Charcoal cavity test

Make a small cavity on rectangular piece of charcoal. Take small quantity of a salt mixture and double quantity of anhydrous Na₂CO₃ or mixture of Na₂CO₃ & K₂CO₃ in the charcoal cavity. Wet the mixture with 2-3 drops of water and heat it in reducing flame. Observe the colour of bead or incrustation formed due to heating.

Observation                                                                  (Probable) Inference

a)Formation of metallic bead

1.White, shining and makes black spot on paper   Ag⁺

2. White and makes black spot on paper               Sn²⁺,Sn⁴⁺,Pb²⁺

3.Red coloured bead                                                     Cu²⁺

4.Purple white bead                                                     Bi³⁺,Sb³⁺

5.Brown coloured metallic spots having magnetic properties  Fe³⁺,Ni²⁺,Co²⁺

b)Formation of incrustation

1.White incrustation                                                       Sb³⁺,SbO

2.Yellow incrustation                                                       Bi³⁺,PbO

3.Yellow incrustation makes black spot on paper            Pb²⁺

4.White but yellow in hot condition                                  ZnO, SnO

5.Yellow but orange in hot condition                                  BiO

6.Brown coloured incrustation                               CdO

7.White incrustation vapourisable

   & having garlic odour                                          As₂O₃

In charcoal cavity test, the basic radicals form metallic carbonates on heating with Na₂CO₃. These metallic carbonates then dissociate in the form of their oxide or metal. For example,

CuSO₄ + Na₂CO₃ ® CuCO₃ + Na₂SO₄

CuCO₃  ® CuO + CO₂

                  Incrustation

CuO + C ® Cu + CO

                   Bead