68 



phate. The latter, when once more moistened, changes back 

 into the hydrate. 



Zinc sulphate + Water <= Hydrate of Zinc sulphate. 



Many common chemicals are in fact such hydrates. Thus com- 

 mon blue-stone, used in gravity batteries, is a hydrate of cupric 

 sulphate. When heated, it loses water and leaves the colorless, 

 anhydrous cupric sulphate. These are cases of simple com- 

 bination and decomposition. 



Efflorescence. Some hydrates are so unstable that the 

 water passes off, even at room temperature, when the hydrate 

 is left in an open vessel. Thus crystals of washing soda (hydrate 

 of sodium carbonate) crumble to powder (effloresce) when not 

 kept in a closed vessel. 



Sodium carbonate + Water = Hydrate of sodium carbonate. 



If a crystal of a hydrate like this is placed in the barometric 

 vacuum (Fig. 33, p. 61) a considerable vapor pressure of water 

 is indicated, so that the tendency of the hydrate to decompose, 

 when this vapor is allowed to escape, is easily understood. The 

 pressure of water vapor in equilibrium with such hydrates, when 

 partially dehydrated at ordinary temperatures, is found to be 

 greater than the average pressure of water vapor in the atmo- 

 sphere. 



On the other hand, when anhydrous cupric sulphate and zinc 

 sulphate, obtainable from the hydrates by heating only, are 

 spread out in the air, they return slowly to the hydrated con- 

 dition. They combine with the moisture in the air. The vapor 

 pressure of water in the air is greater than the pressure of water 

 vapor in equilibrium with these hydrates and their anhydrous 

 products at ordinary temperatures. Calcium chloride absorbs 

 water vapor (p. 59) because of its tendency to form a hydrate. 



Other Chemical Properties. Water combines directly with 

 some oxides. Its union with quicklime yields slaked lime or 



