The setting of curd is a chemical change because it involves a chemical reaction between the milk and the bacteria present in the curd starter. Here are a few ways to show that curd setting is a chemical change:

1. Change in composition: The process of curd setting involves the fermentation of lactose, a sugar present in milk, into lactic acid by bacteria such as Lactobacillus. This chemical reaction changes the composition of milk, resulting in the formation of curd.

2. Formation of new substances: During the curd setting process, the lactic acid produced by bacteria reacts with the proteins present in milk, causing them to coagulate and form a solid mass. This formation of curd is the result of a chemical reaction between the lactic acid and proteins.

3. Irreversibility: Once the curd is formed, it cannot be transformed back into milk by any physical means. Unlike physical changes, such as melting or freezing, a chemical change is usually irreversible.

4. Energy change: The curd setting process is exothermic, meaning it releases heat energy. This energy change indicates that a chemical reaction is occurring.

5. Odor and taste changes: The bacteria present in the curd starter produce certain compounds, such as diacetyl, which give curd its distinct aroma and taste. These changes in odor and taste are the result of chemical reactions that occur during the curd setting process.

Overall, these observations support the notion that curd setting is a chemical change rather than a physical change.