Burning toast is a chemical change. This change occurs when the heat applied to the bread causes a reaction between the ingredients within the bread and oxygen in the air.
During this process, the structure of the bread is altered at a molecular level, leading to the formation of new substances, such as carbon (which gives burnt toast its black color) and various gases. Unlike physical changes, which can often be reversed (like melting ice back into water), chemical changes create substances that cannot easily revert to their original form. Once toast is burned, it cannot be turned back into raw bread, which is a clear indicator of a chemical transformation.