Theory:

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 (i.e. they cannot be brought back to their original form) in nature. These changes are permanent.
Example:
Burning of candle
Rusting of iron
Burning of paper
Burning of matchstick
Formation of curd from milk
Cooking a food
Heating sugar to form a caramel, etc.
Bursting a balloon: If a balloon is blown to its full size and pricked with the pointed tip of a pencil or needle, it cannot regain its original form.
 
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Bursting a balloon
  
Burning of a matchstick: When the matchstick is burned, it turns into ashes when it undergoes complete combustion. The ashes have an entirely new chemical composition than that of the unburnt matchstick.
 
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Burning of matchstick
  
Baking a roti: When the dough is baked into a roti, it cannot be changed into a dough again. These changes cannot be reversed.
 
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Baking a roti
  
Pot making: A potter makes a pot using a lump of clay. The shaped pot can be rolled again into a lump which is a reversible process. But, when the same pot is baked in an oven, it cannot be shaped into a lump of clay again. So, this process becomes irreversible.
 
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Pot making