114 INTRODUCTION TO GENERAL SCIENCE 



83. BOILING FKOM ANOTHER POINT OF VIEW 



Section 23 states that change of state is due to added or 

 subtracted molecular energy, while Section 2 states that molec- 

 ular energy is heat. Since we have just learned that the at- 

 mosphere has pressure, we can now look upon the boiling of 

 water in a different way from formerly. 



As heat is added to water, its molecules move faster and 

 faster, and those which are near the surface tend to pass away 

 into the air. The molecules of the air, however, strike against 

 the outcoming water molecules and force many of them back 

 into the liquid. When the temperature of the water becomes 

 so high that the average velocity of the water molecules is 

 equal to the average velocity of the air molecules, the former 

 possess enough energy to overcome the atmospheric pressure, 

 and the water boils. 



Since boiling is the overcoming of atmospheric pressure by 

 the water molecules, if the pressure is less, the water boils at a 

 lower temperature, for less energy is necessary. Therefore 

 water boils at a lower temperature on a mountain top than in 

 a valley. The altitude may be obtained, approximately, by 

 means of a thermometer. Using a Centigrade thermometer, 

 multiply the difference between 100 and the temperature at 

 which boiling begins, by 295. This gives the altitude in 

 meters. With the Fahrenheit thermometer, multiply the 

 difference between 212 and the temperature at which boiling 

 begins, by 533. This gives the altitude in feet. Since cooking 

 is a chemical change due to temperature, a longer period of 

 boiling is necessary on a mountain than at sea level to accom- 

 plish the same result. 



In the laboratory, by means of the air pump, water may be 



