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Question

The entropy change can be calculated by using the expression, â–³S=QrevT. When water freezes in a glass beaker, choose the correct statement amongst the following:

A
Ssystem decreases but Ssurroundings remains the same.
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B
Ssystem increases but Ssurroundings decreases.
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C
Ssystem decreases but Ssurroundings increases.
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D
Ssystem decreases and Ssurroundings also decreases.
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Solution

The correct option is C Ssystem decreases but Ssurroundings increases.
Freezing of water is an exothermic process (heat releases).
As water freezes from liquid to solid, randomness (or entropy) decreases, i.e., Ssystem decreases. Heat released during the process, is absorbed by the surroundings hence, Ssurroundings increases.

Theory:

Second Law of Thermodynamics:

It states that as energy is transferred or transformed, more and more of it is wasted. The Second Law also states that there is a natural tendency of any isolated system to degenerate into a more disordered state.

Note: Entropy of universe increases in the course of a spontaneous change.

ΔSuniv. is greater than 0
ΔSuniv.=ΔSsys+ΔSsurr > 0
For a spontaneous process in an isolated system, Entropy always increases
Also, When a system is in equilibrium, the entropy is maximum and the change in entropy, ΔS=0.

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