130 GENERAL SCIENCE 



Heat Produced by Solidification. Tubs of water are sometimes 

 placed in cellars to prevent vegetables from freezing. As the tem- 

 perature of the cellar falls, the water begins to freeze first. In so doing 

 it gives out heat enough to prevent the air from falling as far below 

 the freezing point as it otherwise would. Heat continues to be 

 given out by the water as long as it is freezing. 



It is possible to observe these changes more easily in some other 

 substances than it is in ice. When we melt substances and then 

 allow them to crystallize, they give out the same amount of heat 

 which is needed to melt the crystal. This heat, which becomes 

 apparent in solidification, makes the substance warm the contain- 

 ing vessel and surrounding object. Observe the change in tem- 

 perature before, during and after the crystallization process in some 

 melted hyposulphite or soda (" hypo ") 



(a) Fill a test tube half full of crystals of "hypo," saving out a small 

 crystal for use later. Support the test tube by means of a wire holder in 

 the top of a boiler, and heat until the "hypo" is melted and the liquid has 

 cleared up. 



(6) When the "hypo "is melted, fasten the test tube in the lower clamp on 

 the ring stand. Fasten a thermometer in the upper clamp and adjust so that 

 the bulb is well down in the liquid "hypo." Fasten the thermometer in this 

 position, and do not afterward disturb either tube or thermometer. 



(c) Stand a glass of cold water on a ring of the ring stand, and raise it until 

 the liquid "hypo" in the test tube is entirely surrounded by the water. Imme- 

 diately begin to read and record the temperature shown by the thermometer 

 every half minute. (One student should hold the watch while the other 

 watches the thermometer.) 



(d) When the temperature of the liquid "hypo" has fallen to 55 F. or below, 

 drop a small crystal of ' ' hypo ' ' into the liquid ' ' hypo ' ' in the test tube. Continue 

 reading the temperature every half minute as before, and also observe closely 

 what happens in the liquid "hypo" after the crystal is dropped in. 



(e) Continue the reading until the "hypo" has solidified and the tem- 

 perature is again falling steadily or has become stationary. Do not attempt 

 to pull the thermometer out of the solidified "hypo," but put the test tube, 

 thermometer and all,' in some hot water. As soon as the "hypo" is melted, 

 remove the thermometer and wash all the material carefully. 



What would you consider the freezing point of "hypo"? 

 What happens when solidification begins? Why? 



What would be the result if "hypo" or similar material were used in a 

 bottle for a chemical hot water bottle? 



