Page 67 - Lab Manual & Project class 12

SYSTEMATIC QUALITATIVE ANALYSIS
5. Charcoal Cavity Test

Metallic carbonate when heated in a charcoal cavity decomposes to give
corresponding oxide. The oxide appears as a coloured residue in the cavity.
Sometimes oxide may be reduced to metal by the carbon of the charcoal cavity.
The test may be performed as follows:
(i) Make a small cavity in a charcoal block with the help of a charcoal borer.
Do not apply pressure otherwise it will crack [Fig.7.6 (a)].
(ii) Fill the cavity with about 0.2 g of the salt and about 0.5 g of anhydrous
sodium carbonate.

Maxbrain Chemistry

(a) (b)

Fig. 7.6 : (a) Making charcoal cavity (b) Heating salt in the cavity

(iii) Moisten the salt in the cavity with one or two drops of water, otherwise
salt/mixture will blow away.
(iv) Use a blowpipe to heat the salt in a luminous (reducing) flame and
observe the colour of oxide/ metallic bead formed in the cavity both
when hot and cold [ Fig. (7.6 b)]. Obtain oxidising and reducing flame as
shown in Fig. 7.7 a and b.
(v) Always bore a fresh cavity for testing the new salt.

Note : • To obtain oxidising flame
hold the nozzle of the
blowpipe about one third
(a) (b) within the flame.
• To obtain reducing flame
Fig. 7.7 : Obtaining oxidising and reducing flame place nozzle of the blowpipe
(a) Oxidising flame; and (b) Reducing flame just outside the flame.

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24-04-2018