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Surface tension

Water molecules are held together by hydrogen bonds, which give water its unique properties. At the surface of water, molecules are more densely packed because they are not being pulled from above, resulting in stronger intermolecular forces. This creates surface tension, which allows for phenomena such as water droplets maintaining a round shape and insects walking on water.

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Video transcript

- What we have here is a zoom-in of the surface of water. So up here you have the air, this is the air, these are some air molecules, maybe they're nitrogen molecules. They're fairly far apart, in fact, in reality, they would be even more far apart than this. And then over here you have water molecules. We've seen this many times. You have the oxygen atom and it's bonded to two hydrogen atoms, and the oxygen atom likes to hog the electrons more. It's more electronegative, so you have a partially negative charge at this end and partially positive ends at this end. And that attraction between the partially positive ends and the partially negative ends, that's what gives water all sorts of neat properties. Those are the hydrogen bonds. Those are the hydrogen bonds that give water all sorts of neat properties and keep it in its liquid state at a standard temperature and pressure. Now what I want to think about is the surface in particular. And if you look at the surface of water, it might look completely smooth. But if you were to zoom in on a molecular level, you'll see that, well, it's just made up of these molecules. But roughly speaking, roughly speaking, let's just say that this is roughly the surface, the surface of the water. The surface of the water. Now, what's going on at the surface? Well, all these molecules are interacting through hydrogen bonds. Let's say this molecule right over here, it has hydrogen bonds pulling on it upwards, up to this one, pulling it this way, pulling it downwards, pulling it in really, really, to some degree, almost every direction. And they all have their kinetic energy and they're bumping around, but they're flowing past each other. The hydrogen bonds are giving that cohesiveness. The molecules are attracted to each other. But if you look at the molecules on the surface, if you look at the ones on the surface, sure, they might have stuff pulling down on them, they might have stuff pulling them to the side, but they don't have anything pulling on them from above. And because of this, you could imagine that they're able to get a little bit more densely packed, that they're able to get a little closer to their neighbors. And this is what allows them to actually have a stronger, I guess you could say, intermolecular force at the surface than you have within the body, and that causes a phenomenon known as surface tension. So you have stronger, you have kind of a deeper, and this is still just hydrogen bonds, but since they're not being pulled in other directions by, upwards by the air, they're able to get a little bit more closely packed, a little bit tighter, and this we refer to as surface tension, surface tension. And you have probably observed surface tension many, many, many times in your life in the form of, say, a water droplet. A water droplet, it's able to have this roughly round shape because all the little water molecules on the surface of the water droplet, and here the surface might even be on the bottom of the water droplet. They are more attracted to each other than they are to the surrounding air, so they're able to form this type of a shape. You might've seen it if you go to a pond or a stream sometimes, so you see some still water. And let's say, let me do this in blue. So let's say that this is the surface of the water right over here. You might have seen insects that are able to walk on the surface of the water. And I'm not doing a great job at drawing the insects. They don't look exactly like that. But they can walk on the surface of the water. You might've seen or you might've even tried to do something like put a paperclip on the water. And even though this thing is actually more dense than the water and you might expect it to sink, but because of the surface tension, which really forms something of a film on top of the water, the thing won't penetrate the surface, so the paperclip will float, unless you were to push on it a little bit and it allow it to puncture the surface, and then it would actually sink, which is what you would expect because it is actually denser. You'd even see this if you were to take a cup, if you were to take a cup and you were to fill it all the way up to the rim and then a little bit higher, it won't immediately overflow. It won't immediately overflow. If you're very careful, you'll see that you form a bulge here. And that bulges because those individual water molecules are more attracted to each other than they are to the surrounding air. So that allows for something of a little bulge. Obviously if you keep pouring water, at some point, they're just gonna start overflowing because gravity's gonna take over there. Gravity's gonna overwhelm the surface tension. But this bulge will actually form. So surface tension, it is really due to the cohesion of the water. Remember, cohesion is when the molecules are attracted to each other. And it definitely, and especially because they're more attracted to each other than the surrounding air.