This video explains surface tension, viscosity, evaporation, and boiling point.
TRANSCRIPT:
So there’s a couple of different liquid properties that we’re gonna go over today, and surface tension happens to be the first one. So let me just draw out my example first. We have a car surface we’re gonna pretend this is, and a ton of beads of water on top of it. So this is an example of surface tension, because surface tension is caused by the attractions of the outside molecules of a liquid to the inside ones. And I chose water beads because they’re a really good example of this. Let me just draw in the water molecules first. These are the outermost ones, and we’ll say that there’s a couple of inner ones. Alright. You’ve probably seen my other video before on hydrogen bonds, but what they basically are, is that, the partial positive hydrogen on one water molecule are going to the attracted to the partial negative oxygens on another water molecule. So like I wrote on this one, the outer water molecules are going to be pulled in by those inner ones, creating this sort of dome shape.
Basically, surface tension is really the attractive forces between the outermost level molecules and the inner ones, and that’s going to pull the outer ones in and in and in until the surface area is minimized. Alright, so next one is viscosity. Viscosity is basically a liquid’s resistance to flow. So the higher the viscosity, the slower it’s going to be – think of syrup or molasses. The lower the viscosity, the faster it’s going to be. So we would say water has a lower viscosity than syrup because it flows faster. Next one is evaporation – that’s pretty easy. That’s when parts of a liquid evaporate into a gas, so let’s go back to water again. Let’s say we have a little container of water and some of the water molecules are evaporating into water vapor. I’d like to point out that you have an average kinetic energy that you start out with when you have your water, and when you heat it up, it’s obviously going to increase because the energy from whatever source you’re heating the water with is going to go into here, make the water molecules go faster, and some of the fast ones are going to go out. In fact, the fastest water molecules are going to escape and go out into the open air, and that, in turn, is going to decrease your average kinetic energy. Basically, when you heat up a container of water, the fastest molecules leave, and those fast ones – since they left – that is going to decrease the average kinetic energy because the fastest ones left so you’re left with slower ones.
The last one is boiling point. That’s really easy. So I like to think of it in terms of this: when vapor pressure equals atmospheric pressure. So a liquid boils when its vapor pressure equals the pressure in its atmosphere. And I’ll make a whole separate video on vapor pressure for you later, but what vapor pressure really is, is the amount of pressure that builds up when some of your liquid molecules evaporate.
