At absolute zero, the entropy of a pure crystal is zero. This is

Options

A. first law of thermodynamics
B. second law of thermodynamics
C. third law of thermodynamics
D. zeroth law of thermodynamics

Answer

To solve the question "At absolute zero, the entropy of a pure crystal is zero. This is," we need to identify which law of thermodynamics this statement refers to.

### Step-by-Step Solution:

1. **Understanding the Statement**: The statement indicates that at absolute zero (0 Kelvin), the entropy of a pure crystal is zero. This is a specific condition related to the behavior of entropy in thermodynamic systems.

2. **Identifying the Laws of Thermodynamics**: We need to consider the four laws of thermodynamics:
- **First Law**: Law of conservation of energy.
- **Second Law**: Entropy of an isolated system remains constant for reversible processes.
- **Third Law**: Entropy of a perfect crystal approaches zero as the temperature approaches absolute zero.
- **Zeroth Law**: If two systems are in thermal equilibrium with a third system, they are in thermal equilibrium with each other.

3. **Analyzing Each Law**:
- The **First Law** does not relate to entropy directly; it deals with energy conservation.
- The **Second Law** discusses entropy in terms of reversible processes but does not state anything about absolute zero.
- The **Zeroth Law** is about thermal equilibrium and does not pertain to the entropy of a pure crystal at absolute zero.
- The **Third Law** specifically states that the entropy of a perfect crystal approaches zero as the temperature approaches absolute zero.

4. **Conclusion**: Since the statement given in the question directly corresponds to the Third Law of Thermodynamics, we can conclude that the answer is the Third Law.

5. **Final Answer**: The correct answer is **the Third Law of Thermodynamics**.