180 Domestic Science 



117. Dissolved substances also produce an effect 

 upon the freezing-point of the liquid in which they 

 are dissolved, but the alteration is of an opposite 

 character, the temperature of freezing being lowered 

 by the presence of the dissolved matter, the magnitude 

 of the decrease being proportional to the strength of 

 the solution. 



EXPERIMENT 64. Surround a test-tube containing 

 a little water with a freezing-mixture of ice and salt 

 as described in Experiment 55. Insert a thermometer 

 in the water and read the temperature at which the 

 water freezes, as shown by the mercury becoming 

 steady at a certain level. Using the same thermometer, 

 find the temperatures at which 2 % and 5 % solutions 

 of salt become frozen. Compare your observations 

 with the statement given above. 



118. When solid ice becomes liquid water, heat is 

 absorbed. As will be shown later, the dissolving of a 

 solid substance in a liquid may also be attended by a 

 similar absorption of heat, and this action is fairly 

 general. The cause of the production of a very low 

 temperature by the admixture of ice and salt may now 

 be readily grasped. Some of the salt dissolves in the 

 water adhering to the ice, and thus a slight lowering 

 of temperature is caused. A further fall is occasioned 

 by the dissolving of part of the ice in the newly- formed 

 salt solution, attended by the disappearance of the 

 amount of heat rendered "latent " by this process. The 

 continued operation of these two causes brings about a 

 fall of temperature sufficient to make the mixture of 

 great value as a means of producing cold. A thorough 

 admixture of snow with one-third of its weight of salt 

 will effect a lowering of temperature to 21. 



