MCAT Physics Question 44: Answer and Explanation

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Question: 44

14. In experiment A, a student mixes ink with water and notices that the two liquids mix evenly. In experiment B, the student mixes oil with water; in this case, the liquids separate into two different layers. The entropy change is:

  • A. positive in experiment A and negative in experiment B.
  • B. positive in experiment A and zero in experiment B.
  • C. negative in experiment A and positive in experiment B.
  • D. zero in experiment A and negative in experiment B.

Correct Answer: B

Explanation:

When the ink randomly intersperses throughout the water, the final state is more disordered than the initial state, so the entropy change of the system is positive. When the oil separates from the water, the final state is just as ordered as the initial state (because the oil and the water are still completely separate), so the entropy change is zero. You can also answer this question by noticing the reversibility of the two experiments. Experiment A has a positive entropy change because it is irreversible, while experiment B has no entropy change because the reaction is reversible. According to the second law of thermodynamics, the overall entropy change can never be negative in a thermodynamic process that moves from one equilibrium state to another.

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