The third law of thermodynamics states that:
AEntropy of a system cannot decrease
BThe total entropy of the universe is constant
CThe entropy of a perfectly crystalline substance at absolute zero is zero
DAbsolute zero of temperature can be reached in a finite number of steps
Answer & Solution
Correct answer: C. The entropy of a perfectly crystalline substance at absolute zero is zero
Third law: the entropy of a perfect crystalline substance at $T = 0\,\text{K}$ is taken to be zero. This provides an absolute reference for entropy, allowing tabulation of standard entropies $S^\circ$ at $298\,\text{K}$. Option D is the negation of an unattainability statement equivalent to the third law.
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