storing milk by putting it into the freezer milk turning sour when left in a warm place mixing milk into a glass with chocolate powder allowing milk to evaporate, leaving behind powdered milk
Which of these is an example of a chemical reaction?

Options:
- storing milk by putting it into the freezer
- milk turning sour when left in a warm place
- mixing milk into a glass with chocolate powder
- allowing milk to evaporate, leaving behind powdered milk

milk turning sour when left in a warm place