Chemical Concepts Demonstrated: Cr2+, Cr 3+, amd Cr (IV) oxidation states of chromium
Observations and Explanations:
|Dish||Observation / Explanation|
|A + B||
Violet solution of Cr(H2O)6 3+.
The color of the solution changes from violet to an "acid green".
Green Cr(OH)3 precipitates from the green solution. More base will cause the solid to redissolve to give a green chromite Cr(OH)4 -solution.
The green solution changes to orange as the CrO42-/Cr2O72- ions are formed.
|E + HCl||
After the HCl is added a series of erratic color changes are observed. When the reaction is complete the solution is green.
|A, C, & D with HCl||
Solution A remains violet. Solution C changes from green back to violet. Solution D produces another green solution.
|B with HCl & Zn||
When the solution becomes acidic, several pieces of Zn are added to the dish. The bright blue color of Cr2+ (aq) will be visible momentarily, but air oxidation rapidly converts this to a green solution.
|Original solution + BaCl2 + Pb(NO3)2||
A yellow precipitate of BaCrO4 will form. Adding HCl will redissolve the precipitate and produce an orange-yellow solution. Pb(NO3)2 produces another yellow precipitate, PbCrO4.