WHAT ARE THE CHARACTERISTICS OF HEAT? 115 



water until the air has been driven out. Remove the heat. Close 

 the flask with a stopper holding a thermometer. As the steam 

 condenses, the pressure on the water is reduced. After the bubbling 

 has stopped, condense more steam by placing a cold wet cloth or ice 

 around the upper half of the flask. Does the water boil again? 

 At what temperature did you boil water? After the temperature 

 gets below 90 C. or 190 F. and there is no boiling, place the heat 

 under the flask for a moment. Result? 



Why Pressure Cookers Are Useful. At sea level water 

 under average atmospheric pressure boils at 212 F. 

 Both water and steam have the same temperature. 



People who live on . 



high mountains find 

 that water boils be- 

 fore it reaches 212 F. 

 This is because the 

 pressure of the air is 

 less and the boiling 

 point of water de- 

 pends upon the pres- 

 sure on its surface. 

 On some high moun- 

 tains boiling water is 

 not hot enough to 

 cook vegetables, so 

 they must use some 

 other method or in- 

 close the water in a 

 vessel to prevent the 

 escape of steam. The 

 increased pressure 

 raises the boiling point 

 and makes cooking possible. In the pressure cooker where 

 steam is not allowed to escape, the pressure increases and 

 the temperature rises with the rise in pressure. This ac- 

 counts for the more rapid and thorough cooking of foods 



Wright Pierce 



In what ways is a pressure cooker useful? 



Why would it be useful at high altitudes ? What 



scientific principle underlies this value? 



