ELEMENTS AND WATER 65Vapor pressure in milli-
meters of mercury at 20°
Na 2 SO 4 -10H 2 O 13.25
Na 2 HPO 4 -12H 2 O 13.1
Na 2 CO 3 -10H 2 O 12.6
ZnSO 4 -7H 2 O 10.07
SrCl 2 -6H 2 O 5.8
CuSO 4 -5H 2 O 4.8
BaCl 2 -2H 2 O 4.04
The partial pressure of the water vapor in the air varies from
day to day, but the average is about 8-10 mm. Hydrates like
Na 2 CO3-10H 2 O will lose water on exposure to such air, while salts
like BaCl 2 -2 H 2 O will not lose water unless the water vapor in the
air is unusually low.
Heating blue vitriol increases its aqueous tension, and thus
the dehydration by heating is in fact an artificially induced efflor-
escence.
- Deliquescence. Place on a watch glass small lumps of
any of the following compounds: calcium chloride, zinc
chloride, potassium hydroxide, sodium hydroxide, ferric
chloride. Note that in a few minutes the surface becomes
covered with moisture, and that after a longer time the solids
have completely changed to liquid. This liquefaction is due
to condensation of the water vapor of the air at the surface
of the solid and the dissolving of the solid material. All
deliquescent substances are very soluble in water. The ten-
sion of the vapor escaping from water is always lessened by
substances dissolved in it, and when this tension is lowered
to below that of the water vapor existing in the atmosphere,
water is absorbed by the solution from the air. In general,
substances will deliquesce when the aqueous tension of their
saturated solutions is less than the tension of water vapor in
the air.
ELEMENTS AND WATER
When substances like salts and water form compounds the
force of attraction is of a subordinate character, and the formula
of a hydrate is usually written with a period between the formula
of the salt and that of the water, for example, CuSO 4 -5H 2 O.
This indicates that the " primary " valence of the element is satisfied