Video: Understanding Changes in State

Some water vapour in a container is cooled. It first condenses to become water and then freezes to become ice. The temperature of the contents of the container are recorded every minute, and the results are shown in the graph. What state was the water in between 0 and 5 minutes?

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Video Transcript

Some water vapour in a container is cooled. It first condenses to become water and then freezes to become ice. The temperature of the contents of the container are recorded every minute, and the results are shown in the graph. What state was the water in between zero and five minutes?

Before we answer this question, let’s take a look at the graph. It shows us the temperature of the water in degrees Celsius versus the time in minutes after this water vapour has begun cooling. If we look on the graph at the initial moment, time 𝑡 equals zero, we see that the temperature of this water at that point was above 120 degrees Celsius. This confirms what we’re told in the problem statement that this water at this point is vapour. It’s in the gas phase. And, in fact, this is part of the focus of this first question, which asks us what state was the water in between zero and five minutes.

Looking on our horizontal axis, we see zero is right here and five minutes is right there. If we trace those two time values up until they intersect our curve, then we see the portion of the curve we’re interested in. It’s this portion right here. We can see that at a time value of exactly five minutes, that’s when the temperature of this water sample becomes equal to a 100 degrees Celsius. So up until five minutes, the temperature of this sample was always above that. That tells us the water is too hot to be in its liquid phase and certainly too hot to be in its solid phase. This means that over this time interval, from zero to five minutes, the water is in the gas phase. Now let’s look at a few more questions about this graph.

Our next question asks, what state was the water vapour in between 40 and 45 minutes?

To figure this out, we locate those two time values on our horizontal axis. We can see that for this portion of the curve, between 40 and 45 minutes, the temperature of our water is always at or below zero degrees Celsius. And in addition to that, if we look a little bit to the left of 40 minutes, at this flat portion of the curve here, we can tell that what’s going on there is the water is going from a liquid to a solid, liquid to ice. This means that at 40 minutes exactly, we’ve completed that phase transition and our water is now entirely frozen. It’s solid ice. This tells us that for this time interval between 40 and 45 minutes, our water is in the solid state of matter.

Next, we want to know what word describes what is happening between five and 15 minutes.

Between five and 15 minutes, we see that our curve is going through this flat portion. It’s not changing temperature, but time is elapsing. Based on the description in the problem statement, as well as the title of this graph, we know that even though the temperature of our water isn’t changing over this time, there’s still an energy exchange going on. That is, energy is being taken away from the water. Now, if energy is leaving the water but its temperature isn’t changing, that can only mean one thing, that the water at that point is going through a phase transition, a change from one state of matter to another. And indeed, that’s exactly what’s going on between five and 15 minutes on this curve.

Now, we saw earlier that over this portion of the curve, from zero to five minutes, our water was in the gas state. And if we then look at the portion of the curve after 15 minutes, this portion right here, we can see that based on the temperature range of that curve, the water must be in the liquid state. That’s because it’s between zero degrees and 100 degrees Celsius. This indicates that this transition here between five and 15 minutes is from the gas to the liquid phase. Whenever material makes the phase change of going from a gas to a liquid, there is a particular name for that process. The word is condensation. That describes the particular change in phase that we’re seeing between five and 15 minutes from a gas to a liquid.

And finally, our last question asks this: What word describes what is happening between 35 and 40 minutes?

Looking at that portion of the curve, we see once again it’s flat, indicating that this is a period where our water is being cooled. But its temperature is not changing. That’s the hallmark of a phase transition, a change from one state of matter to another. Now in this case, unlike before, this change of phase is happening at a temperature of zero degrees Celsius. For water, that’s the crossover temperature between the liquid and the solid phases. Because our water is being cooled over this time period, we know that it’s moving from a warmer to a cooler state over this interval. In other words, it’s going from a liquid to a solid. The word for describing that phase change, that transition from a material being a liquid to being a solid, is one we’re familiar with. It’s freezing. As the water goes from a liquid phase to a solid phase over this time interval, it’s going through the process of freezing.

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