CHANGES IX STATE OF MATTER 35 



sufficiently cooled. Some change directly from gases 

 into solids, or solids into gases, without assuming the 

 intermediate liquid state. For instance, when crystals, 

 of iodine are heated they pass directly into the gaseous 

 state. On a cold winter day, snow or ice on a cement 

 walk will disappear and yet the walk has not been 

 wet at all. The snow or ice passed from the solid to 

 the gaseous state without becoming water. This water 

 vapor may then pass directly back to the solid from the 

 gaseous state; as, for instance, when snow and frost 

 are formed. It is impossible to convert some solids 

 into liquids, because if heated beyond a certain tem- 

 perature they burn. Wood and paper, for example, 

 do not become liquid when heated. 



Change from solid to liquid, and liquid to gas. 

 Changes in the state of matter are most readily brought 

 about by changes in temperature. In raising a body 

 from a lower to a higher temperature, heat is consumed. 

 For instance, heat is consumed in raising 100 c.c. of 

 water from 25 C. to 35 C., or in raising ice from 

 _10C. to 2C. 



Heat, however, is also consumed in changing a solid 

 to a liquid or a liquid to a gas. If ice at 10 C. is 

 slowly heated, a thermometer, imbedded in the ice, 

 will show a gradual rise in temperature until 0C. is 

 reached. Then for some time the thermometer will 

 show no. rise in temperature. During this time the ice 

 is melting. 



As soon as the ice has melted, the rise in temperature 



