Lesson Video: States of Matter Physics

In this lesson, we will learn how to describe the three common states of matter, their physical properties, and the transitions between them.


Video Transcript

In this video, we’re talking about states of matter. And while there do exist more exotic states of matter, we’ll keep our discussion focused on the three most common, the ones that show up the most, solids, liquids, and gases.

Now, when we use this phrase “state of matter”, what we’re referring to is just how it is that the atoms or the molecules in a material are arranged. Say that we took a pan and we put a solid object in it, in this case a chunk of ice. In this perspective, we see the individual water molecules arranged in a very orderly structured pattern. One characteristic of solid materials is that their particles are fixed in place. They’re not mobile enough to be able to move past one another within that material. It’s also true of solid materials that they’re very hard to compress. If you’ve ever tried to compress, say, a block of wood or a countertop, you know what that means. And additionally, material which is in a solid state does not flow. And that goes back to the fact that the particles aren’t able to move past one another easily.

So we had this solid material, this chunk of ice, and it’s sitting there in our pan. Not much is going on because, after all, the particles in the ice are fixed in place. But then, let’s say we start to add some energy to the system. We put a flame under this pan and we start to heat the ice. If we apply heat for a long enough time so that enough energy is added into this chunk of ice, eventually, it will change its state of matter. By dumping all this heat energy into the ice, what we’ve done is we’ve loosened the bonds between the individual water molecules. As each water molecule is heated and gains energy, it starts to distance itself from its neighbours. The orderly structure of the arrangement of the molecules breaks down. And they start to move in a much more disordered way.

This is a description of a state of matter in the liquid phase. In this phase, particles are able to move past one another. Liquids, like solids, are still hard to compress. Water, for example, is so hard to compress that often as a shorthand description, we just say it’s incompressible. A point of difference though between liquids and solids is that liquids are capable of flowing while solids or not. We went from a solid, a chunk of ice, to a liquid, water, by adding heat energy to our system. If we continue to add even more heat, soon enough we’ll see yet another state of matter. Eventually, the molecules in this water become heated enough and so energetic that they spread apart from one another and completely break any bonds between them.

When this happens, we have a gas, in this case water vapour. In this state of matter, the particles are so far apart that there’s very little resistance to motion in any particular direction. The particles are spread out far enough that they’re easy to compress back together. Gases, in general, are very compressible. And lastly, gases are very capable of flowing. In fact, gases and liquids are both called fluids because of this capacity.

Now, we might wonder what happens if we continue even longer to heat this material. Will we find a new state of matter? Well, if we were doing this with ice, what we would find is that all of these molecules would eventually heat hot enough to just escape from this pan and fly off into the atmosphere. Adding more energy just disperses the gas more.

In this process, we’ve talked about moving from a solid to a liquid to a gas. In other words, we’ve been moving left to right on our screen. But, in general, it’s also possible for states of matter to transition in the other direction. Instead of going from a solid to a liquid, for example, we could go from a liquid to a solid. Or, we could start with a gas and go from a gas to a liquid. It’s even possible to completely skip the liquid phase. We could go from a solid straight to a gas or a gas directly to a solid. Each one of these six transitions between phases has its own name.

When a solid becomes a liquid, like we had when our ice melted into water, then that’s referred to as melting. Whereas when we go the other way, a liquid to a solid, that process is known as freezing. Then, when we go from a liquid state of matter to a gas state of matter, that process is known as evaporation. And then, going the opposite way, from a gas to a liquid, that’s called condensation. We might be familiar with this if you have a cold drink on a hot humid day. In this case, the water vapour from the humid air around the cold drink condenses on the outside of the glass. Like we said, it’s also possible to make a jump across one entire state of matter, the liquid phase. If we were to go straight from a solid phase to a gas phase, that’s known as sublimation. And going straight from a gas phase to a solid phase, without going through the liquid phase at all, is called deposition.

Some of these words are familiar and some, may be, less familiar. But all of them are helpful in describing phase transitions between states of matter. As we talk about different states of matter and phase transitions, that means moving from one state of matter to another, it’s important to connect energy with that process of transition. Recall that with our ice in the pan, the only way it was able to change from ice to liquid water to gas was because we were adding energy into it. We were heating the ice. We can say that for a material to change from one state of matter to another, that’s called a phase change, the material needs to gain or lose energy.

For example, we saw that for a block of ice, a solid, to turn into a puddle of water, a liquid, we needed to add energy into the ice, the solid. But then, let’s say we wanted to go the opposite direction. Say we wanted to freeze some water. In that case, it would be necessary to take energy away from the water. It would have to lose energy to turn into ice. And it’s the same story if we took liquid water and turned it into water vapour or condensed water vapour into a liquid. The water would need to gain energy to become a gas, water vapour. And the water vapour would need to be cooled down. It would need to lose energy to condense into a liquid.

So whenever there is a change from one state of matter to another, we know that there is an exchange of energy, either a gain or a loss. In this process of heating ice to water and then water to vapour, there’s something very interesting to be seen if we look at the temperature of our material, whatever state of matter it’s in, plotted against the total energy added into that material. In our case, that energy was added by putting a flame underneath the pan that was holding our water sample. So we were adding energy into our sample. And overtime, we know that its temperature increased and also that its state of matter changed.

But take a look at this curve of the temperature of our sample, in degrees Celsius, versus the energy added. Notice that there are two flat portions to the curve. In these portions, we were adding energy into our sample. But its temperature wasn’t going up at all. It was staying constant. Why would that be? Well, notice what temperatures on the vertical axis these values correspond to. The lower flat portion corresponds to zero degrees Celsius. And the upper flat portion corresponds to 100 degrees Celsius. These temperatures we know are the boundary temperatures between phases of matter, zero between solid and liquid and 100 degrees Celsius between a liquid and a gas.

The two flat portions of this curve confirm what we said here, that in order to change the state of matter of a material, that material needs to gain or lose energy. In other words, in these parts of this graph, the energy added went not to increasing the temperature of our material, in this case water, but rather to changing its state. Looking at the top flat portion, we could say that here we had liquid water at 100 degrees Celsius. And here, we had water vapour at 100 degrees Celsius. Or, on the bottom portion, here to the left-hand side of this, we had zero-degree ice. And on the right-hand side, we had zero-degree water. Now that we have a sense for what the states of matter are and how they change from one to another, let’s get a bit of practice through an example.

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.

Let’s summarize now what we’ve learned about states of matter. In this lesson, we saw that a particular state of matter — whether a solid, a liquid, or a gas — describes how molecules are arranged in a material. We saw that solid materials have a very orderly structure. Liquids are less ordered, and gases are less ordered even more. We also learned a bit about the properties of these three different states. We saw that solids and liquids are both essentially incompressible and that while solids are incapable of flowing, liquids and gases can flow.

Along with this, we’d learned the names for transitions between different states of matter. We learned that going from a solid to a liquid is melting whereas going the opposite way is freezing. Going from a liquid to a gas is evaporating whereas going from a gas to a liquid is condensing. And we also saw it’s possible to skip the liquid phase entirely. And going from a solid to a gas is called sublimation whereas going from a gas to a solid is deposition. And finally, we saw that there is a connection between a material changing its state of matter and the energy of that material. In particular, where there’s a change in state, a material either gains or loses energy.

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