(A): Entropy of a perfect crystalline substance at absolute zero is zero
(R ): At absolute zero translation kinetic energy of a system is zero.

Options

A. A and R are true, R explains A
B. A and R are true, R does not explain A
C. A is true, but R is false
D. A is false, but R is true

Answer

To solve the question regarding the assertion and reason statements, we will analyze each part step by step.

### Step 1: Understanding the Assertion
The assertion states: "Entropy of a perfect crystalline substance at absolute zero is zero."

- **Explanation**: According to the Third Law of Thermodynamics, the entropy of a perfect crystalline substance approaches zero as the temperature approaches absolute zero (0 Kelvin). At absolute zero, the system is in a state of perfect order, meaning there is no randomness or disorder among the particles.

### Step 2: Understanding the Reason
The reason states: "At absolute zero, the translational kinetic energy of a system is zero."

- **Explanation**: At absolute zero, the thermal motion of particles theoretically comes to a complete stop. This means that the translational kinetic energy, which is the energy associated with the movement of particles, is zero. Therefore, the particles are motionless.

### Step 3: Analyzing the Relationship Between Assertion and Reason
Now we need to determine if the reason correctly explains the assertion.

- The assertion is that the entropy is zero at absolute zero due to the perfect order of the crystalline structure, and the reason given is that the kinetic energy is zero at this temperature. The fact that kinetic energy is zero implies that the particles are not moving, which directly contributes to the lack of disorder (or randomness) in the system. Hence, the reason does indeed explain why the entropy is zero.

### Step 4: Conclusion
Both the assertion and the reason are true, and the reason is the correct explanation of the assertion.

### Final Answer
The correct choice is: "Assertion and reason are true. Reason is the correct explanation of A."

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