CHAPTER XI 



PRESSURE AND CHANGE OF STATE 



61. Boiling Affected by Pressure. In previous studies, we 

 have discovered that heating a substance causes it to expand 

 by pushing its molecules further apart. We know also that if 

 the heating is continued long enough, most substances become 

 gases and take up a much greater space ( 53) . When anything 

 happens to prevent this expansion, how- 

 ever, it naturally affects the change of 

 state and requires much more heat to 

 accomplish it. Under ordinary condi- 

 tions, water boils at sea level at a tem- 

 perature of 100C., but when the pres- 

 sure of the air over water is doubled, we 

 must raise the temperature to 121C. 

 before boiling occurs. Reducing the 

 pressure has of course the opposite effect. 

 If the air over an evaporating liquid is 

 pumped away, it will boil at a very low 



FIG. 22. Boiling under f , ' f . , , , 



diminished pressure effect- temperature; in fact, if the pressure is 



ed by cooling air in bottom properly regulated water may be boiled 



and frozen at the same temperature. 



In boiling syrup and other thick liquids, which there is danger 

 of burning, they are often placed in closed vessels and the 

 vapor pumped away. This not only lowers the boiling point 

 but enables them to boil faster (89, 104). An understanding 

 of these facts enables us to ascertain the height of different 

 places above sea level by simply boiling pure water there. 

 As we ascend above sea level, we rise above part of the air 



66 



