- [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.