Chemical Reactions

In a chemical change, chemical reaction takes place and the substances undergo a change in their state. During chemical reactions, one substance reacts with another to form a new substance. The chemical composition of the new substance is different from that of the reacting species. Due to a chemical change, the chemical properties of matter also change. That means the product is entirely different from either of the reactants. Some chemical reactions may be either exothermic or endothermic in nature. In endothermic reactions, a substance absorbs energy in the form of heat and undergoes a chemical reaction. In exothermic reactions, heat energy is released. In exothermic reactions, heat energy is released.

This video describes some reversible and irreversible chemical changes.