Surface tension | States of matter and intermolecular forces | Chemistry | Khan Academy

Summary

The script delves into the molecular structure of water, highlighting its unique properties due to hydrogen bonding and electronegativity. It explains how water molecules at the surface are more densely packed, creating surface tension that allows phenomena like water droplets maintaining their shape, insects walking on water, and paperclips floating. This cohesive force, where water molecules are more attracted to each other than to air, is the essence of surface tension, a fundamental concept in understanding water's behavior.

Takeaways

• 🌡️ The script explains the molecular structure of water, highlighting the electronegative nature of the oxygen atom which results in a polar molecule with partially negative and positive ends.
• 🔗 It discusses the concept of hydrogen bonds, which are the attractions between the partially positive hydrogen atoms of one water molecule and the partially negative oxygen atoms of another.
• 💧 The script emphasizes the unique properties of water due to these hydrogen bonds, which contribute to its ability to remain liquid at standard temperature and pressure.
• 🔍 When zooming in on the water's surface, it appears to be made up of molecules that are interacting through hydrogen bonds, creating a cohesive structure.
• 🌊 The surface molecules of water experience different forces compared to those within the bulk, as they lack the upward pull from above, leading to a denser packing.
• 📐 This denser arrangement at the surface results in stronger intermolecular forces, which is the phenomenon known as surface tension.
• 💧 Surface tension is illustrated through the example of a water droplet, which maintains a round shape due to the cohesive forces among the surface molecules being stronger than their attraction to the air.
• 🐞 The script mentions insects that can walk on water, demonstrating the effect of surface tension that allows them to do so without breaking the surface.
• 📌 It also describes the ability to place a paperclip on the water's surface without it sinking immediately, again due to the surface tension acting as a barrier.
• 💧 The script describes how a cup filled with water can bulge above the rim without immediately overflowing, another demonstration of surface tension.
• 🌌 Finally, the script concludes that surface tension is a result of the cohesion of water molecules, which are more attracted to each other than to the surrounding air.

Q & A

What is the primary reason for the molecules in the air being far apart?

The molecules in the air, such as nitrogen, are far apart due to the weak intermolecular forces between them, allowing them to be spaced out significantly.

Why does the oxygen atom in a water molecule attract electrons more than the hydrogen atoms?

The oxygen atom is more electronegative, meaning it has a greater tendency to attract electrons towards itself, resulting in a polar covalent bond where the oxygen end is partially negative and the hydrogen ends are partially positive.

What are hydrogen bonds and how do they contribute to water's unique properties?

Hydrogen bonds are a type of dipole-dipole interaction that occurs between the partially positive hydrogen atoms of one water molecule and the partially negative oxygen atoms of another. They contribute to water's unique properties such as high surface tension, cohesion, and its ability to remain liquid at standard temperature and pressure.

Why do water molecules at the surface appear to be more densely packed compared to those within the bulk?

Water molecules at the surface are more densely packed because they lack the upward pull from hydrogen bonds that molecules within the bulk experience. This allows them to be drawn closer to their neighboring molecules, increasing the intermolecular forces at the surface.

What is surface tension and how does it arise?

Surface tension is a phenomenon where the surface of a liquid, like water, behaves like a stretched elastic sheet due to the stronger intermolecular forces at the surface compared to within the bulk. It arises because the molecules at the surface are more attracted to each other than to the surrounding air.

Why can a water droplet maintain a roughly round shape?

A water droplet maintains a roughly round shape due to surface tension, which causes the water molecules on the surface to be more attracted to each other than to the surrounding air, pulling the droplet into the shape with the least surface area.

How do insects manage to walk on the surface of water?

Insects can walk on the surface of water due to the surface tension that forms a sort of 'film' on the water's surface. The cohesive forces between the water molecules are strong enough to support the insect's weight without breaking the surface.

Why does a paperclip float on water even though it is denser than water?

A paperclip floats on water due to surface tension, which creates a barrier that prevents the denser object from penetrating the surface. The paperclip will only sink if the surface tension is overcome by applying enough force to break through the water's surface.

What causes a bulge to form when a cup filled with water is overfilled?

A bulge forms when a cup is overfilled because the cohesive forces between water molecules, due to surface tension, are stronger than the force of gravity pulling the water down, allowing the water to rise above the rim of the cup before it overflows.

How does gravity eventually cause water to overflow from an overfilled cup?

Gravity eventually causes water to overflow from an overfilled cup by overcoming the surface tension. Once the force of gravity is strong enough, it pulls the water molecules apart, breaking the surface tension and causing the water to spill over the edge.

What is cohesion and how does it relate to surface tension?

Cohesion is the attraction between molecules of the same substance. It relates to surface tension as the cohesive forces between water molecules are what create the surface tension, allowing phenomena such as insects walking on water or the formation of a water bulge in an overfilled cup.

Outlines

00:00

💧 Understanding Water's Surface Tension

This paragraph explains the molecular composition of water and the concept of surface tension. It starts by describing the air molecules and water molecules, highlighting the electronegativity of the oxygen atom which results in a polar molecule with partially negative and positive ends. The hydrogen bonds formed between these molecules give water unique properties, including its liquid state at room temperature. The focus then shifts to the water's surface, where the lack of molecules pulling from above allows for a denser packing and stronger intermolecular forces, leading to surface tension. Examples of surface tension in everyday life, such as water droplets, insects walking on water, and the ability to slightly overfill a cup of water without immediate overflow, are provided to illustrate this phenomenon.

Keywords

💡Surface Tension

Surface tension is a property of liquids that results from the imbalance of intermolecular forces at the surface. It causes the liquid to behave as if it is covered by a thin elastic film. In the video, surface tension is explained as the reason why water droplets can form a round shape and why insects can walk on water. The script illustrates this by describing how water molecules at the surface are more attracted to each other than to the air above, leading to a tighter packing and a stronger intermolecular force at the surface.

💡Hydrogen Bonds

Hydrogen bonds are a type of dipole-dipole attraction between a hydrogen atom covalently bonded to a highly electronegative atom and another electronegative atom. In the context of the video, hydrogen bonds are crucial for the unique properties of water, as they are responsible for the partial charges within water molecules, leading to the cohesiveness that contributes to surface tension. The script describes how these bonds pull water molecules together, especially at the surface, where they are not counteracted by molecules from the air above.

💡Electronegativity

Electronegativity is a measure of the tendency of an atom to attract a bonding pair of electrons. In the video, the concept of electronegativity is used to explain why the oxygen atom in a water molecule is partially negative, as it is more electronegative than the hydrogen atoms it is bonded to. This difference in electronegativity between oxygen and hydrogen leads to the formation of hydrogen bonds, which are central to the discussion of surface tension.

💡Cohesion

Cohesion refers to the intermolecular forces that cause molecules of the same substance to stick together. The video script explains that cohesion is responsible for the surface tension in water, as the molecules are more attracted to each other than to the surrounding air. This cohesion is exemplified by the ability of water to form a bulge when a cup is overfilled, as the water molecules' attraction to each other prevents immediate overflow.

💡Adhesion

Adhesion is the attraction between different types of molecules, such as between water molecules and the surface of an object. Although not explicitly mentioned in the script, adhesion is an important concept related to surface tension, as it explains how water can interact with other substances, such as the surface of a paperclip, allowing it to float on water due to the balance between adhesion and the object's weight.

💡Molecules

Molecules are the smallest particle in a chemical element or compound that has the chemical properties of that element or compound. In the video, molecules are the fundamental units being discussed, particularly water molecules, which are composed of hydrogen and oxygen atoms. The script describes how these molecules interact through hydrogen bonds and how their arrangement and behavior at the surface contribute to surface tension.

💡Kinetic Energy

Kinetic energy is the energy that a moving object possesses due to its motion. The video script mentions kinetic energy in the context of water molecules, which are constantly moving and bumping into each other. This kinetic energy is an important factor in the dynamics of the molecules at the surface and within the bulk of the liquid, influencing how they interact and contribute to surface tension.

💡Intermolecular Forces

Intermolecular forces are the forces of attraction or repulsion that act between neighboring particles (atoms, molecules, or ions). In the video, these forces are discussed in the context of how they affect the behavior of water molecules, particularly at the surface where they are responsible for the phenomenon of surface tension. The script explains that these forces are stronger at the surface due to the lack of counteracting forces from above.

💡Water Droplet

A water droplet is a small, roughly spherical mass of water. The video uses the water droplet as an example to illustrate the concept of surface tension. The script explains that the surface molecules of a water droplet are more attracted to each other than to the surrounding air, which allows the droplet to maintain its round shape despite the presence of gravity.

💡Density

Density is the mass per unit volume of a substance. The video script briefly touches on density when discussing the paperclip example, noting that despite being denser than water, the paperclip can float due to the surface tension. This highlights the balance between the object's density and the strength of the surface tension that can support it.

💡Elastic Film

An elastic film is a thin, flexible layer that can stretch and return to its original shape. The video script uses the term 'elastic film' as a metaphor to describe the behavior of the surface of water due to surface tension. It suggests that the water's surface acts like a film that can be stretched or distorted without breaking, which is why it can support the weight of small objects like insects or a paperclip.

Highlights

The air is composed of molecules like nitrogen, which are far apart and more so in reality.

Water molecules consist of an oxygen atom bonded to two hydrogen atoms with a polar nature due to the electronegativity of oxygen.

Hydrogen bonds between water molecules contribute to its unique properties and liquid state at standard conditions.

Molecular view of water's surface reveals a non-uniform structure with molecules interacting through hydrogen bonds.

Surface water molecules experience different forces compared to those within the bulk, lacking upward pull.

Molecules at the water's surface can pack more densely, leading to stronger intermolecular forces.

Surface tension is a phenomenon resulting from the stronger cohesive forces at the surface of water.

Water droplets maintain a round shape due to surface tension and the cohesive attraction of surface molecules.

Insects can walk on water surfaces due to the surface tension creating a 'film' that supports their weight.

Surface tension allows objects denser than water, like paperclips, to float momentarily before sinking.

When a cup is filled with water to the brim, a bulge forms due to the cohesive forces overcoming gravity temporarily.

Cohesion, the attraction between water molecules, is the primary cause of surface tension.

Surface tension demonstrates the difference in intermolecular attraction at the surface compared to the bulk.

The phenomenon of surface tension can be observed in everyday life, such as in water droplets and still water bodies.

The strength of surface tension can be overcome by external forces, such as gravity or physical disturbance.

Understanding surface tension is crucial for grasping the behavior of liquids and their interactions with solids and gases.

Transcripts

- What we have here is a zoom-in

of the surface of water.

So up here you have the air, this is the air,

these are some air molecules,

maybe they're nitrogen molecules.

They're fairly far apart, in fact, in reality,

they would be even more far apart than this.

And then over here you have water molecules.

We've seen this many times.

You have the oxygen atom and it's bonded

to two hydrogen atoms, and the oxygen atom

likes to hog the electrons more.

It's more electronegative, so you have

a partially negative charge at this end

and partially positive ends at this end.

And that attraction between the

partially positive ends and the partially negative ends,

that's what gives water all sorts of neat properties.

Those are the hydrogen bonds.

Those are the hydrogen bonds that give water

all sorts of neat properties and keep it

in its liquid state at a standard

temperature and pressure.

Now what I want to think about

is the surface in particular.

And if you look at the surface of water,

it might look completely smooth.

But if you were to zoom in on a molecular level,

you'll see that, well, it's just made up of these molecules.

But roughly speaking, roughly speaking,

let's just say that this is roughly

the surface, the surface of the water.

The surface of the water.

Now, what's going on at the surface?

Well, all these molecules

are interacting through hydrogen bonds.

Let's say this molecule right over here,

it has hydrogen bonds pulling on it upwards,

up to this one, pulling it this way,

pulling it downwards, pulling it in really,

really, to some degree, almost every direction.

And they all have their kinetic energy

and they're bumping around, but they're

flowing past each other.

The hydrogen bonds are giving that cohesiveness.

The molecules are attracted to each other.

But if you look at the molecules on the surface,

if you look at the ones on the surface,

sure, they might have stuff pulling down on them,

they might have stuff pulling them to the side,

but they don't have anything pulling on them from above.

And because of this, you could imagine

that they're able to get a little bit more densely packed,

that they're able to get a little closer to their neighbors.

And this is what allows them to actually have

a stronger, I guess you could say, intermolecular force

at the surface than you have within the body,

and that causes a phenomenon known as surface tension.

So you have stronger, you have kind of a deeper,

and this is still just hydrogen bonds,

but since they're not being pulled

in other directions by, upwards by the air,

they're able to get a little bit more closely packed,

a little bit tighter, and this we refer to

as surface tension, surface tension.

And you have probably observed surface tension

many, many, many times in your life

in the form of, say, a water droplet.

A water droplet, it's able to have this roughly round shape

because all the little water molecules

on the surface of the water droplet,

and here the surface might even be

on the bottom of the water droplet.

They are more attracted to each other

than they are to the surrounding air,

so they're able to form this type of a shape.

You might've seen it if you go to a pond

or a stream sometimes, so you see some still water.

And let's say, let me do this in blue.

So let's say that this is the surface

of the water right over here.

You might have seen insects that are able to

walk on the surface of the water.

And I'm not doing a great job at drawing the insects.

They don't look exactly like that.

But they can walk on the surface of the water.

You might've seen or you might've even tried

to do something like put a paperclip on the water.

And even though this thing is actually more dense

than the water and you might expect it to sink,

but because of the surface tension,

which really forms something of a film on top of the water,

the thing won't penetrate the surface,

so the paperclip will float, unless you were to

push on it a little bit and it allow it

to puncture the surface, and then

it would actually sink, which is what you would expect

because it is actually denser.

You'd even see this if you were to take a cup,

if you were to take a cup and you were to fill it

all the way up to the rim and then a little bit higher,

it won't immediately overflow.

It won't immediately overflow.

If you're very careful, you'll see

that you form a bulge here.

And that bulges because those individual water molecules

are more attracted to each other

than they are to the surrounding air.

So that allows for something of a little bulge.

Obviously if you keep pouring water, at some point,

they're just gonna start overflowing

because gravity's gonna take over there.

Gravity's gonna overwhelm the surface tension.

But this bulge will actually form.

So surface tension, it is really due to

the cohesion of the water.

Remember, cohesion is when the molecules

are attracted to each other.

And it definitely, and especially because they're

more attracted to each other than the surrounding air.