Interesting online CONFCHEM discussion going on right now on the ChemWiki and
greater STEMWiki Hyperlibary project. Come join the discussion.

ChemWiki: The Dynamic Chemistry Hypertext > Under Construction > Demonstrations > Additional Demos > The Chemistry of Chromium

The Chemistry of Chromium

Chemical Concepts Demonstrated

  • Cr2+, Cr 3+, and Cr (IV) oxidation states of chromium


  1. A Cr3+ solution is prepared.  Some is added to crystallizing dishes A and B.
  2. NaOH is added to the Cr3+ solution. Some is poured into dish C.
  3. More NaOH is added to the solution. Some of this is poured into dish D.
  4. 3% H2O2 is added to the rest of the solution. Some of this is pour into dish E.  HCl is added to this dish.
  • Solution E is compared with AC, and D, and HCl is added to them.
  • HCl and granulated Zn are added to dish B.
  • BaClis added to the remaining solution. HCl is then added.  Finally, Pb(NO3)2 is added.

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.

AC, & 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.



You must to post a comment.
Last modified
10:30, 2 Oct 2013



(not set)
(not set)

Creative Commons License Unless otherwise noted, content in the UC Davis ChemWiki is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 United States License. Permissions beyond the scope of this license may be available at Questions and concerns can be directed toward Prof. Delmar Larsen (, Founder and Director. Terms of Use