Phase changes

0:00- [Instructor] Ice melts at 0 degrees Celsius. But if we take something like gold, then it'll melt only about 1000 degrees Celsius. Similarly, water boils at around 100 degrees Celsius. But if we take something like nitrogen, well, it'll boil at a very low temperature of -200 degrees Celsius, slightly about that.

0:16But the big question is why do different materials have different melting and boiling points? Let's find out. Now, to answer this question, we need to ask a much more fundamental question. What keeps stuff together? Well, if you were to look into, you know, if you could zoom in and look at the molecule at the atomic level, we'll find that all these atoms and molecules are actually being attracted to each other.

0:38There is a force of attraction that's keeping them together. In fact, you've probably witnessed this force of attraction when you've seen two drops merging to form a bigger drop, okay? So this force of attraction keeps all the particles together. And turns out that this force of attraction purely depends on the types of particles.

0:56So for example, the strength of this force of attraction between water molecules would be different than that between gold atoms, right? But another thing you can see is that particles also have kinetic energy. What does that depend on? Well, kinetic energy depends purely on temperature.

1:13In fact, temperature is a measure of the average kinetic energy of the particles. At high temperature, the average kinetic energy is very high. And at low temperature, the average kinetic energy is very low. Kinetic energy only depends on temperature. It has nothing to do with which particles we're dealing with.

1:25So you can see there are two things over here. First, we have the force of attraction that's trying to keep them together. This purely depends on the type of particle, but it has nothing to do with temperature. On the other hand, we have kinetic energy that has nothing to do with the particle type, but it purely depends on temperature.

1:43And what's interesting is that these two are kind of opposite. The attraction force is trying to keep them together, whereas the kinetic energy is trying to make the molecules go farther away from each other. And it's the balance between these two that decide what the melting and the boiling points would be.

1:58So let's take a concrete example to understand that. Let's take ice at a very low temperature, say -10, -15 degrees Celsius. At this temperature, again, if we were to zoom in, we will see the atoms and molecules all stuck together due to the attractive force. And they also have kinetic energy that's trying to make them go apart.

2:13However, at this temperature, it turns out the kinetic energy is very low, so low that the force of attraction actually locks them into places giving us a solid. And the way I like to visualize this is by using some bar graph. So here's the force of attraction of the water molecules over here, and here is the kinetic energy.

2:29Look, the level of kinetic energy is very low relative to the force of attraction. And as a result, you get a solid. But now comes the big question, what happens if we start heating it? Why don't you pause the video and think about what will happen to the force of attraction and the kinetic energy as we start heating it?

2:44Will it increase, decrease, what happens to them? Pause and think about it. All right, what happens with the force of attraction? Nothing, because that only depends on the types of atoms and molecules. It has nothing to do with temperature. Whereas what happens with the kinetic energy, ooh, ooh!

2:59That increases with temperature, which means as we hit this up, the temperature rises and the kinetic energy will start increasing. At one particular point, the kinetic energy of these particles will be high enough that it can partially overcome the forces of attraction.

3:14And when that happens, the atoms and molecules will no longer be locked in place. They will start moving around. This is when solid turns into liquid. In our case, ice starts turning into liquid water. And this temperature at which it happens, for water, it happens to be about 0 degrees Celsius.

3:33And that temperature where liquid turns, sorry, solid turns into liquid, is what we call the melting point. So the melting point of water is 0 degrees Celsius. Now let's keep heating it up further. What happens as we heat it up? Again, nothing happens with the force of attraction, but the kinetic energy will keep rising.

3:49And at one particular point, it will be high enough that it can fully overcome the force of attraction. And then that happens, these molecules will now be free, almost completely free from each other, freely moving about. In other words, our liquid starts turning into gas, water starts turning into steam.

4:08So the temperature at which this happens is what we call the boiling point. And for water, that boiling point happens to be at 100 degrees Celsius. And if you further heat it, well, the steam just gets hotter, nothing else will happen. So when the kinetic energy is too low to overcome any amount of attraction, we have solid.

4:25When the kinetic energy is high enough to partially overcome the force of attraction, we have liquid. And when the kinetic energy is high enough to fully overcome the force of attraction, we get a gas. All right, now let's reverse the whole thing. Let's cool down our gas and see what happens.

4:39Again, nothing will happen to the force of attraction because it does not depend on temperature, but the kinetic energy will reduce and eventually when it goes below the boiling point, look! It will no longer be able to fully overcome the force of attraction, which means the gas will turn into liquid.

4:54We call this condensation, and this point is called the condensation point. And you can clearly see the condensation point is the same thing as the boiling point. And we've seen this before. For example, when you, you know, hold a plate over, say boiling water, we see liquid drops, that's condensation.

5:08The steam over here has temperature lower than the condensation point, lower than 100 degrees Celsius so it condenses into liquid water. And that's why you see the drops over there. Okay, and what happens if we were to reduce the temperature even more? Well, again, the kinetic energy will keep reducing.

5:24And when it's below the melting point, look! It will no longer be able to overcome any force of attraction, which means the liquid will turn back into solid. We call this the freezing point. And you can see the freezing point is the same as the melting point. And therefore, when liquid water, you know, is below 0 degrees Celsius, it freezes into ice.

5:46Okay, so the key thing that we see over here is that the boiling point and the melting point depends a lot on the force of attraction, right? Now, here's a question: What if we consider a material like gold? Well, it turns out for gold, the force of attraction is much higher than that of water.

6:03Now, actually, the attraction is much higher in gold compared to water. So the graph should be much higher over here. But don't worry about that. But this means now the kinetic energy needed to partially and fully overcome the force of attraction would be much higher than before.

6:16And as a result, the freezing point or the melting point and the boiling point would be much higher than before. For gold, it turns out to be, you know, about 1000 degrees Celsius and about like close to 3000 degrees Celsius. That's why for gold, you need a much, much higher temperature for it to melt.

6:36Okay, what about nitrogen? Well, it terms for nitrogen, the force of attraction is much, much lower. And therefore, the melting point and the boiling points would be much lower. And that's why it boils at a much lower temperature of -196 degrees Celsius actually.

6:50That's why at room temperature, nitrogen is a gas. So long story short, the temperature at which the kinetic energy can partially overcome the force of attraction is what we call the melting or the freezing point. That's when you have a phase change from solid to liquid or liquid to solid if you're cooling it down.

7:07And similarly, the temperature at which the kinetic energy is high enough to fully overcome the force of attraction, that's what we call the boiling point. That's when you get a phase change from liquid to gas, or again, if you're cooling it down, from gas to liquid.

7:19And look, since these temperatures purely depend upon how strong or weak the attractive force is and that, in turn, depends upon which types of particles we are dealing with, types of atoms and molecules we're dealing with. That's the reason why the boiling points and melting points of different particles, different substances would be different.