The setting of curd is a chemical change because it involves a chemical reaction where lactose (milk sugar) is converted into lactic acid by bacteria through fermentation. This results in the formation of a new substance—curd—which has different properties such as taste, smell, and composition compared to milk. Importantly, this change is irreversible; once the curd is formed, it cannot be changed back into milk. These facts show that a chemical transformation has taken place, not just a physical change.
Key Points Showing Chemical Change in Curd Setting
- Bacteria like Lactobacillus convert lactose into lactic acid: C6H12O6→2C3H6O3\text{C}6\text{H}{12}\text{O}_6\to 2\text{C}_3\text{H}_6\text{O}_3C6H12O6→2C3H6O3 (chemical fermentation reaction).
- Change in chemical composition from sugars to acid.
- Formation of curd by coagulation of milk proteins due to acid interaction.
- Different taste, smell, and other properties of curd vs. milk.
- Irreversibility: curd cannot be converted back to milk.
These factors collectively confirm that setting of curd is a chemical change because new substances form and the process cannot be reversed by simple physical means.
