When chalk (calcium carbonate, CaCO₃) is heated, it undergoes a thermal decomposition reaction. This reaction breaks down the calcium carbonate into calcium oxide (CaO) and carbon dioxide (CO₂).

Here's the chemical equation for the reaction:

CaCO₃ (s) → CaO (s) + CO₂ (g)

Explanation:

* CaCO₃ (s): Solid calcium carbonate (chalk)

* CaO (s): Solid calcium oxide (quicklime)

* CO₂ (g): Gaseous carbon dioxide

This reaction is also known as calcination and is used to produce quicklime, which has various industrial applications.