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Main content
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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 they 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 keep gives water all sorts of neat properties that 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 just 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 zoom in on a molecular level you'll see there 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 of these molecules are interacting through hydrogen bonds they say this molecule right over here it has hydrogen bonds pulling on it upwards up to this one pulling in this way pulling it downwards pulling it in really really into some degree almost you know every 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 too 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 then you have within the body and that causes a phenomenon known as surface tension so you have you have stronger you have kind of a deeper and this 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 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 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 have seen it if you go to a pond or a stream sometimes so you see some still water and say let's say let me do this in blue so let's say that this is 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 have seen or you might have even try to do something like put a paper clip 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 the top of the water the thing won't penetrate the surface so the paper clip will float unless you were to push on it a little bit and it would allow 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 is 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 going to start overflowing because gravity's going to take over there gravity's going to overwhelm the surface tension but this bulge 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 then the surrounding air
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