WEBVTT

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JESSICA HARROP: Hi, I'm Jessica.

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And I'm going to be talking
about a chemical demonstration

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today that I like
to call let it snow.

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MIT's Dr. John Dolhun--

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here he is-- and his
assistant, Arielle,

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are going to perform two
magic tricks involving

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super absorbent polymers.

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Let's watch as they show us
the first one at the Cambridge

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Science Festival.

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[WHOOSH]

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JOHN DOLHUN: And
what I'm going to do

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is I'm going to pour some
water into the beaker

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that she's holding.

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And you keep your
eyes on the beaker.

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Bring it down just a bit.

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Hold it firmly.

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Turn it to-- we are making snow.

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We're making snow
today, Arielle.

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ARIELLE LUBIN: Oh, boy.

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It's so cold and wet.

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It's just like real snow.

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Look!

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[WHOOSH]

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JESSICA HARROP:
So what happened?

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Well, Arielle was
holding a beaker

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containing some sodium
polyacrylate, which

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is a polymer.

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Now a polymer is a
big molecule that's

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a chain of repeating units.

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This is the unit.

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And this is what it
looks like in a chain.

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Now, different chains are
held together by cross links.

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The et cetera just
means that this could

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be thousands of carbons long.

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This is just a small part of
the chain and the polymer.

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Now when water is
added to the polymer,

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it wants to rush into
the material because

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of the presence of so many
sodium and carboxylate ions.

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The water displaces the
sodium ions leaving negatively

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charged carboxylic ions.

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These repel each other
and the repulsion

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causes the polymer, which
starts out wound together,

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to unwind, swell, and stretch.

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The cross lengths, which connect
the polymer chains together,

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prevent them from breaking
apart in the water.

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And the fake snow that's
created is the same snow

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they use in
Hollywood movie sets.

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Now, let's watch Dr. Dolhun and
Arielle turn this same polymer

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into something
totally different.

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[WHOOSH]

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JOHN DOLHUN: OK, so I've got
another beaker she's holding.

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And I'm going to pour some
water in it and observe.

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Turn it over, Arielle.

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You don't have to hold
your hand under the hole.

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That's the same polymer.

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Only this time
the polymer is not

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as cross-linked
as the first one.

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It's more of the
straight chain polymer.

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And it can absorb many more
times its weight in water.

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You'll recognize this.

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This is the diaper polymer.

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You know now, right?

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[WHOOSH]

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JESSICA HARROP: So Dr.
Dolhun uses the same polymer

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with a different amount
of cross linking.

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The sodium polyacrylate
that makes diaper gel

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has fewer cross-links
and therefore can

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absorb more water than the
sodium polyacrylate that

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turned into snow.

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Overall, this polymer can
absorb up to 800 times

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its own weight in distilled
water and about 30

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times its own weight in urine.

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It's the main ingredient
in baby diapers

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used to keep the baby dry.

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[MUSIC PLAYING]

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Hope you enjoyed the video
and I'll see you next time.

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