in this video we'll talk about surface tension as it applies to water so surface tension it's this property of a liquid like water that lets it resist an external force that's because water molecules they're sticky they kind of stick together due to hydrogen bonding and that forms the surface tension so something like this insect here this water strider it's pushing down gravity is pulling it down but the water is resisting that push because of the surface tension so it can be on top of the water we even see surface tension in outer space here on the space station surface tension that's what's making the water stick together and form this ball so let's look at this diagram here to see how that works we'll assume this black here these black dots they're the water molecules water molecules in the center of the liquid like at the center of that sphere we saw on the space station those water molecules in the center have more opportunity to bond form weak hydrogen bonds with other water molecules but the water molecules on the surface they can only bond with the molecules below them and the ones next to them and because there are fewer of these bonds these bonds are stronger and that's what creates the surface tension let's look at a simulation to really help us understand that so here we have a group of water molecules and they're in three dimensions you can imagine them in three dimensions here if you look at them closely you'll see that the red is always pointing towards a white that's because with water molecules the oxygen the red's a little more electronegative so this is kind of a little more negative the hydrogens are more positive they line up so the positive and the negative attract each other and you'll see that for all of these molecules here let's draw some hydrogen bonds in so the green those are the hydrogen bonds between atoms they're not very strong but they're strong enough to influence the properties of water in real life these molecules would be moving let's do that so there they go and you can see they're moving around hydrogen bonds are breaking and reforming and they're loosely forming the sphere just like we saw on the space station let's add some more water molecules so here we have a bunch of water molecules and you can see the reds are pointed to the whites and the whites are pointed at the reds that's the hydrogen bonds let's draw them in and roughly it again takes the shape of a sphere what you'll notice though the atoms on the outside on the surface of this sphere they can't form as many hydrogen bonds as the atoms on the center that's the surface tension because they can't form as many bonds those bonds are going to be stronger and that'll create surface tension and it will actually force the water to take the shape of a sphere let's go back to our diagram so you can see back at our diagram the water molecules on the surface they don't form as many bonds as those in the middle of the water because of that these bonds are stronger these molecules are actually a little closer together and we have surface tension there are things that will reduce surface tension soap is excellent at reducing surface tension if we put some soap in here that would weaken the hydrogen bonds here and we wouldn't have as much surface tension in the water could be tough for our little bug friend temperature and other types of solvents or solutes put within the water here they also influence surface tension this is dr b with the surface tension of water thanks for watching