Why does CuCO3 plus HCl turn green?

Answer 1

Cu(CO3) + 2HCl --> CuCl2 + H2O + CO2

CuCl2 product, when dry is a yellow/brown powder. However, when wet it forms a co-ordination complex with the water in the solution:

CuCl2.2H2O --> Cu(Cl)2(H2O)2

The electronic structure of transition metals and ligands cause visible colours. In this case the solution is blue/green in colour.

If you dry the solution down you will get a blue/green powder.

Drying further in a dessicator will give the yellow/brown anhydrous CuCl2.

Answer 2

When copper(II) carbonate (CuCO3) reacts with hydrochloric acid (HCl), copper(II) chloride (CuCl2) and carbon dioxide gas (CO2) are produced. The green color observed is due to the formation of copper(II) chloride, which has a greenish color.