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Yes, it is a chemical change (reaction) though it carried out by microorganisms (Lactic acid bacteria):

Out of glucose through the Embden-Meyerhof-Parnas (EMP) pathway yielding two moles of pyruvate.

  • glucose + 2 NAD+ + 2 ADP + 2 Pi → 2 pyruvate + 2 NADH + 2 H+ + 2 ATP + 2 H2O

Intracellular redox balance is maintained through the oxidation of NADH, concomitant with pyruvate reduction to lactic acid.

Two steps in short formula:

C6H12O6 --'EMP'--> 2 C3H3O3- + 2 H+ --'Reduct'--> 2 C3H6O3

and in words:

glucose -> 2 pyruvate (2-oxopropanoate) + 2H+ -> 2 lactic acid (2-hydroxypropanoic acid)

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12y ago
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9y ago

Yes, it is a chemical change. Milk contains sugar in the form of Lactose, which on microbial action breaks down to Lactic acid. This lactic acid lowers the pH of the milk making it sour.

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9y ago

Yes, lactose is converted to lactic acid which sours the milk

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8y ago

It is a chemical change because there is a chemical reaction occurring that is rotting the milk. An example of a physical change would be if the milk was evaporated or frozen.

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11y ago

Yes, milk turning sour is a chemical change because you cannot turn sour milk back into normal milk, for it contains bacteria.

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12y ago

Yes.

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Q: When milk turns sour is that a chemical change?
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