* Change in Composition: Milk contains lactose, a sugar. When bacteria (like Lactobacillus) are introduced, they convert lactose into lactic acid. This changes the chemical composition of the milk.

* Irreversible Process: You can't simply reverse the process by adding heat or other methods. The milk proteins have been altered by the lactic acid, resulting in a new substance (curd).

* New Properties: Curd has a different texture, taste, and smell compared to milk. This is due to the chemical changes that have occurred.

In summary, the conversion of milk to curd involves a change in the chemical composition of the milk, leading to the formation of a new substance with different properties. This makes it a chemical change.