32 OTHER FACTS ABOUT HEAT 



21. The Amount of Heat Absorbed. The amount of heat 

 which must be constantly supplied to water at the boiling point 

 in order to change it into steam is far greater than we realize. 

 If we put a beaker of ice water (water at o C.) over a 

 steady flame, and note (i) the time which elapses before the 

 water begins to boil, and (2) the time which elapses before 

 the boiling water completely boils away, we shall see that it 

 takes about %\ times as long to change water into steam as it 

 does to change its temperature from o C. to 100 C. Since, 

 with a steady flame, it takes 5^ times as long to change water 

 into steam as it does to change its temperature from o C. 

 to the boiling point, we conclude that it takes 5| times as 

 much heat to convert water at the boiling point into steam as 

 it does to raise it from the temperature of ice water to that of 

 boiling water. 



The amount of heat necessary to raise the temperature of 

 I gram of water i C. is equal to I calorie, and the amount 

 necessary to raise the temperature 100 C. is equal to 100 

 calories ; hence the amount of heat necessary to convert 

 I gram of water at the boiling point into steam at that same 

 temperature is equal to approximately 525 calories. Very 

 careful experiments show the exact heat of vaporization to 

 be 536.1 calories. (See Laboratory Manual.) 



22. General Truths. Statements similar to the above hold 

 for other liquids and for solutions. If milk is placed upon a 

 stove, the temperature rises steadily until the boiling point is 

 reached ; further heating produces, not a change in tempera- 

 ture, but a change of the liquid into steam. As soon as the 

 milk, or in fact any liquid food, comes to a boil, the gas flame 

 should be lowered until only an occasional bubble forms, be- 

 cause so long as any bubbles form the temperature is that of 

 the boiling point, and further heat merely results in waste of 

 fuel. 



