Question

Identify the correct statement regarding entropy:
A. At absolute zero, the entropy of a perfectly crystalline substance is positive.
B. At absolute zero, the entropy of a perfectly crystalline substance is zero.
C. At zero degrees Celsius the entropy of a perfectly crystalline substance is taken to be zero.
D. At absolute zero of temperature the entropy of all crystalline substances is taken to be zero.

Answer 1

Hint: It is related to the third law of thermodynamics, which tells about the properties of a system at equilibrium at absolute zero temperature.

Complete step by step solution:
Third law of thermodynamics states that the entropy of a perfect crystal of a pure substance approaches zero as the temperature approaches zero.
Mathematically, the absolute entropy of any system at zero temperature is the natural log of the number of ground states times Boltzmann's constant. The entropy of a perfect crystal lattice is zero, provided that its ground state is unique, because $\ln (1)$ = 0.So the first option is incorrect.
The entropy of a perfect crystal at absolute zero is exactly equal to zero. At absolute zero (zero kelvin), the system must be in a state with the minimum possible energy, and the above statement of the third law holds true provided that the perfect crystal has only one minimum energy state.
So according to the above statement at absolute zero the entropy of a perfectly crystalline substance is zero. So the correct option is 2nd.
The 3rd option is incorrect as written above that at 0 kelvin the entropy of a perfectly crystalline substance is taken zero so it cannot be at 0 degree Celsius.
Similarly the 4th option is incorrect.

So option B is correct.

Note:
It is impossible for any process, no matter how idealized, to reduce the entropy of a system to its absolute-zero value in a finite number of operations.