Chemical bonds are broken by the energy that is released during the chemical reaction. This energy is created by the movement of electrons from one molecule to another as they attempt to achieve a chemical balance. The breaking of chemical bonds can easily be identified because they are often accompanied by physical changes as the original elements used in the chemical reaction take on new forms or become new elements.
According to Chemtutor, the Law of the Conservation of Mass states that there will always be the same amount of matter before and after a chemical reaction. This happens on the chemical level when molecules break apart, join together, or lose and gain electrons to form new elements. It also happens on the physical level when the chemical reaction causes elements to change their form. However, no matter can be created or destroyed during this reaction. It can only change form.
One example of a chemical reaction is the breaking down of simple sugars during fermentation. The carbon in the sugars reacts with oxygen in the air. Some of the carbon bonds in the sugar break and rejoin with the oxygen bonds to form carbon dioxide molecules. The remaining molecules form the chemical C2H5OH, which is ethyl alcohol.