Chalk is primarily composed of calcium carbonate (CaCO3). When subjected to strong heating, it undergoes thermal decomposition, a chemical process where heat causes a compound to break down into simpler substances. In the case of chalk, calcium carbonate breaks down into calcium oxide (CaO) and carbon dioxide (CO2) gas.

The chemical reaction for this process is:

CaCO3 (chalk) → CaO (calcium oxide) + CO2 (carbon dioxide)

During this decomposition, the carbon dioxide gas produced escapes from the heated chalk, causing tiny cracks and fissures to form within the structure. These cracks and fissures result in the slight crumbling of the chalk, leading to a reduction in its structural integrity.

Additionally, as the carbon dioxide gas is released, it can react with moisture present in the air, forming carbonic acid (H2CO3). This process contributes to further erosion and weakening of the chalk structure, making it more susceptible to crumbling.

It's important to note that the extent of crumbling depends on the level of heating applied. At higher temperatures and with prolonged exposure to heat, chalk decomposition accelerates, leading to more pronounced crumbling and even complete disintegration of the material.