Zinc carbonate (ZnCO3)
Zinc carbonate occurs in nature as calamine.
When NaHCO3 is added to Zinc sulphate solution, ZnCO3 is obtained.
ZnSO4 + 2NaHCO3 ® ZnCO3 + Na2SO4 + H2O + CO2
It is a white powder, insoluble in water.
i) Action of Heat
On heating at 300 o C , Zinc carbonate decomposes into Zinc oxide and
ZnCO3 ⎯300⎯® ZnO + CO2 ⎯ o C
i) It is used in making ointment for curing skin diseases.
ii) It is used in the preparation of cosmetics.
iii) It is used a pigment for rubber.
Purple of cassius
Purple of cassius is only a form of colloidal gold. It is purple or red in colour and after the name of Cassius who discovered it.
It is prepared by mixing very dilute solution of gold chloride with stannous chloride solution.
2AuCl3 + 3 SnCl2 ® 2Au¯ + 3SnCl4
The gold thus precipitated is adsorbed by stannic hydroxide formed by the hydrolysis of SnCl4.
SnCl4 + 4H2O ® Sn(OH)4 + 4HCl
It is used in making ruby-red glass and high class pottery.