MCAT General Chemistry Question 207: Answer and Explanation

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Question: 207

5. At 1 atm, deionized water can remain a liquid at temperatures down to –42°C. If a foreign body is added to the supercooled liquid, it will immediately turn into ice. Which of the following is true about this process?

  • A. The reaction is exothermic.
  • B. Tap water could also be supercooled to -42°C.
  • C. The transformation of a supercooled fluid to a solid is nonspontaneous.
  • D. Water's unique phase diagram allows it to be supercooled.

Correct Answer: A

Explanation:

A Upon nucleation with a foreign body, supercooled water will transition from liquid to solid phase. This phase transition (crystallization) requires heat to be released since intermolecular interactions are formed. Choice B is eliminated because tap water contains many dissolved particles that can serve as sites of nucleation. Choice C is incorrect because the supercooled fluids are only kinetically stabilized against freezing, and their transformation to a solid form is thermodynamically spontaneous. Since pressure remains constant, the negative sloped solid-liquid equilibrium line in water's phase diagram does not play a role in supercooling, eliminating Choice D.

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