Calcium carbonate (CaCO3) needs a high temperature to decompose because the reaction is endothermic. This means that it requires energy input to break the bonds in the compound. Here's a breakdown:

* Strong Bonds: The bonds between calcium, carbon, and oxygen in calcium carbonate are quite strong. These bonds require a significant amount of energy to break.

* Decomposition Products: The decomposition of calcium carbonate produces calcium oxide (CaO) and carbon dioxide (CO2). This is a chemical change where the original molecule is broken down into simpler molecules.

* Energy Input: The energy needed to break these bonds comes from heat. The higher the temperature, the more kinetic energy the molecules have, increasing the likelihood of bond breaking.

In short, the high activation energy needed to break the strong bonds in calcium carbonate requires a high temperature to initiate the decomposition reaction.

Here's the reaction:

CaCO3(s) + Heat → CaO(s) + CO2(g)

Note: The decomposition of calcium carbonate is a reversible reaction. At lower temperatures, the reverse reaction (the reaction of calcium oxide with carbon dioxide to form calcium carbonate) is favored.