About this video
This lesson explains why water forms round droplets and why it sticks to surfaces, using two forces between water molecules. Cohesion is attraction between particles of the same substance, and adhesion is attraction between particles of different substances. The video links both to everyday effects such as spilling, the curved meniscus, and fog nets that collect water from air.
You'll be able to
- define cohesion and explain how it shapes water droplets
- define adhesion and explain how it makes water stick to surfaces
- apply cohesion and adhesion to explain a real-world effect such as fog nets
Key words
- Cohesion
- The attraction between particles of the same substance, such as water molecules.
- Adhesion
- The attraction between particles of different substances, such as water molecules and glass.
- Surface tension
- The force that holds a liquid drop together, which pulls it toward a spherical shape.
- Meniscus
- The curved surface of a liquid that rises or dips where it touches the side of a container.
Transcript
- [Instructor] Why does water form these beautiful droplets? And why do I always spill water when I'm pouring it from one glass to another? And what's this all got to do with capturing water from air using fog nets? Well, let's find out. So let's start with this first question, why does water form beautiful droplets?
To answer this question, let's zoom in all the way to the molecular level. So if you could zoom in, we would find these water molecules zooming around at very high speeds. But an important thing about these water molecules is that they attract each other, kind of like magnets.
I'm using these dash lines to show this force of attraction, okay? But they don't stick to each other because they're all moving very fast. But this force of attraction kind of keeps them together. And it's the same reason why we get this beautiful drops, because the water molecules on the surface over here are being pulled inwards by the water molecules below due to this force of attraction.
And we give a name to this force, we call this cohesion. So in general, what is cohesion? Cohesion is when particles of the same substance are attracted to each other. For example, particles of water, which is basically water molecules are being attracted to each other.
Cohesion is the reason why we get this spherical shaped drops. And guess what? Turns out that cohesion is much stronger in water compared to other liquids. For example, look at the drop of, say, an alcohol. You can see that it's much more flattened compared to that of water.
That's because cohesion is less over here. The cohesion over here is more, but if you come back over here, we could see that, you know, the drop is not perfectly round. Why is that? Well, we have the surface tension holding the drop together, but we also have forces acting on the droplet from the leaf pushing up on the bottom.
And that's why, look, it's more flat at the bottom. So here's the question. What if you could see a water droplet that isn't getting pressed against anything? Then you would see the shape surface tension alone creates a sphere. In fact, here is NASA footage of water drops in orbit.
When nothing is pressing up against them, what do you see? It's pretty much spherical. That's awesome, right? All right, onto the next question. Why does water spill when I am pouring it from one glass to another? Well, again, if you were to look at it over here carefully, you can see that the water is kind of sticking to the glass.
And again, the reason for that is if you were to zoom in, and again, we have these water molecules moving around over here. And let's say this is the particles of glass. I've not shown particles of glass over here. But guess what? It turns out that water molecules not only attract each other, but they're also attracted to the particles of glass.
And this force is what makes the water stick to the glass and making it spill. This is called adhesion. So adhesion is when particles of one substance, like water molecules are attracted to another substance like that of glass particles. So water molecules have cohesion, which means they're attracted to each other, and they also have adhesion.
They're also attracted to other particles. Here's another consequence of adhesion. If you were to look at water in a tiny test tube like this, this is the surface of the water, you can clearly see that the level of the water is not exactly the same. It's lower in the middle, and it's higher at the ends, at the edges or where it's touching the glass.
And this curved surface that we see over here, it's called a meniscus. But again, the question is, why is that happening? Why do we see a meniscus over here? Well, again, this is because of adhesion, because water molecules are also attracted to the particles of the glass.
This is the reason why this side's near the glass. That particular side of the water is raised a little higher. And another application of this is seen in something called rain chains. We can see water is sticking to the chain as it flows down. Again, this is adhesion, water's ability to get attracted to other particles.
But there are so many examples of cohesion and adhesion of water. And one of my favorite ones is fog nets. These are used to literally pull out water from air. But how does it work? Well, again, if we were to zoom in over here, well, there is always water molecules in the air.
Now, when they come in contact with these fog nets, which are basically vertical wires over here, because of adhesion, they stick to the wire. And then because of cohesion, more and more water molecules can stick to each other. And then eventually they become big drops.
And because of gravity, they fall down and they can be collected. So this is how fog nets make use of both cohesion and adhesion, to pull out water from air. And it's useful in places where there's scarcity of water, which I find is pretty amazing. So to summarize, we saw two properties of water, cohesion and adhesion.
Cohesion is where particles of the same substance are attracted to each other. And the way I like to remember it is, well, cohesion has the word co. And so co stands for cooperate. And so the particles cooperate and so sort of like I get attracted to each other.
And one of the consequence of that is that we get these beautiful spherical droplets. But they also show adhesion. Here are particles of one substance is attracted to the particles of the other substance. And again, the way I remember it is, adhesion reminds me of the word adhesive.
Adhesives are used to stick two different substances together, right? So that's how I remember what adhesion is. And this is one of the reasons why, for example, you know, when you pour water from one glass to another, you end up spilling it, or at least I end up spilling it.
And remember, both of these arise from the water molecules magnets like properties to stick to each other and to others.
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