Is milk souring a physical or chemical change?
That's a chemical change.
So chemical changes require a change on a molecular level that cannot be reversed because they form some new substance. Souring milk is not something you can reverse, and the process of it souring produces new molecules. Some other examples of chemical changes would be things that involve burning, the creation of a new gas or bubbles, or change in color, like the formation of rust.
Physical changes could also be on a molecular level, but the molecules themselves will always comprise of the same atoms. Water freezing makes the hydrogen and oxygen atoms spread farther apart, but they are all still hydrogen and oxygen atoms.
Hope that makes sense!