Video: Selecting the Portion of a Cooling Graph Corresponding to Condensing

Below is a cooling graph for a pure substance. The substance begins as a gas and ends as a solid; each segment represents a different process. Which segment corresponds to condensing?

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Video Transcript

Below is a cooling graph for a pure substance. The substance begins as a gas and ends as a solid. Each segment represents a different process. Which segment corresponds to condensing?

This figure represents a cooling graph or a cooling curve, which tells us how the temperature and energy of a substance change as we go from a gas to liquid to a solid. We start as a gas and end as a solid. And each segment, A through E, represents a different process along the way. So starting with segment A, let’s try to figure out what each of these segments represent. In segment A, the temperature is decreasing and we’re losing energy. Temperature is proportional to the average kinetic energy of our system. And the average kinetic energy tells us how quickly the particles in our system are moving. A higher average kinetic energy means that the particles are moving more quickly.

So in segment A, we’re starting off as a gas and the temperature is decreasing, so the particles are beginning to move more and more slowly. This corresponds to a gas cooling. Once the gas cools, we reach segment B. In segment B, we’re losing energy but the temperature is staying constant. Since the temperature is staying constant, but we’re still losing energy, this energy loss in segment B must be a decrease in potential energy. Liquids and solids experience higher intramolecular forces than gases do, which means that their particles are more attracted to each other and they stay closer together. So what’s going on in segment B is that the slower moving gas particles began to stick together, which forms a liquid. Since in liquid the particles are closer together and more attracted to each other, the average potential energy goes down.

Since we’re starting with a gas and ending with a liquid, which is a different phase, segment B represents a phase change. The phase change of going from a gas to a liquid is called condensing. The question asks us to identify the segment that corresponds to condensing. So B is the correct answer choice. But I’m going to continue going through this cooling curve, just so we have a good idea of what’s going on. At the end of segment B, all of the gas has been turned into a liquid. Now, we’re at segment C, where the temperature is decreasing and energy is being lost similar to segment A. So this segment must correspond to a liquid being cooled down. Now that our liquid has cooled down, we’re at segment D.

Here, the liquid particles are moving very slowly, so they begin to move closer together and form the more ordered solid. In this segment, the temperature is again staying the same even though we’re losing energy. So it must correspond to a phase change as well. The phase change that corresponds to a liquid going to a solid is called freezing. Finally, at the end of segment D, we have a solid. Now, the temperature and energy began to decrease once more. So segment E must be the solid cooling. Now, we have a good idea of what’s going on throughout this entire cooling curve. But we were ultimately tasked with identifying which segment corresponds to condensing, which was B.

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