UPSC Prelims 2014 Q70: Science & Technology – Chemistry
Which of the following is/are the example/examples of chemical change?
1. Crystallization of sodium chloride
2. Melting of ice
3. Souring of milk
Select the correct answer
using the code given below.
- (a) 1 and 2 only
- (b) 3 only
- (c) 1, 2 and 3
- (d) None
Answer: (b) 3 only
Explanation
Chemical change and physical change
In a chemical change, a new substance is formed and the change is usually not reversible.
In a physical change, only the form or state changes.
| Process | Chemical change? | Reason |
|---|---|---|
| 1. Crystallization of sodium chloride | No | Salt stays salt; only its form changes |
| 2. Melting of ice | No | Water changes state from solid to liquid |
| 3. Souring of milk | Yes | Lactic acid bacteria turn lactose into lactic acid, a new substance |
More examples:
| Physical change | Chemical change |
|---|---|
| Melting, boiling, freezing | Rusting of iron |
| Dissolving sugar in water | Burning of fuel |
| Breaking glass | Cooking of food, ripening of fruit |
Answer: (b) 3 only.
Why each option is right or wrong
- (a) Incorrect. Crystallization and melting of ice are physical changes.
- (b) CORRECT. Only the souring of milk is a chemical change.
- (c) Incorrect. Statements 1 and 2 are physical changes.
- (d) Incorrect. Souring of milk is a chemical change, so 'None' is wrong.
Key point
Chemical change: new substance formed (souring of milk, rusting, burning). Physical change: only state or form changes (melting, crystallization).
Remember
Souring of milk = chemical change.
Common mistakes
- Treating crystallization as chemical because crystals look new.
- Thinking any change of state is chemical.
- Forgetting that souring makes lactic acid.
Exam tip
Ask: is there a new substance? Ice and water are the same substance; curd and milk are not.
Sources
- NCERT Class VII — Science, Ch 5 (Physical and Chemical Changes)
- NCERT Class IX — Science, Ch 2 (Is Matter Around Us Pure?)