130 THE PRODUCTION AND USE OF HEAT 



amount of heat to turn water into vapor after it has been 

 heated to the boiling temperature. We can get a good idea 

 of the amount required by performing an experiment. 



Exercise 39. Amount of Heat Required to Vaporize Water 



Weigh a tin quart cup or small tin basin. Fill the cup half full of 

 water and crushed ice or snow. Weigh again. Stir the water until 

 the ice is just melted. The water should now be at 0C. Quickly 

 place the cup over a flame and note the exact time by the watch. 

 The height of the flame must not be changed during the experiment. 

 Watch the water till it begins to boil. Note the exact time again. 

 Record the number of minutes required to bring the water to the 

 boiling point. Allow the water to continue boiling about twice as 

 long as was required to bring it to the boiling point. Now remove 

 the flame and record the exact time. Weigh the vessel and contents 

 again. 



Record your results as in the table below. 



DISCUSSION OF EXERCISE 39. A student performing this 

 experiment got the following record: 



Weight of 1-qt. tin basin 86 grams 



Weight of basin and water 309 grams 



Weight of the water alone 223 grams 



Began heating the water at 2-33-30 



Water began boiling at 2-42-40 



Time required to boil the water 9 min. 10 sec., or 550 sec. 



Let the water boil till 3-1-40 



Water boiled for 19 min., or 1140 sec. 



Weight of the basin and water after boiling 219 grams 



1 heref ore, weight of the water vaporized 90 grams 



From this record we see that it required only 550 seconds to 

 raise 223 grams of water from 0C. to 100C.; but that it 

 required 1140 seconds, while applying the same heat, to vapor- 

 ize 90 grams of water. Now 90 grams was but 9 %23 of the 

 whole amount of water or 0.403 of it. To have evaporated 

 all of the water would have required 1140 seconds -f- 0.403 or 

 2828 seconds. From this experiment we conclude that it re- 

 quires 282 %5o times, or 5.14 times, as much heat to vaporize 

 a certain quantity of water as is required to raise that 



