Salts of zinc , tin, aluminum and magnesium which give white residue in the charcoal cavity test, on hating with cobalt nitrate form coloured double oxides as

2Co(NO₃)₂     →      2CoO + 4NO₂ + O₂
CoO + ZnO      →      CoO.ZnO
                                       Green

  CoO + ZnO    →     CoOZnO
                                    Dirty greem

CoO + AI₂O₃    →     CoOAI₂O₃
                                           Blue

CoO + MgO      →      CoO. Mgo
                                          Pink

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