What is thermal decomposition?

• A. Addition of water to form polymers
• B. Use of heat to break down large molecules into smaller molecules
• C. Condensation of monomers
• D. Removal of oxygen

Answer: (B)

Thermal decomposition is when heat is used to break a substance apart into smaller substances by breaking chemical bonds. The best answer states that heating breaks down large molecules into smaller ones, because providing enough energy allows the bonds holding the molecules together to be overcome, producing simpler substances. For example, heating calcium carbonate gives calcium oxide and carbon dioxide, and heating a hydrated salt like copper sulfate pentahydrate removes water to give the anhydrous salt and water. The other ideas describe processes that don’t involve breaking the substance into smaller pieces with heat: adding water to form polymers is more like polymer formation or hydrolysis in some contexts, polymerization (condensation of monomers) builds larger molecules rather than breaking them apart, and simply removing oxygen isn’t the general definition of thermal decomposition.