116 



This fact has been utilized in devising a sort of hot-water 

 bottle. The bottle is made of rubber and contains a super- 

 saturated solution of sodium acetate. Whenever the heat is 

 wanted, the stopper is taken out, rubbed with the finger, and 

 screwed back. The rubbing spreads on the inner surface of the 

 stopper, next the liquid, some of the crystals adhering to the screw, 

 and so starts the crystallization. The bottle then becomes warm 

 and remains so for a considerable time. After it has cooled, it is 

 placed, without being opened, in boiling water to redissolve the 

 crystals, and, when cold, is ready for use again. 



Influence of the Solute upon the Solvent. The dissolving 

 of a substance alters the properties of the solvent. The observed 

 changes may be divided into two classes. 



In the first class, the amount of the change varies with the 

 substance dissolved. Very striking and difficult to explain, for 

 example, are the erratic changes in volume which occur when 

 solution takes place. Specific effects of this class show that 

 chemical changes often accompany solution. For example, 58.5 

 g. of sodium chloride (volume 27.5 c.c.) and 10,000 c.c. of water 

 have a volume, totalling 10,027.5 c.c., but, when they are dis- 

 solved, the solution measures only 10,016.5 c.c. This is a very 

 dilute solution (about \ per cent), so that the contraction of 11 c.c. 

 is relatively considerable. On the other hand, 214 g. of ammonium 

 chloride (volume 142.5 c.c.) and 843.5 c.c. of water have a total 

 volume of 986 c.c., but when dissolved give 1000 c.c. of solution. 

 Here there is an expansion of 14 c.c. In the case of table-sugar 

 and water, however, there is almost no change in volume. 



Another important property of solutions in which the influence 

 of the solute is specific is conduction of electricity. Pure water is an 

 exceedingly poor conductor. A solution of table-sugar in water 

 is~also practically non-conducting. But when acetic acid is 

 dissolved in water a solution is obtained which conducts the 

 current fairly well, while a solution of sodium chloride is an ex- 



