HYDBOCHLORIC ACID. CALCULATIONS 129 



nearly as perfect a way of liberating sulphuric acid as (1) is of 

 liberating hydrogen chloride. 



Precipitation. When two soluble substances are dissolved 

 separately in water, and the solutions are mixed, chemical inter- 

 action frequently is evident between the dissolved materials. If 

 one of the products is not very soluble, then a supersaturated 

 solution (p. 115) of this product may be thus produced. As a rule, 

 this substance immediately becomes visible as a fine powder, 

 called a precipitate, suspended in the liquid. More or less rap- 

 idly, according to its fineness of dispersion, this precipitate settles 

 out, leaving the solution clear. Equation (2) in the preceding 

 paragraph is an example of such a reaction. 



Often the precipitated product can be recognized by the physi- 

 cal appearance of the precipitate, and so this sort of action is used 

 as a test for one of the original substances. Thus, precipitates 

 are classified according to their color. Again, precipitates of the 

 same color differ in degree of dispersion, and may be described 

 as gelatinous, curdy, pulverulent, or crystalline. In the two 

 former cases, at least, the precipitation is so sudden that there is 

 not time for crystals to be formed, and the product is amorphous 

 (see p. 95). 



Physical Properties of Hydrogen Chloride. Hydrogen 

 chloride is a colorless gas. It is spur in taste, and has a sharp odor. 

 It is irritating, but not poisonous in small amounts. 



The gas is exceedingly soluble in water, one volume of which, 

 at 15, will dissolve no less than 455 volumes of the gas. The 

 saturated solution at 15 contains nearly 43 per cent of the gas 

 by weight. The concentrated hydrochloric acid of commerce 

 contains about 35 per cent. 



The density of the gas (weight of 1 c.c.) is 0.001628. Of more 

 interest to the chemist is the weight of 22,400 c.c. or 22.4 liters 

 (the gram-molecular volume), .namely 36.468 grams. This is the 



