Video: Identifying the Section of a Temperature-Energy Graph for Water from Which Its Heat of Vaporization Can Be Determined

From which part of the graph could the heat of vaporization of water be determined? [A] Part A [B] Part B [C] Part C [D] Part D [E] Part E

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Video Transcript

From which part of the graph could the heat of vaporization of water be determined?

This graph is called a heating graph or a heating curve. It shows us the change in temperature and energy as a substance, in this case, water, is heated from the solid phase all the way to the gas phase. Each of the segments of this curve correspond to a different process. This question is asking us about the heat of vaporization, which is the energy that’s required to change a substance from a liquid to a gas. Since energy is on the 𝑥-axis of this graph, we would be able to determine the heat of vaporization of water if we could figure out which of the segments in this heating curve corresponded to the process of going from a liquid to a gas, which is, of course, boiling.

If we look at our graph, there are two temperatures that are labelled, zero degrees Celsius and 100 degrees Celsius. Zero degrees Celsius is, of course, the melting point of water. And 100 degrees Celsius is the boiling point. Since segment D corresponds to a process that’s occurring at the boiling point of water, segment D must be boiling. So D must be the segment that we’re looking for. According to our diagram, this amount of energy was required to go from a liquid to a gas. So this amount of energy corresponds to the heat of vaporization. You may have noticed that the segments that we’ve identified is corresponding to phase changes. That is, boiling and melting are flat, meaning that while we’re adding energy to the substance, the temperature isn’t changing.

If we compare a substance that’s a liquid to a substance that’s a gas, in the liquid, the molecules are close together, while, in the gas, the molecules are fully separated. So during a phase change, all of the energy that’s being added to the substance is being used to separate these particles so that the substance can go from a liquid to a gas or from a solid to a liquid. But we’ve identified our answer choice already. Let’s quickly run through the other segments on this heating graph just so we know what they are.

Segment A is a process that’s occurring below zero degrees. Since water can only be a solid below zero degrees, segment A must correspond to a solid heating. We know that zero degrees is the melting point of water. So segment B must be melting. This segment again is flat because all of the energy here is being put into separating the molecules of solid water so that they could become liquid water. After segment B, we now have liquid water. So now, the energy can go towards increasing the temperature of the water again. So segment C corresponds to our liquid heating.

We’ve already identified segment D as boiling, where all of the energy we’re putting in is going towards turning the liquid into a gas. At the end of segment D, we now have gaseous water. And the temperature starts to increase again. So that means segment E corresponds to the gas heating. But as we’ve discussed, in this question we were interested in the segment that we could use to determine the heat of vaporization of water, which is the energy that’s required to go from a liquid to a gas. And we’ve identified segment D as the process of boiling. So segment D is that segment we’re looking for.

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