 Okay, if you listen to the previous section, we were talking about specific heat of different materials. We're going to talk about something a little bit different now, but what I want to emphasize is that it's related to specific heat, and I want to emphasize again, different materials, different types of things have different specific heats. That is just a fancy way of saying some things are easy to heat up and cool down. They don't take that much energy, and other things are more difficult to heat up or cool down. They take more energy to do so, and that's usually numerically written with a specific heat number. So one thing about specific heat is the calculations that we did in the previous video, they only work when you're dealing with a temperature change that keeps the material in the same state. If this doesn't make any sense, if you're saying, huh, I will try to explain below and on the next slide. What I am saying is that the specific heat calculations that we did in the previous video, they only work if you're working with a solid, and it turns into a warmer solid, or if you're working with a warm solid and it cools down, but it still stays a solid. Those calculations also work if you're working with a liquid and it gets warmer, or if you're working with a warm liquid and it gets cooler, but it still stays as a liquid. And finally, they work if you're working with a gas, and it gets to be a warmer gas, or if you start with a warm gas, and it cools down, but it's still a gas. In other words, these calculations for specific heat, they only work if you stay in the same state, if you are a solid and stay as a solid, if you're a liquid and stay as a liquid, or if you're a gas and stay as a gas. The calculations do not work if you are a solid and you get heated up so much that you get turned into a liquid, or if you're a liquid and you get cooled down so much that you're turned into a solid. Same thing, they don't work if you're a liquid and you turn into a gas, or if you're a gas that turns back into a liquid, or any other change of state. If there are calculations that you can do to figure out how much energy it takes to go from a solid to a liquid, or from a liquid to a gas, or vice versa, and we're going to talk about those in a minute, but you can't use specific heat for those specific calculations. So, let's keep going. I will try to explain why you can't use specific heat with a graph on the next slide. Here's the graph. It's called a heating curve sometimes. What we are going to talk about is water. So this is my cartoon version of ice, which is frozen water. On the x-axis here is how much energy or how much heat we added to our water. On the y-axis over here is the temperature of our water in degrees Celsius. So you're going to have to remember that water freezes and melts at zero degrees Celsius and that water boils and condenses at 100 degrees Celsius. So let's start by thinking of ice cubes that are very cold. They are ice cubes that are at negative 20 degrees Celsius. Very cold ice cubes are sitting in the freezer and their temperature is right here. What we're going to do is we're going to add some heat or some energy. And as we add more and more heat or energy to our ice cubes, the temperature of our ice cubes rise gets warmer and warmer. They're still ice cubes because we are below zero degrees Celsius. We're somewhere on this diagonal right here. But eventually we will add enough heat or energy to our ice cubes that they reach zero degrees Celsius. What's special about zero is that's the temperature that ice melts at. And once our ice cubes hit zero, you can basically continue to add more energy, so we're adding more and more energy even still. But a weird thing happens. The temperature doesn't rise anymore. It stays flat. I'm sort of highlighting the flat part here. So you're adding more and more energy, but you would think the temperature goes up, but it doesn't. What's happening is all of the energy that you're adding at this point is being used to break the connections that the water molecules have in the ice and basically melt the ice and turn it into liquid water. So even though you're adding energy here, it's not being used to raise the temperature of the water. It's being used to melt the water. So that's what's going on in this flat part right here. Basically the ice is melting. But eventually you're going to add enough heat or energy that you melted all of the ice, and that's where this corner is. At this corner you've melted all of the ice. If you continue to add heat or energy, what will happen at this point is you'll have very cold liquid water. That's my cartoon for liquid water that's very cold. But if you continue to add heat or energy, the temperature of the water is going to start to go up. So it'll go from 0 degrees Celsius to warmer and warmer temperatures. And you can keep adding heat or energy to your liquid water until it becomes very hot liquid water. So here's our very hot liquid water. And at this point we're at 100 degrees Celsius. So this is very hot water. And the special thing about 100 degrees Celsius is this is the temperature that water turns into steam. In other words, this is the temperature that water gets vaporized at. At this point, at this corner, if you continue to add heat or energy to our very hot liquid water, the temperature doesn't go up again. Again, for me, I would not have expected this if I was a beginning student. But that's basically what happens is you add heat or energy at this point, and the water temperature doesn't go up. What happens is all of that extra heat or energy is being used to convert this very hot liquid water into steam. So what's happening is the water is boiling away into steam. Once you add enough heat or energy, you will have boiled away all of the very hot liquid water. And you'll be at this corner over here. And if you continue to add heat or energy, all of your steam will start to get warmer and warmer as well. So this is called a heating curve. The flat parts tell you where your material, what temperature your material melts and freezes at. So water freezes and melts in this flat part, which corresponds to 0 degrees Celsius. Water boils and condenses in this flat part over here, which corresponds to 100 degrees Celsius. So what I am telling you, this is a long-winded way of telling you, let me erase all of this junk, is that the specific heat calculations only work on these diagonals. They only work when the temperature is going up or down. They do not work on the flat parts. So specific heat calculations work there. They work there. And they work in this flat part as well. I'm sorry, in this diagonal part as well. The flat parts need their own special equations. So basically there is an equation or a concept or an idea dealing with melting and freezing and how much energy it takes to melt and freeze something. And there is an idea or concept or equation that deals with boiling and condensing and figuring out how much energy it takes to boil something away. And that's what we're going to talk about in the next couple of videos.