How is copper carbonate decomposed?
As above, carbonates decompose upon heating to give the metal oxide and carbon dioxide. The mass is lost as carbon dioxide. Given measurement of the masses of calcium carbonate and its residue, before and after the decomposition reaction, you can determine the amount lost as carbon dioxide.
Sometimes, the effluent gas from this reaction is shunted to bubble out thru a solution of calcium hydroxide, which the carbon dioxide will turn milky due to precipitation of calcium carbonate: