EVAPORATION, BOILING TEMPERATURE, ETC. 17 



21. The Temperature of the Steam Arising from Boiling 

 Water. In the last experiment the bulb of the thermometer 

 was in the water, therefore the thermometer recorded the tem- 

 perature of the water itself. Would it record a different 

 temperature if it were raised above the surface of the water, 

 high enough so that no water could touch it, but still be sur- 

 rounded by steam? 



Exercise 14. Temperature of Steam Arising from Boiling Water 



Raise the thermometer higher in the rubber stopper used in Ex. 

 13. The bulb of the thermometer should be up in the neck of the 

 distilling flask but below the side tube. Replace the lamp and bring 

 the water to a boil. What temperature does the thermometer record 

 now? See if the temperature can be raised by causing the water to 

 boil more rapidly. (Caution. Do not permit the flame to reach higher 

 than the surface of water in the flask.} 



The temperature of the steam which escapes from boiling 

 water is always a little lower than the temperature of the boil- 

 ing water itself. This is exactly what we should expect, for 

 the heat is being applied to the glass vessel and then trans- 

 ferred to the water. The water is constantly in contact with 

 glass which is slightly above the temperature of boiling water. 

 Moreover, if there are impurities in the water their pres- 

 ence will tend to change the boiling temperature; generally 

 they raise the temperature, although certain dissolved gases 

 lower it. 



DEFINITION. The temperature marked 100C., or 212F., 

 and called "boiling point" is the temperature of steam arising 

 from boiling water when the pressure is equal to that of the atmos- 

 phere at the sea level. 



In the experiment did your thermometer indicate a tem- 

 perature of 100C., or 212F. ? Can you explain why it did not? 



22. To Change the Temperature Reading from One Scale 

 to the Other. We have seen that there are 100 degrees on 

 the centigrade scale between the freezing point of water and 

 the boiling point of water; on the Fahrenheit scale there are 



2 



