Video: Solids, Liquids, and Gases

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

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Video Transcript

In this video, we will learn about matter and the forms or states it can commonly take, solid, liquid, and gas. We’ll learn to describe the processes where matter changes from one state to another. And we’ll study a model that helps us understand why solids, liquids, and gases behave the way they do, the particle or kinetic theory.

Matter is anything with mass. A boot is matter, a star is matter, and an electron is matter. But electromagnetic waves like X-rays and visible light don’t have mass, so they’re not matter. And a more technical definition is that matter occupies space and has mass at rest. You probably already know lots of different forms of matter. The wood of a desk is very different from the water you drink and the air you breathe. They’re all forms that matter can have. So, what makes them different?

Long before we understood the atom, people understood that different forms of matter have different properties. If you press on a table, it’s hard. It doesn’t change shape. But if you push your hand through water or through air, they just move out of the way. As far as we can tell, the table is always the same size and shape, unless we cut off a leg or something. So, its total volume doesn’t change. The water has a fixed volume as well, but its shape isn’t fixed. So, it adopts the shape of its container. But the air is quite different. You can trap it in a syringe and compress it by quite a lot, but it still looks exactly the same. It just fills the volume that’s available. If we do the same to the table or the glass of water, it doesn’t seem to make them any smaller. So, we can call the table and the water incompressible and the air compressible. “Compressible” simply means that we can make something occupy less volume by applying pressure.

So, we clearly have three forms of matter with very distinct behaviors. We came up with words for these different forms of matter. A table is solid, drinking water is liquid, and air is a gas. We call these states of matter. As well as saying that matter has different states, people noticed that some substances can transform. Hard snow would fall from the sky and gather in piles that kept their shape. The morning sun would turn the snow to flowing water that would quickly gather into puddles and then slowly vanish into the air by the afternoon. It took a long time to understand what was going on, but people still understood that something was changing dramatically.

Water, or H2O, has a solid form, ice or snow; a liquid form, which is what we commonly call water; and a gaseous form, steam or water vapor. Although these two terms mean slightly different things that don’t really matter here. Water doesn’t have a strong color of its own. So, steam is actually invisible. But the word “steam” is sometimes used incorrectly to refer to smoke or to tiny droplets of water in mist.

If you started with a block of ice and applied sufficient heat, you would see it transform from ice into liquid water. We call the process of a solid turning into a liquid, melting. If we continue the heating of liquid water, it will boil and bubble and eventually disappear. We call this process where a liquid spontaneously transforms into gas boiling. Liquids can turn into gases at lower temperatures. And this is called evaporation. But what if we do the reverse? If we take some steam and cool it down, it will turn from a gas to a liquid. We call this process condensing or condensation. If we cool it down even further, it will turn from a liquid to a solid. We call this process freezing.

But what about this? Have you ever woken up to ice on the outside of the windows? That’s odd. It didn’t rain last night, but there’s ice on the windows. Where did it come from? On a cold night, water vapor in the air can turn to ice without even becoming a liquid first. A gas is deposited as a solid. We call this process deposition. The reverse process is a little difficult to see. We don’t tend to notice ice just disappearing without melting first. But it does happen, particularly on mountains at high altitudes, where the mild sunshine and a low air pressure make it a little easier. We call the process of a solid turning into a gas without becoming a liquid first sublimation or sublimating.

Another thing that people notice was that, for some substances, state changes happened at pretty much the same temperature every time. Ice will start to turn to liquid water at about zero degrees Celsius. Once all the water is above zero degrees Celsius, it’ll all be liquid. At 100 degrees Celsius, liquid water will transform to steam. Of course, water evaporates slowly below 100 degrees Celsius, but it’s only at 100 degrees Celsius that all the water will turn to gas at once. Once we’re above 100 degrees, all the water will be gas. We call the temperature at which a substance changes state a point. So, zero degrees Celsius is the melting point of water because it transitions from a solid to a liquid. But we sometimes use different names, depending on whether we’re adding energy or taking it away. So, zero degrees Celsius is also the freezing point of liquid water. So, 100 degrees Celsius is the boiling point of liquid water and the condensing point of steam.

We can use these points to predict what state a substance will be in at a specific temperature. Below the melting point, the substance will be a solid. So below zero degrees Celsius, H2O will be solid. Between the melting and boiling point, the substance will be a liquid. So, between zero degrees Celsius and 100 degrees Celsius, H2O will probably be liquid. This assumes we’re not really worrying about the small amount of water vapor in the atmosphere. Above the boiling point, this substance will be a gas. So, above 100 degrees Celsius, H2O will be a gas.

Boiling points are affected by pressure. So, water boils at only about 71 degrees Celsius on the top of Mount Everest, 9000 meters above sea level. And in the Dead Sea, 400 meters below sea level, water boils at about 101 degrees Celsius. And lastly, we know that sometimes substances don’t melt; they sublime. So, for these substances, they are solid below the sublimation temperature and gases above the sublimation temperature.

Melting, boiling, and so forth are examples of physical changes, where a substance behaves differently, but the internal bonds aren’t affected. The molecules of water may move further apart when they evaporate, but they’re still H2O molecules. A chemical change is one way we generate a new substance. But we don’t produce new substances with physical changes. We’ll just see changes in shape, size, or state.

We know a lot by this point. We know the names for the three most common states of matter and the names for the processes where one turns into the other. We can describe things, but we’re no closer to understanding them. For that, we’ll need the particle theory of matter. Imagine you were asked to explain why water can be a hard solid, a flowing liquid, or an expanding gas. Where would you start? Fortunately, there’s a pretty good theory already. It’s called the particle theory of matter, also known as the kinetic theory or kinetic molecular theory of matter. What this theory suggests is that matter is made of particles that attract each other. These particles are not necessarily atoms, just tiny individual lumps. The size isn’t that important for the theory to work. And the second point is that these particles are constantly moving.

Now, let’s imagine some of these particles, and they have lots of energy to start with. Gravity is pretty weak. So, the particles rocket freely around. Occasionally, they might bump into each other. But they have so much energy that while they might stick together a little, they just bounce off each other. This seems like a good model for a gas like air. If they were made of tiny particles that rocketed around through mostly empty space, almost at random, then we ought to be able to compress it and fit those particles into a small space. Or we could put it into a larger container and watch the particles spread out to fill the space. That’s a clear tick for the particle theory, explaining the behavior of gases. So, what about liquids?

So, let’s imagine our particles start losing energy. And it gets to the point where they don’t have enough energy to overcome the attractive forces they experience when they’re close together. Before long, these lower-energy particles have stuck together in one lump. So, we have a single pool of stuff. The particles are attracted to each other. So, they’re close together, but they can still move around and over each other. So, the substance as a whole flows in response to pressure filling whatever container it’s in. All the particles are moving around a lot, but the particles are too small to see. So, the liquid seems pretty calm unless we disturb it. The particle theory seems to be coping well with gases and liquids, just one more state left.

Imagine we take out so much energy from the particles that they can’t even move around each other. The particles are still close together, but this time they’re fixed in place. Individual particles are still moving around a little, vibrating on the spot. But they don’t have enough energy to escape the attractive forces from their neighbors. This seems to do the job. If matter is made of particles that attract each other and can have a lot of energy or not very much at all or some amount in the middle. Then that would explain the behavior of solids, liquids, and gases. The way particles pack together in solids means that solids are very difficult to compress. And it’s the same with liquids. Generally speaking, we think of solids and liquids as being incompressible. That’s not quite true. But you do need incredibly high pressures to notice even the slightest difference. So, we often ignore that.

This is one of the areas where the particle model needs adjusting to account for the fact that chemicals aren’t simply hard spheres. They’re real particles with complicated shapes, a range of sizes, and they can be compressed and interact with each other in different ways. But broadly speaking, the particle theory of matter does an excellent job at explaining the fundamentals of solid, liquid, and gas behavior. We can apply the particle theory of matter to the state changes as well.

As a solid melts, enough energy is being given to the individual particles to allow them to move over each other. In the reverse process, freezing, we simply see the particles losing energy until they can’t overcome the total of the attractive forces from all their neighbors. Likewise, when a liquid boils, enough energy is being given to the individual particles to allow them to escape the attractive forces of their neighbors completely. In the reverse process, condensation, we see the particles lose energy until they stick together without bouncing off.

The process with deposition and sublimation is similar. Whether a substance melts or sublimes comes down to many factors, like the strength of the attractive forces and the local air pressure. The strength of the attractive forces depends on the material. But it’s the presence of these attractive forces that allows us to explain why melting, freezing, boiling, condensing, and so on occur at specific temperatures. The stronger these attractive forces, the more energy we’ll need to transition from a solid to a liquid or a liquid to a gas or, indeed, from a solid to a gas.

The kinetic energy of the individual particles is proportional to the temperature. So, we need a specific temperature to give the particles a specific amount of energy in order to overcome the specific attractive forces. The particle theory of matter also explains another feature of gases. If you have a hotter gas in the same container, the particles will have more energy. This means, in the hotter container, the gas particles are hitting the sides of the container with more force. So, there’s more pressure on the walls of the hotter container than the cooler container.

Now that we’ve looked at solids, liquids, and gases and explained why they might behave the way they do, let’s have some practice.

The element mercury melts at negative 38.8 degrees Celsius and boils at 356.7 degrees Celsius. What will the state of matter of mercury be at zero degrees Celsius?

Mercury is a transition metal. It has the chemical symbol Hg. And as a metal, we might expect it to exist in a lattice of regular atoms fixed in place or in other words as a solid. But the question tells us that mercury melts at negative 38.8 degrees Celsius. What this means is if we provide enough energy to our solid mercury, the individual particles in mercury will start to move over one another. And the whole thing will become a fluid or a liquid. And the temperature at which this happens is negative 38.8 degrees Celsius. So, below this temperature, mercury is a solid. And above this temperature, mercury is a liquid.

The question also tells us that mercury boils at 356.7 degrees Celsius. What this means is that when we get to 356.7 degrees Celsius, all the liquid will start getting enough energy to escape the attractions between one another. And they’ll become a gas. And the temperature at which this occurs is 356.7 degrees Celsius. So, we have melting, moving from a solid to a liquid, and boiling, moving from a liquid to a gas. And mercury is liquid below 356.7 degrees Celsius and a gas above it. All we need to do is figure out what state of matter mercury will be in at zero degrees Celsius.

Zero degrees Celsius is greater than the melting point of mercury and it’s less than the boiling point of mercury. Therefore, the state of matter that mercury will be in at zero degrees Celsius is liquid.

In the next question, we’ll take a closer look at the properties of solids.

Which of the following is not a property of a solid? A) Made of particles that are perfectly still, B) stable shape, C) turns to liquid at its melting point, D) made of particles that are very close together, or E) turns to gas at its sublimation point.

We can use particle theory to understand the properties of solids. We can start with the fact that solids have a fixed shape. This is explained by modeling a solid as lots of individual particles. And there are attractive forces between each particle that holds the shape together. But solids can only theoretically have zero temperature. So even a really, really cold solid will have some energy and the individual particles will be vibrating on the spot just a little. So, on this basis, we’d expect the answer to be made of particles that are perfectly still. Because we know that even if a solid is very, very cold, the individual particles are going to have some small motion.

We know solids have a stable shape. And we know a melting point is when a solid turns into a liquid. And we know that particles are very close together because they’re touching. And they turn to gas at the sublimation point, which is the temperature at which a solid turns directly into a gas without turning into a liquid first. So, the item that’s not a property of a solid is made of particles that are perfectly still.

To finish off, let’s have a look at the key points. “Solid,” “liquid,” “gas” are states of matter. Properties of solids, liquids, and gases can be understood using particle theory. We learned that we can explain the fixed shape of solids by saying the particles are fixed in place with attractive forces holding them together. And liquids can flow because the particles, although close together, don’t have enough energy to escape. So, they move over each other, but not away from each other. And gases are free to be compressed or expand because they are free particles. And so, they fill space. And we learned the names for converting between solids, liquids, and gases.

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