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

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and stir it,

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you see that the food
color mixes very nicely,

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spreads throughout the water.

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

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stir it, stir it hard.

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And what we find is that,

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hey, that oil is not mixing
throughout the water.

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It's not spreading throughout the water.

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So look, in both cases we mix things,

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but the two mixtures look very different.

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And this difference is
important in chemistry.

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So let's talk about them.

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Before we talk about mixtures,

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Let's quickly recap pure substances.

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Pure substances can either be elements

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which are basically made
of one kind of atoms.

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

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

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

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Or they can be compounds

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where two or more atoms are
chemically bonded together

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in fixed ratios.

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For example, in water molecules,

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we always have two hydrogens
for every one oxygen.

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We call these pure substances,

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

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This is purely made of oxygen.

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This is purely made of gold.

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This is purely made of water molecules.

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This is purely made of carbon dioxide.

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So elements and compounds
are pure substances,

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and they have very specific
properties like boiling points,

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melting points, densities and so on.

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Now, what do you think happens

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when we physically combine
two pure substances,

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like, for example, oxygen
and carbon dioxide,

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or let's say we put gold in water.

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We create mixtures.

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A mixture is a physical combination

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of two or more substances
in any proportion you want.

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

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

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Hydrogen and oxygen are mixed together.

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So shouldn't these be mixtures as well?

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

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but the key point is you get mixtures

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when you physically combine
two or more substances.

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And as a result, because these substances

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have different boiling,
melting points and densities,

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you can physically separate them.

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

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

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or spin them or use magnets,

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if they have magnetic properties,

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but all by physical means,
I can separate them.

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But in contrast,

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compounds are where atoms
are chemically combined,

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they're chemically bonded together.

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You cannot separate these
atoms by physical processes.

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So compounds are still pure substances.

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

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So let's investigate
them a little bit more.

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Here we have mixed oxygen
gas and carbon dioxide gases.

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When you do that,

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the different substances
get evenly distributed

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at a molecular level; and as a result,

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the composition stays pretty
much the same throughout.

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You get a uniform composition throughout.

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And you cannot see any
distinct parts or phases

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with the naked eye.

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And that's exactly what happened

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when we added food color to our water.

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Again, the food color
uniformly distributed itself

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throughout the water at a molecular level;

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and as a result, see,
you cannot distinguish

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

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We call such mixtures
homogeneous mixtures.

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So homogeneous mixtures are the one

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where you cannot distinguish

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between the different
substances that are mixed.

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In contrast. Look at
the gold inside water.

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

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I can see the boundary over here nicely.

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

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I can clearly see there's oil here

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and there's water over there.

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Here, look, the substance

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is not evenly distributed throughout.

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There is nonuniform distribution.

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

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

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Such mixtures where we can easily make out

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the different substances
that are mixed together,

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which we can see with naked eye,

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we call them heterogeneous mixtures.

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Alright, let's take some examples now.

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Why don't you pause the video,

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go through each one of
them and classify them

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as either homogeneous mixtures
or heterogeneous mixtures.

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Pause and try.

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Alright, let's look at salads first.

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I can clearly see the different options

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that are mixed together.

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I can clearly see their boundaries.

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So this is a heterogeneous mixture.

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Okay, what about butter caramel.

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Well, can we see distinctly

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where the butter is and
where the caramel is?

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No, they're nicely uniformly distributed.

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So this is a homogeneous mixture.

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We can clearly see the seashells
and the cement separately.

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So it is heterogeneous mixture.

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This is sand and water.

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We can see the sand here
and water over here.

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I can clearly see the boundaries.

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So it's again heterogeneous mixture.

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What about ink?

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Well, over here there's
a uniform distribution.

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I cannot see any boundaries,
any distinctions.

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This is a uniform distributed mixture,

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so this is a homogeneous mixture.

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What about brass?

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Hmm, this could be tricky.

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What exactly is brass?

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

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But look, it is mixed uniformly.

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I cannot see the distinction
between copper and zinc.

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And so this is again
a homogeneous mixture.

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Finally, what about copper?

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Hmm, well, this is a trick question

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because copper is an element,

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it's not a mixture at all.

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It's a pure substance.

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So in summary, when you combine

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two or more substances physically,

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we call them mixtures.

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If the substances are mixed
uniformly at a molecular level

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so you cannot distinctly
see the different parts,

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we call them homogeneous mixtures.

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In contrast, if the different components

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are not uniformly distributed

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and you can distinctly see them,

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you might even see their boundaries,

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we call them heterogeneous mixtures.