From "North Carolina Measures of Student Learning: NC’s Common Exams Chemistry" (Source, Number 20):

The graph below shows a cooling curve for a sample of gas that is uniformly cooled from $155~^\circ\mathrm{C}$.
enter image description here
Why does the freezing point of the substance occur at $–20^\circ\mathrm{C}$?

  1. because the latent heat energy is absorbed by the substance as it is converted from a liquid to a solid
  2. because the latent heat energy is released into the air as the substance is converted from a liquid to a solid
  3. because the average kinetic energy is increasing for the substance as it is converted from a solid to a liquid
  4. because the average kinetic energy is decreasing for the substance as it is converted from a solid to a liquid

Its really just a simple questioning and I think the answer is 3, because the average kinetic energy is increases when converted from solid to liquid. Any reason why I'm wrong and the answer says it is 2?


closed as off-topic by Loong, jerepierre, ron, Klaus-Dieter Warzecha, Geoff Hutchison Mar 5 '15 at 23:02

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    $\begingroup$ Could you move the question from the file into the above? Also, please provide a more descriptive title. $\endgroup$ – jerepierre Jan 10 '15 at 0:43
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    $\begingroup$ @AwesomeFlame123 As you can see, it was neither a problem to move the question into the posting nor was it difficult to give it a more descriptive title. Please invest some more time in asking your questions, people replying here do that on their own free time and they deserve some appreciation for that they are helping you. Not even trying to type the question into the post, I would consider rather rude. $\endgroup$ – Martin - マーチン Jan 13 '15 at 5:06

The first thing is that it has to be going from liquid to solid because the substance is being cooled. Like tap water put in the freezer: liquid to solid.

On that note, the average kinetic energy goes down (directly assumed because that's what temperature is, and it's going down.)

Which leaves us with two answer choices: latent heat absorbed or latent heat released, where latent heat is the heat of a (thermodynamic system) at a constant temperature (like a phase change!)

If you're gaining energy, particles are going to have more velocity, like those virtual diagrams of gases with fast moving balls. Since we're going to a solid, losing energy makes sense because particles can't move as fast and are trapped in a fixed location (like an ice cube, which has a definite cuboid shape).

So liquid to solid, latent heat because the temperature is constant, and losing energy to become a solid - B is the answer.

  • $\begingroup$ Thanks, didn't know what Latent Heat Energy was. Also I should've thought that temperature doesn't necessarily have to change at a phase change. Sorry if I wasted your time with such a simple question. $\endgroup$ – Asker123 Jan 10 '15 at 0:00

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