Water as a solvent | Water, acids, and bases | Biology | Khan Academy

Summary

This script explores water's unique properties as a universal solvent, driven by its polarity that facilitates hydrogen bonding. It explains how charged ions like sodium chloride dissolve in water due to the attraction between the ions and the polar water molecules, forming a shell around them. The video also contrasts this with non-polar substances like hexane, which are hydrophobic and do not dissolve in water, illustrating the fundamental principle behind the phrase 'oil and water don't mix'.

Takeaways

• 💧 Water is a polar molecule with a partially negative and a partially positive end, which facilitates the formation of hydrogen bonds.
• 🌊 Water's polarity is a key factor in its ability to act as a universal solvent, allowing many substances to dissolve within it.
• 🧪 The cytoplasm in cells, which is mostly water, serves as a solvent for chemical reactions involving various molecules.
• 🧂 Sodium chloride (table salt) dissolves in water because the positive sodium ions are attracted to the partially negative ends of water molecules, and vice versa for chloride ions.
• 🔁 The dissolution of sodium chloride in water involves the formation of a 'shell' of water molecules around the ions, disrupting the ionic bond and allowing the salt to dissolve.
• ⚡ The ability of water to dissolve charged particles or polar molecules is due to its polar nature, making it an effective solvent for ionic and polar compounds.
• 🌿 Hydrophilic substances, meaning 'water-loving', are those that dissolve easily in water due to their charge or polarity.
• 🏜 Hydrophobic substances, meaning 'water-fearing', do not dissolve well in water as they lack charge and polarity, tending to avoid contact with water.
• 🛢️ Hydrocarbons, such as hexane found in gasoline, are hydrophobic and do not mix well with water, often forming beads on the water surface.
• 🔬 Understanding water's role as a solvent is crucial for grasping many biological and chemical processes that occur in aqueous environments.
• 🌐 The concepts of hydrophilicity and hydrophobicity are fundamental to the study of solubility and the interactions between different types of molecules in water.

Q & A

What property of water is discussed in the video script?

The video script discusses water's ability to be a solvent, which is its property to dissolve various substances.

Why is water's solubility important for chemical reactions?

Water's solubility is important for chemical reactions because it allows substances to dissolve and interact with each other, which is essential for many chemical processes, including those occurring inside cells.

What is the key feature of water that makes it a good solvent for many molecules?

The key feature of water that makes it a good solvent is its polarity, which allows it to form hydrogen bonds with other polar molecules or ions.

What happens when sodium chloride (NaCl) is put into water?

When sodium chloride is put into water, it dissolves. The positive sodium ions are attracted to the partially negative ends of water molecules, and the negative chloride ions are attracted to the partially positive hydrogen ends of water molecules.

What is the term used to describe the interaction between the positive sodium ion and the partially negative ends of water molecules?

The term used to describe this interaction is 'hydration', where water molecules surround and interact with the sodium ion.

What is the term for a negative ion, like the chloride ion in sodium chloride?

A negative ion is called an 'anion'.

How does the chloride ion interact with water molecules?

The chloride ion, being negatively charged, is attracted to the partially positive hydrogen ends of water molecules, forming a hydration shell around the ion.

What types of substances are likely to dissolve well in water?

Substances that are charged or polar, such as ions and polar molecules, are likely to dissolve well in water due to their ability to form hydrogen bonds or interact with the polar water molecules.

What is the term used to describe substances that dissolve well in water?

Substances that dissolve well in water are called 'hydrophilic', which means 'water-loving'.

What is the term used to describe substances that do not dissolve well in water?

Substances that do not dissolve well in water are called 'hydrophobic', which means 'water-fearing'.

Why do hydrophobic substances like hexane not dissolve well in water?

Hydrophobic substances like hexane do not dissolve well in water because they lack polarity and charge, which means they cannot form hydrogen bonds or interact strongly with water molecules.

What is an example of a hydrophobic substance mentioned in the script?

Hexane, a major constituent of car gasoline, is given as an example of a hydrophobic substance in the script.

Outlines

00:00

🌊 The Solvent Properties of Water

This paragraph discusses the polarity of water molecules and their ability to form hydrogen bonds, which contribute to water's unique properties as a solvent. It explains how water's polarity allows it to dissolve various substances, especially those with ionic or polar characteristics. The paragraph uses the example of sodium chloride (table salt) dissolving in water due to the attraction between the positively charged sodium ions and the partially negative ends of water molecules, and vice versa for the chloride ions. The process illustrates how charged particles or polar molecules can interact with water, leading to dissolution, which is crucial for many chemical reactions, including those occurring within cells.

05:01

💧 Hydrophilic and Hydrophobic Substances in Water

The second paragraph delves into the concepts of hydrophilicity and hydrophobicity in relation to water as a solvent. It explains that substances with charge or polarity, termed hydrophilic or 'water-loving', dissolve easily in water due to their compatibility with water's polar nature. Examples include ions from salts like sodium chloride. Conversely, hydrophobic substances, which are nonpolar and do not carry a charge, do not dissolve well in water and are described as 'water-fearing'. The paragraph uses hexane, a component of gasoline, as an example of a hydrophobic substance that does not interact well with water, tending to form beads on the water's surface. This distinction is fundamental to understanding the solubility of different types of molecules and the separation of substances like oil and water.

Keywords

💡Polarity

Polarity refers to the property of a molecule where there is an uneven distribution of charge, resulting in regions of partial positive and negative charges. In the video, polarity is a fundamental concept that explains why water molecules have one end that is partially negative and the other partially positive, which is crucial for forming hydrogen bonds and contributing to water's unique properties as a solvent.

💡Hydrogen Bonds

Hydrogen bonds are a type of dipole-dipole attraction that occurs between a hydrogen atom covalently bonded to a highly electronegative atom (like oxygen) and an electronegative atom with a lone pair of electrons. In the context of the video, hydrogen bonds are a direct result of the polarity of water molecules and are key to understanding water's role as a solvent and its various properties.

💡Solvent

A solvent is a substance that has the ability to dissolve other substances, known as solutes. In the video, water is highlighted as a universal solvent due to its polarity, which allows it to dissolve a wide range of solutes, facilitating chemical reactions and biological processes in cells.

💡Cytoplasm

Cytoplasm is the jelly-like substance within cells, composed mainly of water, and serves as the site for many cellular activities. The video mentions cytoplasm to illustrate the environment within cells where water acts as a solvent, enabling various molecular interactions.

💡Sodium Chloride (NaCl)

Sodium chloride, commonly known as table salt, is an ionic compound consisting of sodium ions (Na+) and chloride ions (Cl-). The video uses sodium chloride as an example of a substance that dissolves easily in water due to the ionic interactions with the polar water molecules.

💡Ionic Bonds

Ionic bonds are the electrostatic attractions between oppositely charged ions, typically formed between a metal and a non-metal. In the video, the ionic bond between sodium and chloride ions in NaCl is contrasted with the interactions that occur when these ions are dissolved in water.

💡Anion

An anion is a negatively charged ion, formed when an atom gains one or more electrons. The video explains how the chloride ion, as an anion, is attracted to the positive ends of water molecules, leading to its dissolution in water.

💡Hydrophilic

Hydrophilic substances are those that are attracted to and can dissolve in water due to their polarity or charge. The term is used in the video to describe the affinity of ions and polar molecules for water, facilitating their dissolution.

💡Hydrophobic

Hydrophobic substances are those that repel water and do not dissolve well in it, typically due to a lack of polarity or charge. The video contrasts hydrophobic substances like hydrocarbons, such as hexane, with hydrophilic substances, illustrating their inability to dissolve in water and their tendency to form separate phases.

💡Hexane

Hexane is an organic compound with the formula C6H14, consisting of six carbon atoms and fourteen hydrogen atoms. It is used in the video as an example of a hydrophobic substance that does not dissolve well in water due to its nonpolar nature.

💡Hydrocarbons

Hydrocarbons are organic compounds consisting entirely of hydrogen and carbon atoms. In the video, hydrocarbons like hexane are highlighted as examples of hydrophobic substances that do not interact well with water due to their nonpolar molecular structure.

Highlights

Water's polarity is fundamental to its ability to form hydrogen bonds, leading to unique properties.

Water acts as a universal solvent, facilitating the dissolution of many substances which is crucial for chemical reactions.

The cytoplasm in cells, primarily composed of water, allows for various molecular interactions, highlighting water's role in biological systems.

Polarity of water is the key feature that makes it an effective solvent for a wide range of molecules.

Sodium chloride (NaCl), commonly known as table salt, dissolves in water due to the attraction between the ions and the polar water molecules.

The positive sodium ion is attracted to the partially negative oxygen end of water molecules, causing dissolution.

The negative chloride ion is attracted to the partially positive hydrogen ends of water molecules, aiding in the dissolution process.

Charged or polar substances dissolve easily in water, a property referred to as being hydrophilic.

Hydrophilic substances, such as ions, are 'water-loving' and mix well with water due to their compatibility with water's polarity.

Substances that lack charge or polarity, like hydrocarbons, do not dissolve well in water and are termed hydrophobic.

Hydrophobic substances, such as hexane, avoid contact with water, illustrating the difficulty in mixing oil and water.

The self-attraction of water molecules and their lack of attraction to non-polar substances contribute to the separation of oil and water.

Understanding water's role as a solvent is essential for grasping many chemical and biological processes.

The video provides a visual explanation of how ionic bonds are broken and ions are stabilized in water, aiding in the dissolution process.

The concept of hydrophilic and hydrophobic substances is crucial for predicting solubility in water and has practical applications in various fields.

The video demonstrates the molecular interactions that occur during the dissolution of salt in water, emphasizing the role of polarity.

The dissolution of sodium chloride in water is a classic example of how water's polarity enables it to act as a solvent for ionic compounds.

The video concludes by reinforcing the importance of water's polarity in its ability to dissolve a wide variety of substances.

Transcripts

- [Voiceover] We've already talked about the notion

that a water molecule has polarity to it.

One end has a partially negative charge,

and the other end has partially positive charges.

And we've talked about how this leads to hydrogen bonds,

and we alluded to the fact

that maybe these hydrogen bonds

give us all sorts of neat properties of water.

And what I want to talk about in this video

is one of those very important properties,

and that's water's ability to be a solvent

Water's ability to be a solvent.

And this means that it's easy for certain things

to be dissolved inside of water.

And that's super important,

because that's how a lot of the chemistry occurs,

by things getting dissolved in water

and then interacting and bumping with other things.

And this is actually what's happening inside of cells,

in the cytoplasm.

The cytoplasm, which is mostly water,

is a solvent which allows a bunch of interactions

to happen between different types of molecules.

But let's think about why water is a good solvent,

and what types of things it can dissolve easily

and what types of things it might not

be able to dissolve so easily.

So the key feature that makes water a good solvent,

or at least a good solvent for a large class of molecules,

is its polarity.

If I were to take some sodium chloride,

often known as table salt,

so let me...

So if I were to take sodium NaCl, sodium chloride.

Sodium chloride, the sodium and chloride

are attracted by ionic bonds.

The sodium right over here has a positive charge,

it has an electron stripped from it,

and the chloride has a--

let me write that in a different color.

So the sodium has a positive charge,

because it has an electron stripped from it,

and the chloride, it is an anion,

it has a negative, it is a negatively charged ion,

because it gains an extra electron.

But they are attracted to each other.

This has a positive charge, this has a negative charge.

This is called an ionic bond.

But if you put sodium chloride in water,

something very interesting happens.

This is something that we've all experienced.

Take some table salt and put it in water

and see what happens.

It will dissolve.

And why does it dissolve?

Well, let's draw it out.

So this is the sodium right over here.

So that's the sodium.

It has a positive charge.

It has a positive charge.

And then this is the chloride right over here.

It has a negative charge.

What's gonna happen if you put it inside of the water?

Let me do that negative charge in the green.

What's gonna happen when you put it inside of water.

Well, you can imagine.

The negative ends of the water molecules

are going to be attracted to the sodium ion.

So it's going to look something like this.

So you have the oxygen,

oxygen, oxygen, oxygen, oxygen,

I'm clearly not drawing things to scale,

but this'll just give you the idea.

This end of the water molecule

all has a partially negative charge.

Partially negative charge.

So it's going to be attracted to the positive sodium ion.

And then the hydrogen ends,

the hydrogen ends,

are going to have a partial positive charge,

and then they're going to be repelled.

They are going to be repelled from the positive sodium ion.

So it's gonna look something like this.

And it's gonna look something like this.

And you're gonna have partial positive charge

on the outside.

Partial positive charge on the outside.

And now these hydrogens over here,

this will just interact with water

the way that it would typically,

with the hydrogen bonding,

the molecules just flowing past each other.

So the fact that the sodium ion here,

it's an ion, it has charge.

It is able to dissolve in the water very easily,

because it is attracted to the partially negative ends

of the water molecule.

Now, a similar thing is going to happen

with the chloride ion.

And we call a negative ion an anion.

So over here, over here,

and actually let me get some,

let me move it over a little bit

so that I have some space.

So the chloride anion,

let me move it over a little bit.

So right, or maybe I'll move it over here.

So the chloride,

the chloride anion,

let me see, I'm having trouble with my selection tool.

Alright. So there we go.

The chloride anion has a negative charge

that's going to be attracted to the positive end

of the water molecules.

So you could imagine something like this.

So the hydrogen ends, the hydrogen ends,

are going to be attracted to it,

they have a partially positive charge.

They have a partially positive charge.

And of course you have the oxygen end

that has a partially negative charge.

It has a partially negative charge.

And I could draw more of these.

Hydrogen, hydrogen,

attracted there,

you have the oxygen over,

let me do that, I wanna keep my colors consistent.

The oxygen right over there,

we have the hydrogen,

once again this isn't drawn to scale.

Hydrogen, it is bonded to the oxygen.

And so once again, you can form this,

so you could almost imagine this shell of water molecules

is going to be attracted to it,

it's going to be attracted, or I guess you could say

the partially positive end,

which is where the hydrogen atoms are,

is what's going to be attracted to this negative ion.

So this is partial positive over here.

And then on the partially negative side,

outside of this shell,

you can imagine it's just gonna interact with the water

just the way any water molecule would,

and so it's gonna be able to flow very easily.

So you probably see something interesting here.

If something has charge,

if it's an ion,

or if something has some polarity,

it's very easy to dissolve it inside of water.

And in this case, and just to have some terminology here,

in this case, water is the,

water is the solvent,

so water is the solvent.

So the solvent is water.

And the thing that's being dissolved in the water,

we call that a solute.

So we call this the solute.

So the sodium chloride.

That is, you could use sodium chloride as a solute,

or you could say that the sodium ions are a solute

and so are the chloride ions.

That is also considered to be the solute.

And so you say, well what kind of things dissolve well?

Well, things that that have charge or that are polar.

And things that are charged and polar

and tend to dissolve well in water,

there's another word we use for them.

We say that they are hydrophilic.

So we could say that this right over here is hydrophilic.

And if you look at the word root,

hydro is referring to water.

So hydro is referring to water.

And philic means loving.

So this literally means water loving.

Water loving.

Hydrophilic.

And so you might be asking, okay,

everything we've talked about,

you know, we've seen water molecules,

that's polar, we're looking at charged ions,

okay we could get that,

we can get why they'd be hydrophilic,

they can incorporate themselves well into the water,

but what are examples of things

that would not incorporate themselves well in water?

Well, in general, things that don't have charge,

or that aren't polar, aren't going to be able to dissolve

in water all that well.

And a good example is hydrocarbons.

So if you took some hexane.

And hexane is a major constituent of car gasoline.

So hexane, hex the prefix means six carbons.

So let's see, one,

let me do this in another color.

So we have one, two, three,

four, five, six carbons.

So one two three four five six carbons.

And then all the other bonds are with hydrogens.

So let me draw this as well as I can.

Carbon, at least typically, forms four bonds.

So hydrogen, hydrogen, hydrogen, hydrogen,

hydrogen, hydrogen, hydrogen.

This right over here is hexane.

This thing has no polarity to it.

It doesn't form hydrogen bonds,

it doesn't have any polarity.

And so if you were to take hexane

and throw it into water,

it's not going to dissolve that well.

It's actually going to kind bead up.

And you would see that if you actually threw

some gasoline inside of water.

So things like hexane we would call hydrophobic.

Hydrophobic.

So this right over here is hydrophobic.

Hydrophobic.

It'll literally ball up to avoid getting in touch,

to minimize its contact with water.

Because the water is attracted to itself,

and it is not so attracted to this stuff right over here.

And hydrophobic?

You still have hydro, meaning water,

and then phobic, it means fearing.

So this right over here is water fearing.

That's why it's hard to mix things like water and oil

or gasoline and water.