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

Summary

This script delves into the fascinating phenomenon of meniscus formation in liquids, particularly water and mercury, within a glass container. It explains how the surface of water in a glass beaker forms a concave meniscus due to the adhesion of water molecules to the glass, driven by the polarity of the glass and hydrogen bonding in water. The script further explores the concept of capillary action, demonstrating how water climbs up a thin glass tube against gravity, a result of intense adhesion to the polar glass surface. The explanation is enriched with analogies to everyday life, such as the absorption of spills by paper towels, highlighting the relevance of these scientific principles in our daily experiences.

Takeaways

• 🍸 The surface of water in a glass beaker is not flat but forms a meniscus due to the interaction between the water and the glass material.
• 🌀 A concave meniscus occurs when the fluid adheres more to the container than to itself, which is common with water in a glass container.
• 🔍 The polarity of glass, typically made of silicon oxide, contributes to the formation of a concave meniscus due to the stronger electronegativity difference between silicon and oxygen compared to hydrogen and oxygen in water.
• 🔬 The phenomenon where water climbs up a thin glass tube against gravity is known as capillary action, driven by the adhesive forces between water and the polar glass surface.
• 💧 Capillary action is not limited to glass; it also occurs in our body's capillaries, playing a crucial role in our circulatory system.
• 🧪 The process of capillary action can be observed in everyday scenarios, such as a paper towel absorbing a spill due to the water's attraction to the paper material.
• 🤔 The script encourages viewers to conduct experiments, like using a thin glass tube, to observe the effects of adhesion and capillary action firsthand.
• 🔝 The script explains that adhesion is the attraction of water to the glass, causing the water level to rise slightly in the container, while cohesion is the attraction of water molecules to each other, maintaining the integrity of the liquid.
• 🌐 The script uses the example of mercury to illustrate a convex meniscus, where the metal adheres more to itself than to the glass, resulting in a bulge in the center of the container.
• 🚫 The script points out that the type of material used for the container significantly affects whether a meniscus is observed, with polar materials like glass showing a more pronounced effect.
• 🔬 The electronegativity difference and the resulting partial charges on atoms within the material influence the type of meniscus formed, with water showing a concave meniscus in glass due to stronger attraction to the glass's oxygen atoms.

Q & A

What is the shape of the water surface when a glass beaker is filled with water?

The water surface forms a meniscus, which is higher near the glass than it is away from the glass, creating a concave shape.

What is a meniscus and why does it occur?

A meniscus is the curved surface of a liquid in contact with a solid, such as the water surface in a glass beaker. It occurs due to the intermolecular forces between the liquid and the solid, which can be stronger than the forces within the liquid itself.

What is the difference between a concave and a convex meniscus?

A concave meniscus is when the liquid surface is higher near the container, like water in a glass beaker. A convex meniscus is the opposite, where the liquid bulges away from the container, such as mercury in a glass tube.

Why does water form a concave meniscus in a glass container?

Water forms a concave meniscus in a glass container because the partial positive charges at the hydrogens of water molecules are attracted to the partial negative charges of the oxygen atoms in the glass, which is due to the higher electronegativity difference between silicon and oxygen in the glass compared to oxygen and hydrogen in water.

What property of water allows it to adhere to the glass and form a concave meniscus?

The property of water that allows it to adhere to the glass and form a concave meniscus is its polarity, which results in hydrogen bonding and partial charges that interact with the polar glass surface.

What is the term used to describe the phenomenon where a liquid sticks to the walls of its container?

The term used to describe the phenomenon where a liquid sticks to the walls of its container is adhesion.

What is the term used to describe the attraction between molecules of the same substance?

The term used to describe the attraction between molecules of the same substance is cohesion.

What is capillary action and how does it relate to the meniscus?

Capillary action is the phenomenon where a liquid rises in narrow tubes or porous materials against gravity due to the combined effects of adhesion and cohesion. It relates to the meniscus as the liquid's interaction with the container's surface causes it to climb up the tube.

Why does capillary action occur in a thin glass tube filled with water?

Capillary action occurs in a thin glass tube filled with water because the water molecules are attracted to the polar glass surface more strongly than to each other, causing the water to rise up the tube.

How is capillary action similar to the absorption of a spill on a counter with a paper towel?

Capillary action is similar to the absorption of a spill on a counter with a paper towel because both involve the liquid being drawn into small gaps or pores due to the attraction to the material, in this case, the paper towel.

What is the relevance of capillary action in the human body?

Capillary action is relevant in the human body as it occurs in the capillaries, which are the thinnest blood vessels. This action allows for the exchange of nutrients, oxygen, and waste between the blood and the body's tissues.

Outlines

00:00

💧 The Meniscus Phenomenon and Adhesion

This paragraph explores the unexpected behavior of water in a glass beaker, where instead of being flat, the water forms a meniscus that is higher near the glass. The concept of a meniscus is introduced, with a distinction made between a concave meniscus, where the fluid is higher near the container, and a convex meniscus, which occurs with mercury and bulges away from the container. The explanation delves into the molecular interactions between water and glass, highlighting the role of polarity and hydrogen bonding. The stronger attraction of water to the glass due to the electronegativity difference between silicon and oxygen in the glass is discussed, leading to the phenomenon of adhesion where water molecules adhere to the glass surface.

05:02

🌱 Capillary Action and Cohesion

The second paragraph delves into the concept of capillary action, a fascinating property of liquids in narrow tubes or gaps, demonstrated by the water's ability to climb up a thin glass tube against gravity. This occurs due to the intense adhesion of water to the polar glass, where water molecules are attracted to the glass surface and form hydrogen bonds, allowing them to rise. Cohesion, the force that causes molecules of the same substance to stick together, is also discussed in the context of hydrogen bonding within water. The paragraph further explains how capillary action is not just a scientific curiosity but a fundamental process in everyday life, such as the absorption of a spill by a paper towel, and in the human body's capillaries, which are essential for circulation and life.

Keywords

💡Meniscus

A meniscus is the curved surface of a liquid in contact with a solid, such as the shape of water in a glass. In the video, the meniscus is central to understanding capillary action and adhesion. The script describes a concave meniscus where the liquid level is higher near the container's wall, which is a result of water's attraction to the glass being stronger than its attraction to itself.

💡Adhesion

Adhesion is the process where different substances stick to each other, in this case, water molecules sticking to the glass. The video explains that adhesion occurs because the water's partially positive hydrogen ends are attracted to the partially negative oxygen ends in the glass, causing the water to rise slightly in the container.

💡Cohesion

Cohesion refers to the attraction between similar molecules, such as the hydrogen bonds between water molecules. The script mentions cohesion as the reason why water forms a concave meniscus, as the water molecules are attracted to each other, maintaining the integrity of the liquid's surface.

💡Capillary Action

Capillary action is the ability of a liquid to flow in narrow spaces without the assistance of external forces like gravity, driven by the intermolecular forces. The video script illustrates this concept by describing how water climbs up a thin glass tube due to the stronger adhesion to the glass compared to the cohesion within the water.

💡Electronegativity

Electronegativity is a measure of the tendency of an atom to attract a bonding pair of electrons. The script explains that the electronegativity difference between oxygen and silicon in the glass is higher than that between oxygen and hydrogen in water, which contributes to the adhesion of water to the glass.

💡Silicon Oxide Lattice

A silicon oxide lattice is a structure composed of silicon and oxygen atoms, commonly found in glass. The video script uses this term to describe the composition of glass and how the arrangement of atoms with different electronegativities leads to partial charges that affect the adhesion of water.

💡Hydrogen Bonding

Hydrogen bonding is a type of dipole-dipole attraction between a hydrogen atom in one molecule and an electronegative atom, like oxygen, in another molecule. The script mentions hydrogen bonding as the reason for water's cohesive properties, which is crucial for understanding the formation of the meniscus.

💡Polarity

Polarity refers to a separation of electric charges leading to a molecule or substance having opposite charges at two ends. The video script discusses how the polarity of glass molecules contributes to the adhesion of water, as the partially negative oxygen atoms attract the positively charged hydrogen atoms in water.

💡Mercury

Mercury, a heavy metal, is used in the script as an example of a liquid that exhibits a convex meniscus in a glass container. This is due to mercury's strong cohesive forces compared to its adhesive forces with the glass, resulting in a bulge in the center of the liquid surface.

💡Glass Beaker

A glass beaker is a container made of glass used for holding liquids. The script uses the glass beaker as an example to demonstrate the formation of a meniscus and the principles of adhesion and capillary action in liquids like water and mercury.

💡Paper Towel

A paper towel is a disposable wiper made from paper used for cleaning purposes. The video script relates the concept of capillary action to everyday life by mentioning how a paper towel absorbs spilled liquid, illustrating how the liquid is drawn into the small gaps of the paper due to capillary action.

Highlights

The surface of water in a glass beaker forms a concave meniscus, where the water is higher near the glass than in the center.

A convex meniscus forms when using a substance like mercury, where the liquid is higher in the center than near the edges.

The phenomenon where a liquid's surface curves near the edges of a container is called a meniscus.

The concave meniscus occurs because the water is more attracted to the glass container than to itself, due to the polarity of the glass molecules.

Glass, typically made up of a silicon oxide lattice, has a higher electronegativity difference between oxygen and silicon than water has between oxygen and hydrogen.

Water molecules stick to the glass surface because the partial positive charges at the hydrogen atoms in water are attracted to the partial negative charges at the oxygen atoms in glass.

Adhesion is the phenomenon where water molecules stick to the glass surface, causing the water level to rise at the edges.

Cohesion refers to the attraction between water molecules due to hydrogen bonding, which causes them to stick together.

Capillary action occurs when water climbs up a thin glass tube due to the intense adhesion between the water and the glass.

Capillary action is the result of water molecules being attracted to the polar glass lattice and sticking to it, causing the water to rise against gravity.

This capillary action is more pronounced in a thin tube because more water molecules come into contact with the glass.

Capillary action is not only a fascinating phenomenon but also a practical one, as seen in everyday tasks like using a paper towel to absorb spilled liquid.

Paper towels absorb liquid through capillary action, where the water climbs into the small gaps in the towel due to its attraction to the paper fibers.

Capillary action is essential in biological processes, such as the movement of blood in the thin capillaries of our circulatory system.

Understanding adhesion, cohesion, and capillary action reveals how the properties of materials at the molecular level affect observable phenomena like the meniscus and liquid absorption.

Transcripts

- 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.