CHANGES IN STATE OF MATTER 39 



the liquid state to the solid. As this change is taking 

 place the temperature gradually rises, as is indicated 

 on the thermometer. 



When a liquid changes to a solid, heat is given off 

 or liberated. The amount of heat given off is equal 

 to the amount consumed in changing the substance 

 from a solid to a liquid. For example, the amount of 

 heat lost by freezing water is exactly equal to the 

 amount of heat absorbed by melting ice. 



The temperature of the atmosphere near a lake or 

 river in which the water is freezing, is higher than that 

 of the surrounding atmosphere because of the heat 

 given off by the freezing water. When the water at 

 the surface of a lake or river freezes, it gives out its 

 heat to the air immediately above. Ice and water are 

 both poor conductors of heat, and the layer of ice once 

 formed over the surface prevents the underlying water 

 from giving off any more heat. This causes the freez- 

 ing to be very slow. It also accounts for the compara- 

 tively slight depth to which the water freezes. 



Farmers sometimes make use of this fact by placing 

 in their cellars tubs of water, which, as it freezes, gives 

 out heat and thus prevents vegetables from freezing. 

 Fortunately, most winter vegetables do not suffer from 

 cold until a temperature of several degrees below the 

 freezing point is reached. 



Melting* points. At ordinary pressure of the air, 

 ie melts at 0C. This temperature is known as the 

 meltina voint of ice. Other substances do not melt at 



