54 CHEMICAL PHYSICS. 



densation of the gas into a liquid, by causing the gas to pass through 

 a cooling device or condenser, whereby the heat that was made latent 

 and is necessary for maintaining the gaseous state, is extracted. Dis- 

 tillation is usually carried on by boiling the liquid under atmospheric 

 pressure, but sometimes it is done under reduced pressure by exhaust- 

 ing the air from the apparatus, and then the liquid boils at a much 

 lower temperature. Distillation is a very useful process for purify- 

 ing liquids, as, thereby, non- volatile impurities may be easily removed 

 from liquids. Moreover, mixtures of liquids having different boiling- 

 points may be separated approximately into the constituents by dis- 

 tillation (see Fractional Distillation in Index). 



The process of vaporization, known as sublimation, has been men- 

 tioned under Crystals, page 21. 



The determination of the boiling-point is best done in a flask, as 

 shown in Fig. 69, If inflammable or noxious vapors are likely to 

 be evolved, they may be condensed to the liquid state by connecting 

 the exit tube of the flask with a condenser. It is important that the 

 thermometer should not be immersed in the liquid, but should be 

 surrounded only by the vapor of the liquid. Heat is applied to the 

 flask until the liquid assumes a state of active ebullition, and when 

 vapor is escaping freely and the temperature ceases to rise, the latter 

 is noted. This is the boiling-point of the liquid at the atmospheric 

 pressure prevailing at the time. 



Pure liquids, under the same pressure, always have the same boil- 

 ing-points, and this property is very important in chemistry in 

 judging of the purity of liquids. Impure liquids not only have dif- 

 ferent boiling-points from the pure substances, but also the tempera- 

 ture rises during boiling instead of remaining constant. 



The boiling-points of different liquids vary widely. Thus, mer- 

 cury boils at 357 C., water at 100 C., alcohol at 78 C., chloroform 

 at 61 C., ether at 35 C., oxygen at -182.5 C., hydrogen at 

 252.5 C., under standard atmospheric pressure. 



Latent heat of vaporization. When a liquid passes to the gas- 

 eous state, heat energy is absorbed or becomes latent^ and, conversely, 

 when a vapor is condensed to a liquid, the same heat energy is given 

 out. If the heat is not added to the liquid from the outside, the heat 

 is absorbed from the liquid itself during vaporization and its tem- 

 perature falls. The sensation of cold produced by the evaporation 

 of water, alcohol, ether, chloroform, etc., from the skin is familiar to 

 every one. It is by evaporation of moisture from the skin that the 



