Identify the correct statement regarding entropy

Options

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

Explanation

To identify the correct statement regarding entropy, we can follow these steps:

### Step 1: Understand the Third Law of Thermodynamics
The Third Law of Thermodynamics states that as the temperature approaches absolute zero (0 Kelvin), the entropy of a perfect crystalline substance approaches zero. This means that at absolute zero, a perfectly ordered crystalline structure would have no disorder, hence its entropy is zero.

### Step 2: Analyze the Statements
Now, let's examine the statements provided in the question (though they are not explicitly listed here, we can infer them based on the explanation given in the video transcript):

1. **Statement A**: At absolute zero, the entropy of a perfectly crystalline substance is zero.
2. **Statement B**: The entropy of a substance is always positive at absolute zero.
3. **Statement C**: The entropy of a substance is zero at 0 degrees Celsius.
4. **Statement D**: All crystalline substances have zero entropy at absolute zero.

### Step 3: Evaluate Each Statement
- **Statement A** is correct according to the Third Law of Thermodynamics.
- **Statement B** is incorrect because the entropy of a perfectly crystalline substance is zero at absolute zero.
- **Statement C** is incorrect because the correct reference point is absolute zero (0 Kelvin), not 0 degrees Celsius.
- **Statement D** is incorrect because the Third Law applies specifically to perfectly crystalline substances, not all crystalline substances.

### Step 4: Conclusion
The correct statement regarding entropy is **Statement A**: "At absolute zero, the entropy of a perfectly crystalline substance is zero."

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