The explosion of fireworks is classified as a chemical change because it involves a transformation that results in new substances being formed. When fireworks are ignited, the chemical reactions that occur involve the rapid oxidation of various compounds, typically metals and oxidizers, which release energy in the form of heat and light.
During this process, the chemical bonds between atoms in the reactants are broken and new bonds are formed in the products. For instance, when potassium nitrate (an oxidizer) reacts with charcoal and sulfur (which serve as fuel), the result is a variety of new compounds and gases, including carbon dioxide and nitrogen gases, among others.
This transformation is irreversible under normal conditions; once the fireworks have exploded and the materials have reacted, you cannot return them to their original states. The bright colors that we see are also due to the combustion of metal salts, which emit light at specific wavelengths when heated. This further demonstrates that new substances have been created during the explosion.
In summary, the explosion of fireworks is a chemical change because it involves the formation of new substances, the release of energy, and an irreversible reaction process.