Capillary action and why we see a meniscus | Chemistry | Khan Academy

– If you were to take a glass beaker, so let me draw it right over here. If you were to take a glass beaker and you were to fill it up with water, you might expect that the surface of the water would be flat. But it’s actually not the case and I encourage you to try it. You might have even observed this before. The surface of the water will not be flat. The surface of the water
will actually be higher near the glass than it is
when it’s away from the glass. It forms a shape that
looks something like that. And so the first thing we might ask is what’ll we call this thing. And this right over here
is called a meniscus. Meniscus. And in particular this meniscus, because the fluid is
higher near the container than it is when you’re
away from the container, we would call this a
concave, concave meniscus. And you might say, “Well if
this is a concave meniscus, “are there any situations where might have “a convex meniscus?” Well sure, you can have a convex meniscus. If you were take that same glass beaker, instead of filling it with water if you filled it with say, mercury. If you filled it with mercury, you would get a meniscus
that looks like this where there’s a bulge near the center when you’re further
away from the container than when you’re at the container. And so let me just label this. This is a convex, convex meniscus. But it’s one thing to just
observe this and to name them. To say, “Hey this is a meniscus.” So this is a concave meniscus. But a more interesting question is why does it actually happen. And so you might imagine
this concave meniscus is because the fluid is more attracted to the container than it is to itself. And you might be saying, “Wait, wait. “Hold on, hold on a second here. “We’ve been talking about how water “has this polarity, it
has partial negative end. “Each water molecule
has a partially negative “and has partially positive
ends at the hydrogens.” So let me write this down. Partial positive charges at the hydrogens. And that causes this
hydrogen bonding to form and that’s what kind of gives water all of these special properties. “You’re telling me that
it’s more attracted to the glass than it is to itself?” And I would say, “Yes,
I am telling you that.” And you could imagine
why it is going to be more attracted to the glass than itself, because glass actually has, the molecules in glass
actually are quite polar. Glass, typically made up
of silicon oxide lattice. For every one silicon atom,
you have two oxygen atoms. You see that right over here. For every one silicon,
you have two oxygen atoms. And it turns out that the
electronegativity difference between oxygen and silicon is even higher than the electronegativity difference between oxygen and hydrogen. Silicon is even less
electronegative than hydrogen. So the oxygens are really able
to hog silicon’s electrons. Especially the ones that
are involved in the bonding. So you have partial charges,
partial positive charges form at the silicon
and then you still have partial negative charges
form around the oxygens. Form around the oxygens. So these are partial negative. And partial positive at the silicon. And so you could imagine what’s going to happen at the interface. And let me make this
clear what’s going on. This, what I am circling
right now, that is the water. This right over here,
that’s the water molecules. And what we see over here, what we see over here, these
are the glass molecules. So this is the glass right over here. And sure the water is attracted to itself because of the hydrogen bonds. But it has some kinetic energy, remember these things are jostling around, they’re bouncing around,
we’re in a liquid state. And so you can imagine all of a sudden, maybe this, let me see,
maybe this character, this water molecule right over here. Maybe a moment ago it was right over here but it popped up here. It just got knocked by another molecule, it had enough kinetic
energy to jump up here. But once it came up, came in contact with the glass surface right
over here, the glass molecules. It stuck to them. Because its partially positive end, its partially positive
end at the hydrogens. Let me do it in that green color. The partially positive
end at the hydrogens would be attracted to the
partially negative ends of the oxygens in the glass. And so it’ll stick to it. This is actually a stronger partial charge than what you would
actually see in the water because there’s a bigger
electronegativity difference between the silicon and
the oxygen in the glass than the oxygen and the
hydrogen in the water. So these things just keep bumping around. Maybe there’s another water molecule that just get knocked in the right way. All of a sudden for, you know, a very brief moment it
gets knocked up here. And then it’s going to stick to the glass. And this phenomenon of something sticking to its container, we
would call that adhesion. So what you see going on here, that is called adhesion, adhesion. And adhesion is the
reason why you also see the water a little bit higher there. When you talk about
something sticking to itself, we call that cohesion. And that’s what the hydrogen bonds are doing inside the water. So this right over here, that over there, that is co-, that is cohesion. So that’s why we have things, why we observe a meniscus like this. But there’s even more fascinating
properties of adhesion. If I were to take, if I were
to take a container of water. If I were to take a container of water. And just to be clear what’s
going on here with the mercury, the mercury is more attracted to itself than it is to the glass container, so it bulges right over there. But let’s go back to water. So let’s say that this
is a big tub of water. I fill it. So, I fill the water right over here. And let’s say I take a glass tube, and the material matters. It has to be a polar material. That’s why you’ll see
the meniscus in glass, but you might not see
it or you won’t see it if you were dealing with a plastic tube because the plastic does
not have that polarity. But let’s say you were
to take a glass tube, a thin glass tube this time. So much thinner than even a beaker. So you take a thin glass tube
and you stick it in the water, you will observe something very cool. And I encourage you to do this if you can get your hands
on a very thin glass tube. You will notice that the
water is actually going to defy gravity and start climbing
up this thin glass tube. And so that’s interesting. Why is that happening? Well this phenomenon which
we call capillary action. Capillary, capillary action. The word capillary, it’ll
refer to anything from you know, a very, very narrow tube and we also have capillaries
in our circulation system. Capillaries are our
thinnest blood vessels, those are very, very, very, very thin. And there’s actually capillary action inside of our capillaries. But what we’re seeing here, this is called capillary,
capillary action. And it’s really just this
adhesion occurring more intensely because more of the water
molecules are able to come in touch with the polar glass lattice. And so you can imagine we have glass here. If you also had glass over here. And actually it would be very hard to find something that thin that’s on the order of only a few molecules. But this is, I’m not
drawing things in scale. You can imagine now okay, maybe another water
molecule could jump up here and stick to the glass there. And one just gets bumped the right way, jumps up and jump there. And if we didn’t have a polar container, if we didn’t have a hydrophilic container, well then the thing might
just jump back down. But because it went up there,
it kind of just stuck to it. And then it’s vibrating there and then maybe another water
molecule gets attracted to it because of its hydrogen bonds. Then it gets bumped the right way. And then it gets bumped with the higher part of the container
but then it sticks there. And so it starts climbing the container. And that’s what capillary action is and it’s not just some neat parlor trick, we actually probably use capillary action in our every day lives all the time. Beyond the fact that
it’s actually happening in your capillaries in your
body that allows you to live, but if you have a, if you spill
something on your counter. So let’s say that’s a
spill right over there. You spill some maybe,
you spill some water, or you spill some milk. And if you take a paper towel. If you take a paper towel. In fact, if you took a
paper towel like this. If you held it vertically,
you will see the water start to be absorbed into the paper towel. This kind of absorption
action that you see, that actually is capillary action. It’s the water going into
the small little gaps of the paper towel, but
that’s because it is attracted to the actual paper towel.

51 thoughts on “Capillary action and why we see a meniscus | Chemistry | Khan Academy

  1. I have 2 questions? One what draws water to paper towel and 2 why doesn't capillary action happen on the other side of the of the tube (The outer area).

  2. This fire mixtape on capillary action is better than that other fire mixtape about balancing chemical equations.

    dont ever take me seriously

  3. I have one question then why does distilled water doesn't have enough adhesion instead it forms flat meniscus

  4. This is painful to watch. The silica surface is nothing like this guy thinks it is, it is completely covered with hydroxyl groups, in fact you have to go to high temperatures to get rid of them and they reappear instantly when exposed to the ambient environment. The wetting and the capillary forces are caused by the water hydrogen bonding to the silanols (Si–OH) groups on the surface.

  5. I have 1 question; as capillarity is due to adhesion then while calculating capillary rise, why we take surface tension in free body diagram instead of adhesive force

  6. But just adhesion doesn't explain capillary action. If it were the case, then the height that the water could rise to would be independent of the tube's length. But that would mean a perpetual fountain if the tube was short enough, which is not what we see. It also has to do with air pressure being lower inside the tube because of the small space that impedes high velocity air particles. Inside the tube there's a gradient of pressure, with a minimum in the middle of it, and normal air pressure at the ends.

  7. That so cool. I never realized it is the positive polarity of the glass that attracts the negative polarity of the water to it.

  8. I immediately subscribed without hesitation. this dude explains so well without using convoluted language. this is actually helping me

  9. The 13yr old voice in my head says: Imma start callin’ my junk Freddy Mercury because it’s bulging!
    The 17yr old nerd voice in my head says: My bulge puts the Hg in Huge!

  10. I’m taking Fluids Mechanics this semester and I really need to understand this forces. Thank you so much Saul!!!!!

  11. So if I had a tube made of something that Mercury "Adheres to" stronger than it does itself; would it have a concave meniscus? and be sucked up the tube (a tiny bit because it's heavy)? I think yes, so that means capillary action relies on not only the size of the tube, but the liquid and tube materials used!?!?!?! Huh….Whut?

  12. I need help a lot about my science project that is why I am around these things I am one of the good kind of things in the school science fair I love participating in it you get prizes that is awesome

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