Souring of milk is a chemical change.
A chemical change is defined as a change which results in the formation of a completely new substance or you can say a chemical change is nothing but simply a chemical reaction.
Souring of milk is said to be a chemical change because during that process acidification of normal milk takes place and soured milk is produced as a result.
Sour milk can be produced by either adding an acid in milk like lemon juice, vinegar etc or through bacterial fermentation.
This is how sour milk looks like.
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