HYDROGEN DIOXIDE. 153 



when equilibrium is established and evaporation of the liquid apparently 

 ceases. This means that vapor particles fly back into the liquid at the same 

 rate that liquid particles leave the surface of the liquid to become vapor. If 

 the vapor pressure in any way becomes greater than the vapor tension of the 

 liquid, some vapor will be condensed to liquid. On the other hand, if the 

 vapor pressure is constantly lower than the vapor tension of the liquid, evapo- 

 ration will go on until no more liquid is left. One way of accomplishing this 

 is by free exposure of liquids to the atmosphere. Of course the amount of the 

 vapor pressure varies with the temperature. Now it is found that substances 

 containing water of crystallization have a vapor tension just as water has. 

 For example, when a crystal of sodium sulphate (Na 2 SO 4 .10H 2 0) is allowed 

 to rise to the top of a barometer tube at 9 C., it exerts a vapor tension of 5.5 

 mm. that is, the pressure of the water vapor given off by the crystal is enough 

 to depress the mercury column 5.5 mm. Those substances crystallizing with 

 water which at ordinary temperature exert a vapor tension greater than the 

 pressure of the water vapor in the atmosphere, are efflorescent and must be 

 kept in closed containers, just as water must be to prevent loss of water. 

 When the vapor tension of the substances is about the same or less than the 

 atmospheric vapor pressure, the substances are stable and need not be carefully 

 bottled, except to keep them clean. The average pressure of the water vapor 

 in the atmosphere at 9 C. is about 5 mm. At this temperature crystals of 

 sodium sulphate have a vapor tension of 5.5 mm. and are efflorescent, while 

 those of copper sulphate have a vapor tension of 2 mm. and are stable in the 

 air. 



Deliquescent substances are always very soluble in water. A layer of 

 moisture condenses on these just as it does on all bodies exposed to the atmo- 

 sphere. In the case of extremely soluble substances, the condensed moisture 

 forms a thin layer of very concentrated solution upon their surfaces. Concen- 

 trated solutions have a vapor tension less than that of water, and much less 

 than the atmospheric vapor pressure. The result is that water vapor continues 

 to condense from the atmosphere upon the substances until they dissolve and 

 form solutions so dilute that their correspondingly increased vapor tension bal- 

 ances the vapor pressure of the moisture in the atmosphere. 



0-H 

 Hydrogen dioxide, Hydrogen peroxide, H 2 O 2 , or I This 



O H 



compound may be obtained in aqueous solution from several metallic 

 dioxides which, when treated with an acid, yield a portion of their 

 oxygen to water. 



Sodium dioxide and barium dioxide are the compounds chiefly 

 employed in its manufacture, the acid used being either carbonic, 

 hydrochloric, hydrofluoric, sulphuric, or phosphoric acid. The de- 

 composition, when sulphuric acid and barium dioxide are used, is 

 this : 



BaO 2 + H 2 SO 4 = BaSO 4 + H 2 O 2 . 



