Toasting bread is a chemical change since changing the bread to toast cannot be reversed. A chemical change affects the very essence of the object since the substances available at the beginning of the process are no longer available after completion of the process.
Chemical changes affect the nature of the object in question and normally occur through a chemical reaction. A physical change on the other hand can be reversed in such a way that the original object is created once again. Both physical and chemical changes do not affect the mass of the object in question since the law of conservation states that mass remains the same before and after a physical or chemical change.
Another way to look at chemical change is that the particles that make up an object are broken down into finer particles that form a new object. Once these particles have been broken down, there is no way to join them together once again. On the other hand, physical change involves rearranging f particles that make up a given object or substance.
Other examples of chemical change include the rusting of metal, milk turning sour, fuel burning, the body digesting food that has been eaten and when a piece of paper is set on fire.