* Chemical Composition Changes: Curdling involves the breakdown of milk proteins (casein) by acids (like lactic acid produced by bacteria) or enzymes (like rennet). This alters the chemical composition of the milk, creating curds and whey.

* Irreversible: Curdling is irreversible. You can't simply "un-curdle" milk by reversing the process. This indicates a permanent change in the milk's chemical structure.

* New Substances Form: Curds and whey are distinct substances with different properties than the original milk. This formation of new substances is a hallmark of chemical change.

In contrast, a physical change:

* Doesn't alter the chemical composition of the substance.

* Is usually reversible.

* Doesn't create new substances.

Examples of physical changes to milk would be freezing it (changing its state of matter) or heating it (changing its temperature).