CHAPTER XIII 

 HEAT OF FUSION AND DISSOLUTION 



65. If water is placed over a fire, its temperature 

 slowly rises until it reaches the boiling point. The heat 

 then applied is used to change the water into steam. The 

 temperature of the steam is the same as that of the 

 water. What is the heat required to change water into 

 steam called? If heat is taken from water its tempera- 

 ture will continue to decrease until it reaches the freezing 

 point. Under ordinary conditions the temperature of 

 liquid water cannot be lowered below the freezing point, 

 or o C. If heat is taken from water at o C. it begins 

 to freeze and change to a solid form. The temperature 

 of the ice just frozen is the same as the water in which 

 it is, that is, o C. A relatively large amount of heat is 

 given out when a unit mass of water freezes. 



If you want to change ice into water a relatively 

 large quant 'ty of heat will have to be applied just to 

 make it melt, without changing its temperature. The 

 temperature of the melting ice cannot be changed. It 

 will remain o C. until all is melted. All the heat added 

 is used to change it from a solid to a liquid form. The 

 heat required to melt one gram of ice is the same as the 

 amount of heat given out when one gram of water freezes. 

 (What law does this suggest?) 



66. Heat of Fusion. The heat of fusion of a substance 

 is the number of calories of heat required to melt one gram 

 of that substance without changing its temperature. The 

 amount of heat required to melt a substance is the same 



