Hint: Physical changes can only affect or cause modification to a substance's form, but never affects the overall chemical structure. Chemical changes allow a substance to transform into a completely different substance that has a new chemical structure.
The changing of milk into a sour form can be explained in detail.
Lactose is a form of sugar that is generally found in milk. Milk also includes lactobacillus bacteria, which are bacteria that are completely normal to be found in milk and they do not cause any harm.
As milk begins to turn sour, the lactobacillus bacteria begin turning the lactose sugars found in the milk into a form of energy. However, during the conversion lactic acid is generated as a by-product of energy production. Note that when any reaction produces a new product, it becomes a chemical phenomenon, and so milk turning into a sour form is in fact a chemical process since new compounds such as the lactic acid have been produced.
Therefore milk souring is known as a chemical transition or chemical change because it ends up forming a new product that is the lactic acid, hence leaving the milk sour.
The regular scenarios where the souring of milk takes places are:
If we place the container carrying milk in a place that experiences moisture content, this causes the milk to sour quickly.
Another case is if the container having the milk is exposed to high temperatures, it will enhance the process of souring, so this should be avoided.
Also if the container is not closed properly or if any dust enters it and contaminates the container then the milk present in it turns sour easily.