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why cannot a chemical change be reversed

Characteristic of the physical change

1.     No new product is formed. The molecular composition remains totally unaltered. During the physical change only the arrangement of molecules gets altered leading to the change in state. For example, the molecular compositions of ice and water remain same.
2.     The change is temporary and reversible. If the cause producing the change is removed, then the reaction gets reversed. For example, ice on melting forms water and water on freezing produces ice.
3.     No energy changes take place as a result of physical change. The energy required to bring about a physical change is generally equal to the amount of energy required to reverse the change. Therefore there is no change in energy.
4.     The mass of the substance remains same during a physical change. There is no involvement of mass during physical change but only energy is added or removed. No matter is added or removed during a physical change and hence the mass of the substance remains the same.

Characteristic of Chemical Change

1. A change during which the molecular composition gets totally altered, that is change in which a new product is always formed is called a chemical change.
2.     Chemical change results in the formation of a new product. An entirely new product is formed with complete difference in the molecular composition.
3.     The change is permanent and irreversible. It means that the chemical change cannot be reversed by altering or changing the experimental conditions.
4.     The mass of the substance gets altered during a chemical change. During a chemical change either the mass is added or removed. It results in either the increase in weight or decrease in weight of the substance.
5.     Energy changes take place during a chemical change. There is a difference in energy in the breaking of old bonds in reactants and formation of new bonds in products. Energy is either absorbed or released during the chemical change.
6.     If the energy is released then the reaction is known as exothermic reaction and if the energy is absorbed then the reaction is known as endothermic reaction.