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21. METALS OF GROUPS IV-VIII.
C. Group VI. (Chromium) Dem. 638-Dichmmaie
Tested Overhead ProjectionSeries Compiled by HUBERT N. ALYEA Princeton Univerriy
Observations: The Cr+6 in dichrornate is reduced to Cr+3, in the form of green CrSOa+.
continued Volcano
Dem. 642-Perchmmate
Test
T o show: The perchromate test for chromium compounds.
T o show: A miniature volcano from the thermal decomposition of ammonium dichromate.
Materials: H-3.
Materials: H-4, (NH4)2Cr207 (HANDLE WITH CARE, WASH HANDS AFTERWARDS); filter paper, ethanol.
Procedure: Project culture tube half-full of water. To it add a dropperful of dil. H2S01,then a dropperful of KpCr207-aq. Now add half a dropperful of H202 solution and quickly dump in 5 ml of ether, shake and project.
Procedure: Insert a 3 X 1 cm strip of filter paper in a coke-cap full of ammonium dichromate on the asbestos stage of H 4 . Moisten the paper with a few drops of ethanol. Project. Ignite. Observations: A mountain of green CrzOaforms; also fires flash from the erupting mass, simulating a volcano.
Note: Place a small lump of Dry Ice in a coke cap, surround it with a pile of ammonium dichromate, insert the ethanol-soaked filter paper and ignite. When the volcano begins to erupt, put a few drops of warm water on the Dry Ice to simulate "smoke." Dem. 639-Electro-lorming
PbCrOr
T o show: The formation of PbCrOa by electro-forming. Materials: E-l with P b strips for electrodes; I
