SOLUTION. 159 



AVhen 95.35 grammes of sulphuric acid (corresponding to H 2 SOj are 

 thus dissolved, 38,880 calories of heat are liberated, or enough heat 

 to raise over 38 liters of water 1 C. in temperature. When 284.11 

 grammes of crystallized sodium carbonate (Na 2 CO 3 .10H 2 O) are dis- 

 solved, 16,038 calories are absorbed, but solution of an equivalent 

 weight of anhydrous sodium carbonate (Na 2 CO 3 ), or 105.31 grammes, 

 liberates 5,598 calories of heat. If a substance absorbs heat during 

 solution, it develops the same amount of heat when it comes out of 

 solution as by crystallization. For example, a supersaturated solu- 

 tion of sodium sulphate, when crystallizing suddenly, produces quite 

 an appreciable rise in temperature. 



Solution of gases. Henry's Law. Gases dissolve in liquids to a vari- 

 able degree. The range of solubility is quite wide, as may be seen from the 

 following examples: At C. and 760 mm. pressure, 1 volume of water dissolves 

 0.02 volume of hydrogen, or nitrogen, 0.04 volume of oxygen, 1.8 volumes of 

 carbon dioxide, 80 volumes of sulphur dioxide, 550 volumes of hydrochloric 

 acid gas, and 1050 volumes of ammonia gas. 



The solubility of a gas varies with the nature of the solvent; thus, at C., 

 a volume of alcohol dissolves twice as much carbon dioxide as the same volume 

 of water does. It also varies with the temperature, decreasing, as a rule, as the 

 temperature increases. Thus, 100 volumes of water dissolve about 4 volumes 

 of oxygen at C., 3 at 20 C., 1.8 at 50 C., and none at 100 C. In many 

 instances a gas is completely removed from its solution by boiling, but this is 

 not possible in the case of certain very soluble gases, like hydrochloric acid. 

 There is in such cases a chemical action, in part at least, between the gas and 

 the solvent. A 20.2 per cent, aqueous solution of hydrochloric acid distills 

 unchanged under normal atmospheric pressure. 



The solubility of a gas increases with increased pressure on the gas. Com- 

 mercial aerated water is a good illustration. The water is charged with carbon 

 dioxide under considerable pressure. When drawn from the container and 

 exposed to the atmosphere the excess of gas, which cannot remain dissolved 

 under the diminished pressure, escapes, causing effervescence. Such a solution 

 is often called soda water. Solutions of gases in liquids fall into two classes : 

 (1) those from which the gas is completely removed by heat or by decrease of 

 pressure ; (2) those from which the gas is not thus completely removed. Very 

 soluble gases give rise to the second class of solutions, in which a complete 

 chemical and physical independence of the molecules of solvent and gas is 

 lacking. In solutions of the first class there is a fixed relationship between the 

 solubility of a gas and pressure, which is known as Henry's Law. It may be 

 stated thus : The quantity of a gas dissolved by a given quantify of a liquid is pro- 

 portional to the pressure of the gas. Since the volume of a gas is inversely 

 proportional to the pressure, another form in which the law may be stated is : 

 A given quantity of a liquid dissolves the same volume of a gas at all pressures. 



In the case of a mixture of gases in contact with a liquid, each gas dissolves 

 as if it were present alone, and in proportion to its own partial pressure in the 

 mixture, 



