SOLUTIONS 175 



substances unite chemically with a portion of the solvent. 

 Evidence of this is derived from the fact that heat is evolved 

 when many substances are placed in contact with a liquid 

 capable of dissolving them, the evolution of heat being a common 

 sign of chemical union. 



In many cases also changes of colour are observed, and when, 

 from the resulting solution, crystals are deposited, these crystals 

 contain definite molecular proportion of water or alcohol or other 

 liquid, in the midst of which they have been formed. An example 

 may be found in the crystals of common washing soda, which is 

 represented by the formula Na 2 C0 3 .10H 2 0. If some of these 

 crystals placed in a saucer are gently heated they melt easily 

 and soon give off steam. After a time the liquid dries up to a 

 white powder which consists of the soda without the water. I/ 

 the dry powder is now allowed to become quite cold and an 

 equal weight of cold water is poured on it the mass becomes hot, 

 and no solution is produced, for the water unites with the salt 

 to reproduce the original substance. If more water is now added 

 the salt passes again into solution, and after some time, if too 

 much water has not been added, crystals appear having the 

 same composition and properties as the original washing soda. 

 In most cases the solubility of a salt in water is, like that of 

 sugar, continuously greater with rise of temperature up to the 

 boiling-point of the solution or even much beyond that point. 

 But this is not always the case. Glauber's salt (sodium sulphate) 

 is an excellent example. If some of this salt, which is sold in 

 small crystals for medicinal use, is mixed with less than its own 

 weight of water and very gently warmed it dissolves freely, 

 forming a clear solution. If the latter is then heated to the 

 boiling-point a shower of small crystals will be observed falling 

 within the solution, and these crystals consist of sodium sulphate, 

 Na 2 S0 4 , in the anhydrous state, that is, without combined water. 

 The crystals of Glauber's salt contain ten molecules of water, 

 Na 2 S0 4 .10H 2 0, and the solubility of these increases with rise 

 of temperature up to 32 0> 5 C., when a portion of this hydrate 

 splits up into water and the anhydrous salt which is less soluble, 

 and consequently a portion of it is deposited from the liquid. 

 Such a separation of two components of a compound is called 

 " dissociation," and in such a liquid as the solution referred to 

 the process is doubtless continuous with rise of temperature. It 

 certainly cannot be inferred that below the critical temperature 



