Skip to main content
Chemistry LibreTexts

The Chemistry of Chromium (Demo)

  • Page ID
    3058
  • Chemical Concepts Demonstrated

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

    Demonstration

    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.
    chrom.gif

    Observations and Explanations

    Dish Observation / Explanation
    A + B

    Violet solution of Cr(H2O)6 3+.

    C

    The color of the solution changes from violet to an "acid green".

    D

    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. 

    E

    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.

    Contributors