When matter changes, there are also changes in energy.
During chemical reactions, the atoms or molecules rearrange to form new substances.
This involves the breaking of the bond between the atoms and making new bonds.
Breaking of bonds requires energy while making of bonds releases energy.
If more energy is required to break the bond than making the bonds, the reaction is endothermic (overall fall in temperature).
If less energy is required to break the bond than making the bonds, the reaction is exothermic (overall increase in temperature).
Example of endothermic reaction : photosynthesis
Example of exothermic reaction : combustion