Properties of homogeneous mixtures | Middle school chemistry | Khan Academy

- [Instructor] I used to
always think of solutions

as liquids of some kind,

but turns out that it's wrong.

Not all liquids are solutions.

And more importantly,
solutions can be solids,

liquids, or gases.

So, in this video,

let's understand exactly
what solutions are.

So, let's take a concrete example.

Let's put some salt in water
and stir it very nicely.

We get salt water, but
what exactly is salt water?

Let's analyze.

Do you think salt water is an element?

Elements are made of
just one kind of atoms.

For example, you have oxygen

which is made of only oxygen atoms

or gold made of only gold atoms.

Well, salt water has different
kinds of atoms in them.

There's water, therefore
there's hydrogen and oxygen,

and we also put salt.

So, there's sodium and
there's also chlorine.

So, since salt water is made
of several different elements,

it is not an element.

What else could it be?

Hey, what about compounds?

Remember in compounds,

two or more atoms are chemically
bonded in fixed ratios.

For example, water is a compound

because hydrogen and oxygen
are chemically bonded

in the ratio two is to one,
two hydrogen for every oxygen.

Carbon dioxide is a compound,

because carbon and oxygen
are chemically bonded

in the ratio one is to two,

one carbon for every two oxygens.

So, what about salt water?

Is that a compound?

No, it's not, and this is important,

because salt and water
are not chemically bonded,

they are physically mixed over here,

and they don't have a fixed ratio.

You can have different ratios
of salt and water here.

Now, of course, salt is a compound,

because sodium and chlorine
are chemically bonded in.

One is to one fixed ratio,
and water is a compound.

But salt water?

That's not a compound at all.

And more importantly,
compounds are new substances

with different properties altogether.

When hydrogen and oxygen form a compound,

you get water, which is a new substance.

It has its own properties,
it has its own taste,

it has its own boiling point.

Similarly, sodium and
chlorine coming together,

you get salt, which is a
new substance altogether.

It has its own taste,

it has its own melting
point, boiling point.

But when you dissolve salt in water,

you don't get a new substance.

You basically get salt water,

and you don't have new properties.

It's still salty, it's still watery.

And finally, remember, the
atoms of that compounds

cannot be physically separated.

For example, when you heat
water to its boiling point,

the hydrogen and oxygen
atoms do not separate.

The water molecule just move
farther away from each other

as it changes to gas.

But here, salt and water
can be physically separated,

just evaporate the water,
salt will be left behind.

So, hopefully it's clear
that this is not a compound.

But what is this?

Well, when you mix two
substances together,

we get what is called a mixture.

And since this mixture
is uniformed throughout,

you cannot, with your
naked eyes, see the salt

and the water to be different over here.

We call such mixtures homogenous mixtures.

In contrast, mixtures in
which the different components

can be easily seen, like
for example, water and oil,

you can clearly see oil over here,

you can see water here,
you can see the boundary.

In such cases, we call them
heterogeneous mixtures.

But anyways, in our example,

salt water is a homogenous mixture.

And guess what?

These homogenous mixtures
are what we call solutions.

So, let's look at solutions in
a little bit more detail now.

Solutions usually contain two things.

We have the solute and the solvent.

Here, you can think of solute
as something that is dissolved

in the solvent to form a
solution, but in general,

you can think of solute as a substance

which is in less quantity,

and solvent is is the substance
that is in more quantity.

But now, let's look at more examples.

What about air?

Well, air is a homogenous
mixture of oxygen, nitrogen,

carbon dioxide, and many other gases.

So, air is a solution.

So, look, solution need
not be just liquids.

Solutions can be gases.

And then think about alloy,
which are made by mixing metals.

Like, say, steel is made
by mixing iron and carbon,

and bronze is made by
mixing copper and tin.

These are also homogenous mixtures.

The metals are mixed throughout uniformly.

You cannot see them separately,

and therefore these are also solutions.

So you can see, solutions
can also be solids.

Now, a key thing about solutions
is their concentration.

If you have a lot of solute, relatively,

compared to the solvent,

we say it's a very concentrated solution.

And if you have less solute
compared to the solvent,

we say it is a dilute solution.

But why should we care
about the concentration?

Well, here's the thing,

although individual components will have

their own separate properties
because it's a mixture,

it turns out that the property
of the solution itself

can vary, depend upon the concentration.

For example, usually, water freezes

at about zero degrees
Celsius, but guess what?

When you add salt to it, the
freezing point of this solution

is below zero degrees Celsius.

Similarly, if there is
water molecules in the air,

we say there is humidity.

A lot of water molecules means
there is a lot of humidity.

Humidity changes the
properties of the air.

For example, the speed of sound changes,

its density changes, and so on.

And similarly, steel is made
by adding carbon to iron.

And by increasing the
carbon concentration,

we can make steel harder.

We can make it stronger and more durable.

So, look, by changing the
concentrations of the solute,

we can vary the properties
of the solution itself,

and that is pretty awesome.

So, putting it all together,

matter can be classified into two parts.

We have pure substances,
which are basically elements

which are made of just one kind of atoms,

or they can be compounds

which are made of atoms that
are chemically bonded together

in fixed ratios, like
water or carbon dioxide.

And when you physically
mix substances together,

we get mixtures.

Now, mixtures can be heterogeneous.

If you can see the
individual component apart

with our naked eye, you can
see the boundary, like say,

when you add oil to water,
or they can be homogenous.

In this case,

the components are distributed
uniformly throughout,

and so you cannot tell them apart.

We also call them solutions.

The component which is
in the less quantity,

we call them as the solute,

and the one with more quantity,
we call them as a solvent.

What's important is that the
individual physical properties

of the components, like
their boiling point,

melting point, that does not change.

And so, we can use that to
separate them out physically.

But a cool thing about solutions

is that by varying the concentration,

we can vary the properties
of the solutions

to our advantage.