The event that represents a chemical change is milk turning sour.
A chemical change occurs when a substance combines with another to form a new substance, this procces in general are not reversibles.
In the case of the milk it contains a type of sugar called Lactose and also harmless bacterias called lactobacillus which uses the lactose to produce energy and creates lactic acid. This is the thing which makes milk taste sour.
I hope that the answer help you.