The entropy of a perfectly crystalline solid is zero at ……

Answer

To solve the question regarding the entropy of a perfectly crystalline solid, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Entropy**:
- Entropy is a measure of the disorder or randomness of a system. The higher the entropy, the greater the disorder.

2. **Perfectly Crystalline Solid**:
- A perfectly crystalline solid has a well-defined structure with minimal disorder at absolute zero temperature.

3. **Third Law of Thermodynamics**:
- According to the third law of thermodynamics, the entropy of a perfect crystalline substance approaches zero as the temperature approaches absolute zero (0 Kelvin).

4. **Temperature Reference**:
- Absolute zero is equivalent to 0 Kelvin, which is also -273.15 degrees Celsius.

5. **Conclusion**:
- Therefore, the entropy of a perfectly crystalline solid is zero at 0 Kelvin (or -273.15 degrees Celsius).

### Final Answer:
The entropy of a perfectly crystalline solid is zero at **0 Kelvin** (or **-273.15 degrees Celsius**).

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