UPSC Prelims 2014 · Question 50 of 100

UPSC Prelims 2014 question on Chemical Change Examples

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.

  1. 1 and 2 only
  2. 3 only
  3. 1, 2 and 3
  4. None
Show answer

Answer: B. 3 only

Verdict

The answer is 3 only. Souring of milk is a chemical change, while crystallisation of sodium chloride and melting of ice are physical changes.

Analysis

Statement 1 is INCORRECT. Crystallisation of common salt from solution rearranges the same substance into a solid lattice; no new substance appears and the change is reversible by dissolving. Statement 2 is INCORRECT. Melting of ice is a change of state of the same chemical compound. Statement 3 is CORRECT. Bacteria convert lactose into lactic acid, a substance not previously present, the change cannot be reversed, and the taste, texture and pH all shift as a result.

Source

NCERT Class VII Science, Chapter 6.

How to crack it

Apply one test to each item, is a new substance formed, and support it with two secondary checks, is the change reversible and is there an energy or property change beyond state. Changes of state and changes of physical form almost always come out physical, while anything involving souring, rusting, burning, digesting or fermenting comes out chemical. Learning the verbs that signal chemical change is faster than learning examples.

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