Identify the correct statement regarding entropy

Options

A. At `0^@C`, the entropy of a perfectly crystalline substance is taken to be zero.
B. At absolute zero of temperature, the entropy of a perfectly crystal line substance is + ve.
C. At absolute zero of temperature, the antropy of all crystalline substances is taken to be zero.
D. At absolute zero of temperature, the entropy of a perfectly crystal line substance is taken to be zero.

Answer

To identify the correct statement regarding entropy, we will analyze the provided statements based on the principles of thermodynamics, particularly the third law of thermodynamics.

### Step-by-Step Solution:

1. **Understanding the Third Law of Thermodynamics**:
- The third law states that the entropy of a perfect crystal approaches zero as the temperature approaches absolute zero (0 Kelvin). This means that at absolute zero, a perfectly crystalline substance has only one microstate, leading to zero entropy.

2. **Analyzing Each Statement**:
- **Statement 1**: "At zero degree Celsius, the entropy of a perfectly crystalline substance is taken to be zero."
- This statement is incorrect. The correct temperature for zero entropy is 0 Kelvin, not 0 degrees Celsius.

- **Statement 2**: "At absolute zero temperature, the entropy of a perfectly crystalline substance is positive."
- This statement is also incorrect. According to the third law, the entropy at absolute zero is zero, not positive.

- **Statement 3**: "At absolute 0 of temperature, the entropy of all crystalline substances is taken to be 0."
- This statement is misleading. While the entropy of a perfectly crystalline substance is zero at absolute zero, not all crystalline substances may be perfectly ordered, so this statement is not universally true.

- **Statement 4**: "At absolute 0 temperature, the entropy of a perfectly crystalline substance is taken to be 0."
- This statement is correct. It aligns with the third law of thermodynamics.

3. **Conclusion**:
- After evaluating all statements, we find that the correct statement regarding entropy is **Statement 4**: "At absolute 0 temperature, the entropy of a perfectly crystalline substance is taken to be 0."

### Final Answer:
**The correct statement regarding entropy is: "At absolute 0 temperature, the entropy of a perfectly crystalline substance is taken to be 0."**

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