Is decomposing a chemical or physical ch ange?

Both, usually. Consider a compost heap in the back garden - this is decomposing plant matter. The decomposition is chemical as compounds break down into smaller, simpler organic molecules and heat is given off (exothermic). Decomposition is also physical as some of the simpler molecules produced by decomposition includes liquids and gases so a solid decomposes into solids, liquids and gases.