Question

For which of the following processes entropy change (\(\Delta S\)) is negative?
I) Sublimation of dry ice
II) Freezing of water
III) Crystallisation of the dissolved substance
IV) Burning of rocket fuel

• A. I & II only
• B. II & III only
• C. III & IV only
• D. I & IV only

Hint:

Processes that involve phase change from disorder to order (e.g., freezing, crystallisation) usually show a negative entropy change.

Answer 1

The Correct Option is B

Step 1: Understand entropy (\(\Delta S\)) concept. 
Entropy is a measure of disorder or randomness in a system. When a system becomes more ordered, entropy decreases (\(\Delta S<0\)). 
Step 2: Analyze each process. 
I) Sublimation of dry ice: Solid \(\rightarrow\) gas — disorder increases \(\Rightarrow \Delta S>0\) 
II) Freezing of water: Liquid \(\rightarrow\) solid — particles become more ordered \(\Rightarrow \Delta S<0\) 
III) Crystallisation of dissolved substance: Ions/molecules settle into a lattice \(\Rightarrow \Delta S<0\) 
IV) Burning of rocket fuel: Produces gases and heat \(\Rightarrow \Delta S>0\) 
Conclusion: Only II and III lead to a decrease in entropy.