If its physical change thn we can undo it....iff its sours in warm room then it's a chemical change....its bcuz of some lactobacilli survive and can cause spoilage eventually.

Physical Change
A physical change is a change in which the substance retains its original chemical composition and properties. In the case of milk, the process of heating it to make it soar is a physical change.

Explanation
- Heating milk causes the proteins in it to denature, leading to the separation of the whey and casein proteins. This results in the curdling or souring of the milk.
- The changes that occur during the process of souring milk are purely physical in nature. The chemical composition of the milk remains the same; only its physical properties, such as texture and taste, are altered.
- The souring of milk is reversible, as it can be returned to its original state by heating it again or by adding certain substances to neutralize the sour taste.
- Unlike a chemical change, where new substances are formed, a physical change does not involve the creation of any new compounds.

Conclusion
In conclusion, the souring of milk is a physical change because it only affects the physical properties of the milk without altering its chemical composition. This process is reversible and does not involve the formation of new substances.