The option that represents a chemical change is souring of milk.
Here's the reasoning for each option:
-
Melting of chocolate: This is a physical change because it involves a change in state from solid to liquid, but the chemical composition of chocolate remains the same.
-
Condensation of water: This is also a physical change where water vapor changes to liquid water. Again, the chemical composition remains unchanged.
-
Breaking of an egg shell: This is a physical change where the structure of the egg shell is altered, but it does not change the chemical composition.
-
Souring of milk: This is a chemical change because it involves the fermentation of lactose into lactic acid, resulting in a change in the chemical composition of the milk.
Therefore, the correct answer is souring of milk.