Milk turning sour is a chemical change ?
a) Physical change b) Chemical change c) Reversible change d) None of these
The correct answer and explanation is:
The correct answer is b) Chemical change.
When milk turns sour, it undergoes a chemical change. This process is typically caused by the activity of bacteria, such as Lactobacillus, which convert lactose (the sugar in milk) into lactic acid. The accumulation of lactic acid lowers the pH of the milk, causing the proteins, particularly casein, to coagulate and form curds. This is what gives sour milk its characteristic texture and taste.
A chemical change is defined as a process in which one or more substances are transformed into different substances with distinct properties. In the case of souring milk, new chemical compounds are formed, including lactic acid, which was not present in the milk initially. This transformation is not reversible by simple physical means. For example, you cannot “unsour” milk by removing the lactic acid or reconfiguring the proteins back to their original form.
The process of milk souring is also irreversible in the sense that once the milk has soured, it cannot return to its fresh state without significant intervention, such as pasteurization. This irreversible change in the chemical composition of the milk distinguishes it from physical changes, where no new substances are formed.
Therefore, milk turning sour is a clear example of a chemical change. It also demonstrates the role of microorganisms in altering the composition of a substance, which is a common example of biochemical processes in everyday life.