Baking a cake is a chemical change, not a physical change. A physical change does not produce a new substance, as it only involves a change in state, like butter melting or water freezing. A finished cake, on the other hand, is entirely different from the ingredients that were used to make it.
Baking a cake involves many different chemical reactions that result in a delicious dessert. The leavening agent reacts with the other components and heat to create carbon dioxide. This gas and any steam generated are then trapped in tiny chambers throughout the protein structure created by the liquid, flour and eggs. These reactions create the texture of the cake and give it lightness.