WEBVTT

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[MUSIC PLAYING]

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JESSICA: Hi.

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I'm Jessica.

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And today, I'm going to be
talking about a chemical

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demonstration I like to
call " "Mirror, Mirror."

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In this demo, MIT'S
Dr. John Dolhun

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will demonstrate Tollens'
Test, a chemical test created

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by this man, Bernhard
Tollens, a German chemist born

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in the late 1800s.

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So let's watch Dr. Dolhun
perform this demonstration here

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at MIT as part of the
Cambridge Science Festival.

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JOHN DOLHUN: OK, so I'm
going to be careful.

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And I'm going to pour my first
substance into the flask.

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My second substance,
down to the last drop.

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And my third substance.

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And we're going to
do a little shaking.

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We're mixing some
things together.

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JESSICA: So what
did Dr. Dolhun do?

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Well, he added three
solutions to the flask.

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Solution 1 was glucose in water.

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Solution 2 was a mixture of
silver nitrate and ammonium

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nitrate.

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And Solution 3 was sodium
hydroxide, which is a base.

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So let's focus on the
second two solutions first.

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When you mix solutions 2
and 3, two things happen.

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First, you form Ag2O, which
is insoluble in water.

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So it precipitates
out as a brown solid.

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The sodium hydroxide reacts
with ammonium and H4 plus

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to produce ammonia, NH3.

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This dissolves the precipitate.

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And you end up with Ag(NH3)2,
which all has a positive

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charge.

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This is soluble in water, and
it's called Tollens Reagent.

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Now, the positive charge
is borne by the silver.

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So what we have in solution
is essentially silver ions.

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Now looking at Solution 1,
glucose looks like this.

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We don't really care about this
whole part of the molecule.

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So we're going to call
it R for "the rest."

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So glucose is basically this.

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And this part of
the glucose is going

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to start reacting
with the silver ions.

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Let's watch what happens.

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JOHN DOLHUN: I don't
see anything happening.

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Do you?

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OK, still working on it.

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I'm getting tired here.

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Something happening?

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AUDIENCE: It's silver.

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It's

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JOHN DOLHUN: Oh, my
goodness, I can see myself.

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Well, this is a real
workout, I'll tell you.

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So what we've created
here is a beautiful silver

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mirrored flask.

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JESSICA: So look at that.

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Dr. Dolhun has created a
beautiful silver mirror.

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It almost looks like a
Christmas tree ornament.

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And a silver ornament could,
in fact, be made this way.

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So here's what's happening.

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Electrons from
here, the glucose,

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get transferred to the silver,
reducing it to silver zero.

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Silver zero is
insoluble in water.

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It's just silver metal,
and it's precipitating out

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on the inside of the flask,
forming the beautiful silver

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mirror that we see.

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Now, this process is
called "reduction."

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And whenever
something is reduced,

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something else is oxidized.

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So the glucose must
be getting oxidized.

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It becomes this.

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An oxygen was added here.

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Now, this process is similar
to metabolism in our bodies.

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We remove electrons
from the sugar we eat,

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and transfer them to the
oxygen that we breathe.

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In a reduction reaction
like mirror, mirror,

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there might be energy released.

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In your body, this energy is
harnessed in chemical bonds,

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stored for later.

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The point of Tollens' Test was
not to produce silver mirrors,

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although a similar reaction
is used for that today.

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The point was to distinguish
between two types of sugars--

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ones containing aldehyde
and ones containing ketones.

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Let me draw those.

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The difference is
that in an aldehyde,

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the carbon is connected to
a hydrogen. And in a ketone,

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the carbon is connected
to another carbon.

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Aldehydes are more
reactive than ketones,

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so they will react
with the silver ions,

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while the ketones will not.

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That's it for me.

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Hopefully, you
enjoyed this video.

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And I will see you next time.

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[MUSIC PLAYING]

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