THE PHYSICS OF FREEZING MIXTURES 



113 



furnish an incentive for many a wood- 

 land ramble. 



Finally, I procured a copy of Gray's 

 Manual (Seventh Edition). Then, for 

 the first time, I realized how^ little I 

 knew about botany. But it wasn't so 

 much botany that I was after as flow- 

 ers and trees, and I already knew those 

 fairly well. So I survived my perusal 

 of that valuable work. 



The Physics of Freezing Mixtures. 



BY FREDERICK H. GETMAN, PH.D., STAM- 

 FORD, CONNECTICUT. 

 [Written by Request of the Editor of this Magazine.] 



It is well known that the process of 

 solution is accompanied by a thermal 



DIAGRAM— .SEE LAST PARAGRAPH OF NEXT 

 COLUM,N. 



change. In general, when a salt dis- 

 solves in water the temperature of the 

 system falls. For example, a marked 

 lowering of temperature occurs when 

 potassium iodide is dissolved in water, 

 while if ammonium thiocyanate is dis- 

 solved in water (20 grams of salt in 25 

 cubic centimeters of water) sufficient 

 heat will be absorbed in the process to 

 freeze the containing vessel to the sur- 

 face of a block of wood which had pre- 

 viously been wet with water. 



There is abundant evidence to show 

 that when a substance dissolves its 

 condition is analogous to that of a gas. 

 The essential difference between a sub- 

 stance in the solid, liquid and gaseous 

 states is its energy content. In other 

 words, when a solid is converted into 

 a liquid, or when a liquid is converted 

 into a gas, energy in the form of heat 

 is required. Consequently when a salt 

 goes into solution heat is taken up from 



the solvent and a fall of temperature 

 results. 



When sodium chloride (NaCl) or 

 common salt and finely crushed ice are 

 intimately mixed, as in the familiar 

 freezing mixture used in making ice- 

 cream, a small amount of the salt dis- 

 solves in the liquid water present and 

 the thermal equilibrium of the system 

 is disturbed. Assuming the system to 

 be well insulated from the warmer en- 

 vironment of the room, further solution 

 of the salt can only take place by the 

 melting of more ice with a consequent 

 lowering of temperature. The liquid 

 water resulting from the melting of the 

 ice. dissolves more salt, and more ice 

 is thus forced to melt with a further 

 reduction in temperature. This proc- 

 ess will continue until ultimately one 

 of three things will occur, viz., (i) all 

 of the salt will dissolve, or (2) all of 

 the ice will melt, or (3) a mixture of 

 salt and ice having the same composi- 

 tion as the solution will be formed. 

 This mixture is termed a cryohydrate 

 and the temperature of the system cor- 

 responding to this particular mixture 

 of ice and salt is called the cryohydric 

 temperature. The lowest temperature 

 which can be obtained with any defi- 

 nite freezing mixture is the cryohydric 

 temperature. 



A clear insight into the mechanism 

 of freezing mixtures can be obtained 

 from the accompanying diagram rep- 

 resenting the solubility data for sodium 

 chloride. The solubility of sodium 

 chloride expressed in grams per 100 

 grams of water is plotted on the verti- 

 cal axis OY, and the corresponding 

 temperatures are plotted on the hori- 

 zontal axis OX. Starting at the point 

 A, representing the melting point of ice 

 (0°C.), and adding increasing amounts 

 of salt, the temperature falls along AB 

 until the concentration of salt corres- 

 ponding to B is reached. At this point 

 both salt and ice separate together and 

 the cryohydric temperature, — 22.4° C. 

 is reached. This is the lowest temper- 

 ature attainable with salt and ice. 

 Again, starting at the point D (6o°C.) 

 and lowering the temperature, the solu- 

 bility of salt in water diminishes as 

 indicated by the line DC. At C 

 ( — 0.i5°C.) the salt which separates 

 from the solution holds in chemical 



