Which of the following is an example of a chemical change?

Question

Question

This question was previously asked in

RRB JE CBT 2 (Mechanical Engineering) ReExam Official Paper (Held On: 04 Jun, 2025)

Answer 1

The Correct answer is Curdling of milk.

Key Points

The curdling of milk is an example of a chemical change.
A chemical change occurs when the original substance undergoes a transformation and forms a new substance with different properties.
In the case of milk curdling, the lactic acid bacteria present in the milk convert the lactose (milk sugar) into lactic acid.
The increase in lactic acid causes the milk proteins, specifically casein, to denature and coagulate, forming curd.
This process is irreversible, indicating that it is a chemical change.
The formation of curd also involves the release of a new characteristic smell and a change in texture, which are signs of a chemical reaction.
Curdling is widely used in the preparation of various dairy products like yogurt and cheese.

Additional Information

Dissolution of sugar in water
- This is a physical change as no new substances are formed during the process.
- Sugar dissolves in water to form a homogeneous solution, and the process is reversible by evaporation of water.
Freezing of water
- Freezing of water is a physical change because water changes its state from liquid to solid (ice) without altering its chemical composition.
- This process is reversible, as ice can melt back into water.
Tearing of paper
- Tearing of paper is a physical change because the chemical structure of the paper remains unchanged.
- Although the shape and size of the paper may change, its chemical properties stay the same.