Types of mixtures | Middle school chemistry | Khan Academy

- [Instructor] When we add
some food color to water

and stir it,

you see that the food
color mixes very nicely,

spreads throughout the water.

But now let's add some oil to the water,

stir it, stir it hard.

And what we find is that,

hey, that oil is not mixing
throughout the water.

It's not spreading throughout the water.

So look, in both cases we mix things,

but the two mixtures look very different.

And this difference is
important in chemistry.

So let's talk about them.

Before we talk about mixtures,

Let's quickly recap pure substances.

Pure substances can either be elements

which are basically made
of one kind of atoms.

These are the elements that
you find in the periodic table.

For example, look, oxygen
is made of all oxygen atoms.

You have gold, which is
all made of gold atoms.

Or they can be compounds

where two or more atoms are
chemically bonded together

in fixed ratios.

For example, in water molecules,

we always have two hydrogens
for every one oxygen.

We call these pure substances,

because, well, they're purely
made of the same stuff.

This is purely made of oxygen.

This is purely made of gold.

This is purely made of water molecules.

This is purely made of carbon dioxide.

So elements and compounds
are pure substances,

and they have very specific
properties like boiling points,

melting points, densities and so on.

Now, what do you think happens

when we physically combine
two pure substances,

like, for example, oxygen
and carbon dioxide,

or let's say we put gold in water.

We create mixtures.

A mixture is a physical combination

of two or more substances
in any proportion you want.

But wait a second, aren't
compounds also mixtures?

I mean, here, carbon and
oxygen are mixed together.

Hydrogen and oxygen are mixed together.

So shouldn't these be mixtures as well?

No, and this used to
confuse me a lot, okay?

but the key point is you get mixtures

when you physically combine
two or more substances.

And as a result, because these substances

have different boiling,
melting points and densities,

you can physically separate them.

For example, I can just pick
this cold bar out from water.

But in other cases, I can
heat them or cool them

or spin them or use magnets,

if they have magnetic properties,

but all by physical means,
I can separate them.

But in contrast,

compounds are where atoms
are chemically combined,

they're chemically bonded together.

You cannot separate these
atoms by physical processes.

So compounds are still pure substances.

Now, guess what? Even mixtures
can have different types.

So let's investigate
them a little bit more.

Here we have mixed oxygen
gas and carbon dioxide gases.

When you do that,

the different substances
get evenly distributed

at a molecular level; and as a result,

the composition stays pretty
much the same throughout.

You get a uniform composition throughout.

And you cannot see any
distinct parts or phases

with the naked eye.

And that's exactly what happened

when we added food color to our water.

Again, the food color
uniformly distributed itself

throughout the water at a molecular level;

and as a result, see,
you cannot distinguish

where the food color is and
where the water is, right?

We call such mixtures
homogeneous mixtures.

So homogeneous mixtures are the one

where you cannot distinguish

between the different
substances that are mixed.

In contrast. Look at
the gold inside water.

I can clearly see where the
gold is and where the water is.

I can see the boundary over here nicely.

And the same thing happens
when we add oil in water.

I can clearly see there's oil here

and there's water over there.

Here, look, the substance

is not evenly distributed throughout.

There is nonuniform distribution.

There's a lot of oil here and
hardly anything over here.

There's a lot of gold here
and nothing over here.

Such mixtures where we can easily make out

the different substances
that are mixed together,

which we can see with naked eye,

we call them heterogeneous mixtures.

Alright, let's take some examples now.

Why don't you pause the video,

go through each one of
them and classify them

as either homogeneous mixtures
or heterogeneous mixtures.

Pause and try.

Alright, let's look at salads first.

I can clearly see the different options

that are mixed together.

I can clearly see their boundaries.

So this is a heterogeneous mixture.

Okay, what about butter caramel.

Well, can we see distinctly

where the butter is and
where the caramel is?

No, they're nicely uniformly distributed.

So this is a homogeneous mixture.

We can clearly see the seashells
and the cement separately.

So it is heterogeneous mixture.

This is sand and water.

We can see the sand here
and water over here.

I can clearly see the boundaries.

So it's again heterogeneous mixture.

What about ink?

Well, over here there's
a uniform distribution.

I cannot see any boundaries,
any distinctions.

This is a uniform distributed mixture,

so this is a homogeneous mixture.

What about brass?

Hmm, this could be tricky.

What exactly is brass?

Well, brass is a combination
mostly of copper and zinc.

But look, it is mixed uniformly.

I cannot see the distinction
between copper and zinc.

And so this is again
a homogeneous mixture.

Finally, what about copper?

Hmm, well, this is a trick question

because copper is an element,

it's not a mixture at all.

It's a pure substance.

So in summary, when you combine

two or more substances physically,

we call them mixtures.

If the substances are mixed
uniformly at a molecular level

so you cannot distinctly
see the different parts,

we call them homogeneous mixtures.

In contrast, if the different components

are not uniformly distributed

and you can distinctly see them,

you might even see their boundaries,

we call them heterogeneous mixtures.