Q. Which of the following is/are the example/examples of chemical change?
- Crystallization of sodium chloride
- Melting of ice
- Souring of milk
Select the correct answer using the code given below. (UPSC Prelims 2014)
Answer:
3 only
Notes: The correct answer is
[B] 3 only.To distinguish between these types of changes, it is essential to look at whether a
new chemical substance is formed or if the change is merely in the physical state or appearance of the matter.Analysis of the Processes:
- Crystallization of sodium chloride (1 — Physical Change): Crystallization is a technique used to separate a solid from a liquid. When salt (NaCl) is dissolved in water and then crystallized out (through evaporation), the chemical structure of the salt remains exactly the same. No new substance is created; it simply changes from a dissolved state to a solid crystal state.
- Melting of ice (2 — Physical Change): This is a classic change of state. Both ice and liquid water are chemically H2O. Melting can be easily reversed by freezing, and the chemical bonds within the water molecules are not broken or rearranged to form something else.
- Souring of milk (3 — Chemical Change): This is a chemical process (fermentation). Bacteria (Lactobacillus) convert the lactose (sugar) in milk into lactic acid. This process changes the chemical composition of the milk, creates a new substance, and results in a change in taste, smell, and texture that cannot be reversed by physical means.
Key Differences: Chemical vs. Physical Change| Feature | Physical Change | Chemical Change |
| New Substance | No new substance is formed. | One or more new substances are formed. |
| Reversibility | Usually easily reversible. | Usually irreversible. |
| Energy | Small energy changes involved. | Large energy changes (heat/light) often occur. |
| Examples | Boiling, Shredding, Mixing sand/water. | Burning, Rusting, Digestion, Ripening of fruit. |