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- [Voiceover] If you look at most substances, so this is most things, right over here. As we go from a liquid state where things are literally fluid, the molecules are moving past each other, to a solid state, where things are rigid or more rigid, you typically have something getting more dense. So the liquid state would be less dense. Less dense. And the solid state would be more dense. Would be more dense. Let's think about what would happen if water were like most things here. So imagine a body of water. So let's say that's the land right over here, this is a cross section of a body of, I don't wanna draw brown water, I wanna do blue water. So this is a cross section of body of water, like say, we see in this picture here. Now let's say it is very cold. That the air is extremely cold. Let's say it's below the freezing temperature of water. Well what would happen is the water up here would freeze. And this is actually what does happen. Water at the surface would freeze, but if this ice were more dense, the solid water were more dense than the liquid water, well then the ice would sink. The ice would sink and collect at the bottom. The ice would collect at the bottom right over here. And then the water over here would freeze again. It would freeze again. But if that were more dense, then it would sink and collect at the bottom. And you keep going with this process on and on and on, and eventually the entire body of water, the entire lake would freeze solid. Would freeze... It would freeze solid. Now you can imagine this wouldn't be that good for the animals that are living in the water. If you imagine some fish in here. Those fish would then freeze. And most living things, there are a few simple organisms that can survive being frozen. But most living things would just die. And so this would not be a good environment for animals to live in, or for even biology to take place. But what's neat about water, it does not follow this pattern. When we're talking about water, when we're talking about water, when we go to the liquid state, when we go from liquid water to solid ice, to solid ice, we actually get, we actually get less dense. So this right over here is less dense. This is why ice floats. This is more dense, more dense. And this is less dense. And to think about why that is, it all goes back to the hydrogen bonding. So we've seen in previous videos. So, I'm gonna do it all in one color. Oxygen, hydrogen, hydrogen. Let's say this is the liquid state that I'm drawing right over here. This is liquid water. Liquid water. So then you have oxygen, and you have oxygen and hydrogen and hydrogen. And you have oxygen and hydrogen and hydrogen. We've already talked multiple times about the fact that you have partial negative charges at the side away from the hydrogens. Partial negative, partial negative because oxygen is so electro-negative. And you have partial positive charges on the hydrogen ends. Partial positive charges at the hydrogens and these partial negatives and partial positives attract each other and this is called hydrogen bonding. Now the liquid state, you have enough energy. The temperature is just really average kinetic energy that these molecules are able to bounce around and flow past each other. These hydrogen bonds get broken and get reformed over and over again. These things flow past each other and also they have enough energy to kind of push even closer to each other than even the hydrogen bonds would dictate. Sometimes they go closer, sometimes they're further, sometimes they're pushing around. But as we get-- As we get cooler, as we get cooler and we lose heat, then they don't have the kinetic energy to kind of-- They get closer and bump up against each other and move right and flow right past each other and they form a lattice structure where it will look more like this. Where it will look like oxygen, hydrogen, hydrogen, oxygen, hydrogen, hydrogen, oxygen, hydrogen, hydrogen. And so you actually have the molecules being further apart from each other because they don't have the kinetic energy, once again, to keep breaking and reforming these hydrogen bonds, or even to kind of have the kinetic energy to push up against each other. And so you form this lattice structure. You could even have another oxygen. And I'm obviously drawing it in two dimensions, but ice, things in our universe, in our reality are three dimensions. But hopefully this gives you the big picture of it. Oxygen, hydrogen, hydrogen. And so you have these hydrogen bonds. I wanted to do those in white. These hydrogen bonds. And remember, these hydrogen bonds, they're still, it's still the case, electrons are still moving around, and so you still have a partial positive charge here and partial negative charge there, but there's not enough kinetic energy to push them close to each other and flow past each other and so you have ice right over here. You have solid water. You have ice over here being less dense. And this is what keeps ice floating, this is what keeps icebergs floating, this is what keeps lakes like this from freezing solid. This is why the surface freezes, but you have water below it. In fact, that ice on the top protects the water below from... You can imagine, the ice, it protects it from getting even further frozen and so you have animals able to just hang out. Hang out down here and go through the winter and then everything will thaw once you get to the spring and the summer.