64 WATER AND SOLUTION
crucible on top of a good-sized beaker and allow to cool.
Again weigh the crucible and contents. Repeat the heat-
ing, and if a further loss of weight occurs repeat until two
successive weighings are the same. This is to ensure that all
the water is driven off. The cover is used for a double pur-
pose: to prevent fragments of the crystals snapping out of
the crucible from sudden expansion of the steam, and to
keep the heat in.
Calculate the percentage by weight of water in gypsum.
From this result calculate the number of molecules of water
of crystallization in gypsum, assuming the formula to be
CaSO 4 -nH 2 O (atomic weights on front cover page).If the experimental work is carefully done it will be found that
n is an even whole number. Thus the substances calcium sulphate
and water combine chemically in amounts proportional to simple
integral multiples of their molecular weights just as elements com-
bine in amounts proportional to simple integral multiples of their
atomic weights. Although the crystal hydrate is just as truly a
chemical compound as is calcium sulphate or water, nevertheless
the force holding the substances together in this compound is
secondary to that holding the elements together in the simpler
compounds.
- Efflorescence. Place on a watch glass some crystals of
sodium carbonate I^a 2 CO 3 -10H 2 O, zinc sulphate ZnSO 4 -7H 2 O,
or disodium phosphate Na 2 HPO 4 -12H 2 O, and leave them
exposed to the air for some time.
Observe that the surface of the crystals soon becomes white
and powdery and finally the whole crystal changes to a white
powder.
This change which is known as efflorescence is caused by loss of
water of hydration from the crystal, leaving either the anhydrous
salt or a less hydrated salt (of definite composition, however).
Every crystal hydrate has a definite vapor pressure at a given
temperature. If this vapor pressure is greater than the partial
pressure of the water vapor in the air the crystal hydrate will lose
water to the air. This will continue until all the water is lost
or equilibrium between the water vapor in the air and in the
hydrate is reached.