When calcium carbonate is heated, it undergoes thermal decomposition to form calcium oxide and carbon dioxide. The reaction is called thermal decomposition, which is a type of chemical reaction that occurs when a substance is heated and breaks down into two or more simpler substances. The reaction is represented by the equation:

CaCO3(s) → CaO(s) + CO2(g)

The reaction is reversible, and at room temperature, calcium oxide will react with carbon dioxide in the air to form calcium carbonate. However, above about 1200 K, the opposite reaction occurs, and calcium carbonate decomposes to form calcium oxide and gives off carbon dioxide. This is what happens in lime kilns where limestone (calcium carbonate) is heated to form lime (calcium oxide) . The reaction is highly exothermic, and the small amount of water added is partly converted to steam in the process.