How do you know when a decomposition reaction is complete?
A decomposition reaction is complete when the mass of the container and its contents no longer changes on heating.
For example, you may wish to determine the mass percent of CO₂ in CaCO₃. You heat the CaCO₃ to decompose it into CaO and CO₂ according to the equation:
CaCO₃ → CaO + CO₂
You cool the container and and determine the mass of the container + contents. Then you heat some more and repeat the cooling and weighing. Continue the cycle until there is no further loss of mass on heating.
At this point, you know that the decomposition is complete..