 Now let's look at phases of matter from an atomic viewpoint. So people are pretty familiar with phases. We've got the three most common phases, solid, liquid, and gas. There is actually one more phase called the plasma, but we're not going to be dealing with that fourth phase very often. We're going to stick with solid, liquids, and gases. To help explain this, we're going to use a simulation from PHET at the University of Colorado. And this one is the states of matter basics. So this is a Java program. And when I open it up, I've got a few controls here. And I'm not going to go through this in detail, but we can represent different types of materials. I'm actually going to start with oxygen here, where I've got two individual oxygen atoms. So each oxygen atom is an orange dot, and the two oxygen atoms combined together is an oxygen molecule. Now for my three phases, I can have a solid, a liquid, or a gas. In a solid, I want you to notice that the molecules, although they are vibrating, stay in the same relative positions to one another. In a liquid, they're still up close to each other, but they're allowed to sort of turn around and slip past one another, so they don't stay in the same positions. In a gas, they're actually bouncing off of each other, not connected at all. Now let me show you just a couple more details here. If I start off with a solid and I cool it down, it's a little bit of a crude animation here. As I take the temperature down, notice that the individual little molecules are vibrating less and less as it gets colder and colder. Remember that absolute zero, zero K, had no energy, meaning no vibrations. As I warm it up, we'll see that they start to vibrate more and more at a higher temperature. Now as I continue to add heat, the temperature would continue to go up, but only to a certain point. Now here's where the simulation is not perfect. As they start to vibrate, we're still keeping in the same relative positions, but eventually I start to get a few on the edges, which are starting to slip past each other. It's hard to do this transition smoothly on this simulation, but I've just transitioned from being a solid into a liquid. Notice that they're able to move around past each other. As I continue to heat it, not only do they move around faster and slip by each other more easily, the more I continue to heat it, eventually I'm going to get a few of them which vibrate so hard they're able to break free from the surface. This is the start of the transition from a liquid to a gas. And as I continue to add heat, then it will continue to have a transition with more and more molecules breaking free and being in a gaseous state. Now, I can do something similar to this if I continue to cool it off. Instead of bouncing off of each other, since they're not vibrating quite as much as I cool it, they tend to kind of stick to each other a little bit more instead of bouncing off. And eventually as they're sticking to each other, they're going to condense back down into a liquid. And as they condense back down into that liquid, and it takes a while for the individual droplets to kind of coalesce due to gravity, eventually I can get to the point where not only are they together, but they're not moving back and forth relative to each other either. So I'm going back into a solid state. Now, just to summarize then from the atomic point of view, I can think of the phase properties for solid, liquid, and gases having to do with their vibrations but how those individual things are bound to each other. The atoms and molecules are always vibrating and a higher temp means more vibration. And that phase change needs energy. So I need energy to change the temperature, but I also need energy to change the phase. We'll be looking at the equations for these in more detail as we go along. So this is the phases of matter from an atomic perspective in physics.