EXPERIMENTS 203- Burn a strip of magnesium ribbon, held with iron pincers,
and let the ash fall in a porcelain dish. Wet the magnesium
oxide with a single drop of water and place the moist mass on
a strip of red litmus paper. A small fleck of blue shows on
the reverse of the litmus paper when the moist magnesium
oxide is applied.
Magnesium has a very strong affinity for oxygen as shown by
the intensity with which the metal burns. However, the residual
affinity of magnesium oxide for water is much smaller than that of
calcium oxide. The solubility of magnesium hydroxide is so
small that the saturated solution acts but slowly in turning
litmus blue.- To some magnesium chloride solution, add (a) some am-
monium hydroxide; (6) some ammonium chloride and then
some ammonium hydroxide. In (a) a copious white precipi-
tate is observed; in (6) no precipitate.
The weakly ionized NH4OH furnishes a sufficient concentration
of OH~ ions to cause the precipitation of Mg(0H) 2. However, as
the reaction progresses, NH4+ ions accumulate in the solution andMg++ 2C1"
2NH4OH ^ 2OH " 2NH 4 +Mg(OH) 21cause a continual lessening of the OH~ ion concentration. Since
the solubility of Mg(0H) 2 although small, is still appreciable
(0.0002 F.W. per liter in pure water) it is clear that with the OH ~
ion concentration sufficiently depressed a fairly high Mg++ ion
concentration can prevail without reaching the solubility product
of Mg(OH) 2.
In part (6) an initial addition of NH4+ ion depresses the OH ~
ion concentration so that no precipitation of Mg(0H) 2 at all takes
place.
- Dip a clean platinum wire in solutions of such of the chlo-
rides of the. alkali and alkaline earth metals as are at hand,
and observe the color imparted to the Bunsen flame when the
wire is inserted into the lower part of the flame.