Answer:
The decomposition reaction is a reaction in which a compound is broken down into its constituents or parts (two or more than two).
If we talk about reaction of CuCO3 (solid copper(II) carbonate) and Limewater it is a decomposition reaction in which CuCO3 when heated, it decomposes into Copper 2 oxide (CuO) that is black in color, the reaction also produces carbon di oxide CO2 .
CuCO3 (s)-----------------> CuO (s) + CO2 (g)
If the burning of CuCO3 was performed in a container whose outlet lead to the vessel containing lime water Ca (OH)2, what happens is, the lime water reacts with CO2 to form Calcium carbonate.
Ca(OH)2 (aq) + CO2 (g)-----------------> CaCO3 (s) + H2O (l)
So, here we see that as a result of this reaction, CuCO3 is decomposed into several constituents, and the reaction was performed in the presence of heat, that is why it is called a decomposition reaction.
Hope it help!
