Which one of the following is a chemical change?

A chemical change involves the formation of new substances with different chemical properties. Milk turning sour is a chemical change because the lactose present in milk is converted into lactic acid by the action of microbes. This process, known as fermentation, results in a new substance that cannot be easily reversed to its original state. In contrast, dissolving sugar in water is a physical change as it forms a homogeneous mixture without altering the chemical identity of sucrose. Melting of ice is a physical change involving a change of state from solid to liquid, which can be reversed by freezing [1]. Crystallization is also classified as a physical change where a solid is obtained from a solution through cooling or evaporation without forming a new chemical species. Therefore, only the souring of milk represents a chemical transformation.

Sources

1. [1] Science-Class VII . NCERT(Revised ed 2025) > Chapter 5: Changes Around Us: Physical and Chemical > 5.5 Are Changes Permanent? > p. 66