characteristics of hydrochloric 

 acid. 



Experiment 25. Make a solution of litmus by boiling 

 some of that substance in water. After the liquid has 

 ned a deep blue or purple colour, add a very small 

 portion of liquid ammonia, to remove any free acid ; 

 and then dip into the solution some pieces of white 

 blotting or filtering paper. They will, of course, be- 

 come of a blue colour, and will produce the general 

 test employed by chemists for detecting the presence of 

 a free acid. Drop some hydrochloric acid on the blue 

 paper, and it will at once become of a red tint. If such 

 paper be brought into contact with almost any acid, the 

 same effect will be produced. In the case of carbonic 

 acid, the effect is slight, because that has but feeble acid 

 properties, and is readily removed by evaporation. Most 

 of the blue colours of vegetables are thus affected ; but 

 indigo is an exception. The acids found in fruit, <tc., 

 generally called organic such as the acetic, tartaric, 

 oxalic, citric, <kc. are also detected by their producing 

 a red tint when tested by infusions of litmus or red 

 We may here suggest a very interesting occu- 



pation in connection with plants, fruit, &c. It is, that 

 the student should supply liimself with blue litmus 

 paper, and the same coloured red, by means of a little 

 weak vinegar. By the blue paper he may discover the 

 presence of acid in vegetable juices ; and, by the red. 

 the alkaline character of others may also be detected. 



We may, after this digression, return to consider some 

 of the properties of hydrochloric acid. With potas- 

 sium, sodium, and ammonia, it respectively forms the 



