MELTING-POINT MAXIMUM. 193
pronounced, but the beginning of crystallization can always be
observed distinctly by the clouding of the liquid.
To establish the other branch of the curve, determine the
freezing-points in a new series of mixtures: First, take a weighed
amount (about 30 g.) of the hexahydrate, and free it from a part
of its water of crystallization by heating it in an open dish upon
the water-bath for five or six hours, or until the loss in weight
corresponds to about 10%. Pour the sirupy liquid while still
warm into a porcelain mortar, and stir it with a pestle while it
solidifies. When cold, powder the porcelain-like mass and com-
pute the amount of water present by igniting a weighed sample
until it is completely changed to MgO. For the freezing-point
determinations, commence as in the first series with the pure
hexahydrate, and add successively portions of 1 —3 g. of the par-
tially dehydrated salt just prepared. After each addition, bring
about complete solution by fusing the mixture, and then observe
the point at which crystals begin to separate on cooling. More
highly concentrated liquids are sirupy and become turbid on
account of air bubbles, and thus the freezing-points are not so
sharply defined; it is sufficient, however, to carry the observa-
tions to about 70° on this branch of the curve.
To construct the melting-point diagram, compute for each
experiment the total amount of anhydrous magnesium nitrate
present, the total amount of water, and from these the number of
parts of anhydrous salt in 100 parts of the mixture. Plot the
latter values as abscissas and the observed temperatures as ordi-
nates. Between points corresponding to 42% and 58% of the
anhydrous salt the curve rises from about 18° to 89°, whereas
between 58% and 65$, it falls from 89° to 70°. The maximum
point of the curve corresponds to the hexahydrate, and it is thus
proved thermo-analytically that this hydrate exists as a definite
compound.
- If the magnesium-nitrate-hexahydrate/water diagram were to be carried
out to the freezing-point of pure water, it would show the existence of a second
hydrate with nine molecules of water, and of a eutectic point at —29°, between
the latter and pure water. The characteristics of the eutectic point are more
conveniently studied in the case of the system barium-chloride-dihydrate/water.
Place 10 c.c. of water in the apparatus used above and determine
the freezing-point; then add about 1 g. of crystallized barium
