Hi, and welcome back to my videos for General Chemistry 1. For the last dozen or so videos we've been exploring atoms in the ways they behave and combine to form molecules, and we've now got a pretty good idea of the properties of molecules and what they look like. Now we're ready to step back and look at the properties of solids, liquids and gases, and how what the molecules are doing at the atomic scale affects the world we see with our eyes every day. As you probably know, the atoms and molecules in solids are locked into a particular position, like the sodium and chlorine molecules in this sodium chloride crystal. This is because the attractions between the molecules is so strong that they're locked into place. In order to break some of those attractions, we have to put in energy in the form of heat. When we heat a solid, we overcome some of those attractions and the substance melts into a liquid. In a liquid, the molecules are able to move around because they move fast enough to overcome some of the attraction they feel for their neighbors. But not all the attraction. They do still stick together enough so that gravity can pull them to the bottom of a container. We can see that in this simulation. The molecules in this liquid are able to move and slip past one another, but they can escape one another. We'll talk about the forces that caused the molecules to stick together in a liquid when we get to the general chemistry to course, so I hope you'll come back for that. However, in a gas, we've heated the substance so much that the molecules move too fast to attract each other as in liquids or solids, so the molecules in a gas separate from each other. That makes gases a lot simpler to think about than liquids and solids, so we'll spend the rest of this course talking about the gases and how they work. For hundreds of years, it was very difficult to study gases because it takes some pretty sophisticated technology to make airtight containers where you can store gases and learn more about them. But that started to change into the 17th century, and by the end of the 19th, we had a pretty sophisticated idea of how gases behave and why. The most important theory about gases is called the kinetic theory, and in a nutshell, the basic idea is that the molecules in a solid are strongly attracted to each other, and the molecules in a gas are only weakly attracted. Kinetic theory was developed over a long period of time, but two of the most important founders of it were the Dutch physicist Daniel Bernoulli in 1738, and the German physicist Rudolf Clausius, over a hundred years later, in 1857. He are actually pretty familiar with some of the work of Daniel Bernoulli, even if he never had studied him in class before. He developed what's called the Bernoulli principle, which says that an increase in the speed of a gas causes a drop in the pressure. It's one of the most important concepts that allows airplanes to fly. The top of an airplane's wing is shaped so that air flows faster over the top of the wing than over the bottom, so the pressure above the wing is lower than on the bottom, and that pushes the airplane up. Bernoulli's principle is also what makes your shower curtain attack you when you take a hot shower. The hot air rising inside the shower causes a slight drop in pressure, so the air outside the shower pushes the curtain in. The other creator of kinetic theory was Rudolf Clausius, who built on Bernoulli's theories and stated the basic theory of gases that we still use today. The kinetic theory has five important points that will help us in the rest of this course. First it says that the molecules in a gas are always in motion and they move in random directions. The fact that they move in random directions is actually pretty important. Imagine if the air molecules in your room could all move in the same direction at once. That would mean the sight of the room that the molecules had moved away from would suddenly contain a vacuum. We know that things like that don't happen. The next idea we get from kinetic theory is that the molecules in a gas are far apart from each other relative to the size of the molecules. That means that gases are mostly just empty space. The molecules themselves take up almost no volume. That's why when you boil just a tiny amount of water, the water vapor can take up a huge volume compared to the liquid you boil to get it. The third idea we get from kinetic theory is that the molecules in a gas feel almost no attraction or repulsion for each other. You might remember that we said that the molecules in a liquid stick together because they feel forces that attract them to each other. According to kinetic theory, the gas molecules don't feel those forces anymore. The next idea from kinetic theory is that when gas molecules collide, energy might be transferred from one molecule to another, but the average kinetic energy of the molecules is constant. Another way of saying that is that the total amount of heat in the gas doesn't change. You can see that in this simulation. These gas molecules are colored according to the amount of their kinetic energy. Red molecules have the most energy and blue ones have the least. In a high energy molecule collides with a low energy one, you can see that their energies change, but they still have the same average energy. You might recognize that as a very similar law to the law of conservation of energy, which we've talked about before. The last idea we get from kinetic theory is that this average kinetic energy we've been talking about is proportional to the temperature. So if the temperature of a gas is high, what that means is that the molecules have a high kinetic energy. That means we need to talk a bit more about the temperature now. By now you're used to using Celsius temperatures in your chemistry courses. If you live in the US, that probably took a little getting used to, since non-scientists in the US tend to use the Fahrenheit scale instead. But from now on, in this course, we won't want to use either one of those temperature scales. The reason has to do with something we just mentioned. We just saw that the kinetic theory tells us that a gas' temperature is proportional to its kinetic energy. So when we lower the kinetic energy, the temperature goes down. But suppose we had a gas at one degree Celsius, and we then dropped its kinetic energy a bit. If we did that, the temperature would decrease, perhaps to negative one degree Celsius. But that doesn't make sense. The temperature should be proportional to the kinetic energy, and kinetic energy is always a positive number. So in order to make sense, our temperature scale should never give us negative number. The person who came up with that temperature scale was the Scottish engineer William Thompson, better known as Lord Kelvin. Kelvin is also one of the many people you have to thank a few years of cell phone. Kelvin was one of the engineers that helped to lay the very first transatlantic cable in 1866. That was a telegraph cable, but it was definitely a direct ancestor of modern global telecommunications. It's also the reason why we call him Lord Kelvin, and not just William Thompson. For his work on the transatlantic cable, he was given a knighthood. He suggested what he called the absolute temperature scale, where the temperature decreases as the kinetic energy goes down, until the temperature reaches zero when the kinetic energy is zero. Nowadays, we call this the Kelvin scale in his honor, and he actually proposed it in 1848 when Rudolf Clausius was still putting the finishing touches on kinetic theory. It's actually very easy to convert temperatures from Celsius to Kelvin and back. Zero degrees Celsius is 273.15 Kelvin. That means that in order to convert a Celsius temperature to Kelvin, we just add 273.15. So, for example, 25 degrees Celsius would just be 298.15 Kelvin, and negative 70 degrees Celsius would be 203.15 Kelvin. One thing that's important to notice is that although temperatures have different numbers in the Celsius and Kelvin scales, temperature differences do not. For example, suppose we changed the temperature of a gas from 35 degrees Celsius to 120 degrees. That gives us a temperature change of 85 degrees. And now let's try that again in Kelvin. Our beginning temperature was 35 degrees Celsius, which is 308.15 Kelvin. Meanwhile, our final temperature was 120 degrees Celsius, and that's 393.15 Kelvin. If we take the difference, you can see that what we get is 85 Kelvin. That's the same as the temperature change we got in Celsius. So the change in temperature is the same number in both the Celsius and Kelvin scales. That's going to be very important to remember, and a lot of the calculations will be doing in the rest of this course. And in the general chemistry, too, of course. The temperature is a different number in Celsius and Kelvin, but the temperature change isn't. Well, that's enough for now. In the next video, we'll be ready to talk about pressure and what pressure really is, and that'll give us a chance to talk about the very first modern chemists. They're the people who first changed chemistry from the disorganized and secretive practice of alchemy, and turned it into something that we'd recognize as science. It's a fun bit of history and covers some of the very earliest material that we'll talk about in this course. So I hope you'll join me for that. Until then, have a good week.