A decomposition reaction is a chemical reaction in which some chemical bonds in a compound are broken and simpler substances are formed. The breaking of chemical bonds requires the addition of energy, usually in the form of heat.
When a compound is heated, its atoms move about more vigourously. This movement can break chemical bonds. For example, if calcium carbonate is strongly heated, it decomposes into calcium oxide and carbon dioxide.
CaCO₃(s) → CaO(s) + CO₂(g)
In some compounds, the energy needed for decomposition is so small that it can be supplied by a minor shock, such as a physical impact. For example, nitrogen triiodide, NI₃, decomposes explosively into purple iodine vapour and nitrogen gas when touched by a feather.
2NI₃(s) → 3I₂(g) + N₂(g)
This video shows how heating a sample of sodium bicarbonate will cause its decoposition.
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..