* Chemical Changes: The process of toasting bread involves the breakdown of complex carbohydrates (starches) into simpler sugars. This happens due to the Maillard reaction, a chemical reaction between sugars and amino acids. This reaction is responsible for the browning and flavor development in bread.

* New Substances: The Maillard reaction creates new chemical compounds, such as melanoidins, which give toasted bread its characteristic color and aroma. These are not present in the original bread.

* Irreversible: The browning and flavor changes are irreversible. You can't easily turn toasted bread back into its original state.

While there are some physical changes involved (like the bread becoming crispy), the significant changes are chemical.