7. Exothermic and Endothermic chemical changes

Physical change: In the physical transformation, only the physical properties such as colour, structure, size are changed. No new substances are formed. They are reversible changes.

Chemical change: Any substance that changes its chemical properties is known as chemical change. In this change, a new substance is formed, and these changes are irreversible in nature. These changes are permanent.

1. Exothermic change:

Combustion, condensation, freezing, neutralisation reaction, dissolving an acid in water are best examples of exothermic changes as heat is released for the change to occur.

 

 

This is due to the bond formation, which causes to release of heat. The formation of bonds requires less energy.

 

 

 

The burning of wood and paper releases heat energy to their surroundings, which is the best example of exothermic chemical change. The product formed is different from the reactants. This is an irreversible change.

 

  

 

The water molecules release heat to their surroundings and freezes to form ice. This is an example of exothermic physical change as this process can be reversed.

  

2. Endothermic change:

Cooking, photosynthesis, evaporation are the best examples of endothermic changes as heat is absorbed for the change to occur.

 

 

The breaking of the bond requires more energy which causes heat to be absorbed.

 

Making dosa from batter is the best example as heat is absorbed from the pan, making the batter turn into the dosa. Similarly, making vada is also an example of endothermic chemical changes.

 

  

 

The water molecule absorbs heat and evaporates as vapours. This is an example of endothermic physical change as the process is reversible.

In case if heat is absorbed, the enthalpy is positive and negative if heat is released.