WEBVTT

00:00:00.000 --> 00:00:07.105 align:middle line:90%
[MUSIC PLAYING]

00:00:07.105 --> 00:00:08.980 align:middle line:84%
JESSICA HARROP: Hi, I'm
Jessica and today I'm

00:00:08.980 --> 00:00:11.860 align:middle line:84%
going to shed some light on
a chemical demonstration I

00:00:11.860 --> 00:00:15.490 align:middle line:84%
like to call anatomy
of a glow stick.

00:00:15.490 --> 00:00:18.400 align:middle line:84%
There are many different
kinds of chemical reactions.

00:00:18.400 --> 00:00:21.190 align:middle line:84%
Some absorb energy and
some release energy.

00:00:21.190 --> 00:00:24.400 align:middle line:84%
Some release energy as
heat and others as light.

00:00:24.400 --> 00:00:26.110 align:middle line:84%
And when a reaction
produces light,

00:00:26.110 --> 00:00:28.570 align:middle line:84%
the process is called
chemiluminescence.

00:00:28.570 --> 00:00:31.240 align:middle line:90%


00:00:31.240 --> 00:00:34.480 align:middle line:84%
Let's watch MIT's Dr. John
Dolhun, his assistant,

00:00:34.480 --> 00:00:37.840 align:middle line:84%
Shannon, and volunteers,
Sarah, Catherine, and Trevor.

00:00:37.840 --> 00:00:40.030 align:middle line:84%
They are going to show
us some chemiluminescence

00:00:40.030 --> 00:00:42.066 align:middle line:84%
at the Cambridge
Science Festival.

00:00:42.066 --> 00:00:42.565 align:middle line:90%
[WHOOSH]

00:00:42.565 --> 00:00:44.860 align:middle line:84%
SHANNON MOREY: So not all
reactions have this ability

00:00:44.860 --> 00:00:45.650 align:middle line:90%
to produce light.

00:00:45.650 --> 00:00:48.880 align:middle line:84%
And so we have to use some
very special chemicals in order

00:00:48.880 --> 00:00:49.420 align:middle line:90%
to do this.

00:00:49.420 --> 00:00:52.690 align:middle line:84%
And we have some light sticks
that I broke apart here,

00:00:52.690 --> 00:00:56.440 align:middle line:84%
if you guys just want
to take these solutions.

00:00:56.440 --> 00:00:57.620 align:middle line:90%
Hold them carefully.

00:00:57.620 --> 00:01:01.690 align:middle line:90%


00:01:01.690 --> 00:01:04.510 align:middle line:84%
And so in a light
stick, usually there's

00:01:04.510 --> 00:01:05.920 align:middle line:90%
actually hydrogen peroxide.

00:01:05.920 --> 00:01:08.499 align:middle line:84%
And it oxidizes different
chemicals-- oh, yeah,

00:01:08.499 --> 00:01:09.790 align:middle line:90%
you can just face the audience.

00:01:09.790 --> 00:01:10.566 align:middle line:90%
That's cool.

00:01:10.566 --> 00:01:12.190 align:middle line:84%
And it will oxidize
different chemicals

00:01:12.190 --> 00:01:15.310 align:middle line:84%
depending on what color
you want to give off light.

00:01:15.310 --> 00:01:15.810 align:middle line:90%
All right.

00:01:15.810 --> 00:01:17.268 align:middle line:84%
So just pour one
solution together.

00:01:17.268 --> 00:01:19.989 align:middle line:90%
It doesn't matter which way.

00:01:19.989 --> 00:01:20.530 align:middle line:90%
AUDIENCE: Oh.

00:01:20.530 --> 00:01:23.550 align:middle line:84%
SHANNON MOREY: And
there we go, OK.

00:01:23.550 --> 00:01:25.090 align:middle line:90%
Beautiful, beautiful.

00:01:25.090 --> 00:01:25.950 align:middle line:90%
All right.

00:01:25.950 --> 00:01:28.110 align:middle line:84%
So in each of these, we
have a different chemical

00:01:28.110 --> 00:01:32.740 align:middle line:84%
that gets oxidized to
give us a different color.

00:01:32.740 --> 00:01:34.820 align:middle line:84%
All right, thank
you guys so much.

00:01:34.820 --> 00:01:37.568 align:middle line:90%
[APPLAUSE]

00:01:37.568 --> 00:01:38.490 align:middle line:90%
[WHOOSH]

00:01:38.490 --> 00:01:40.820 align:middle line:84%
JESSICA: So how does a
reaction produce light?

00:01:40.820 --> 00:01:42.540 align:middle line:90%
Let's start with two molecules.

00:01:42.540 --> 00:01:45.230 align:middle line:84%
Let's call them A
and B. They react

00:01:45.230 --> 00:01:50.300 align:middle line:84%
to produce C and D. Energy
that's produced in the reaction

00:01:50.300 --> 00:01:53.270 align:middle line:84%
causes the electrons
in D to become excited.

00:01:53.270 --> 00:01:55.310 align:middle line:84%
As the electrons
relax back down,

00:01:55.310 --> 00:01:57.890 align:middle line:90%
they give off energy as light.

00:01:57.890 --> 00:02:01.520 align:middle line:84%
So basically, chemical energy is
transformed into light energy.

00:02:01.520 --> 00:02:05.060 align:middle line:84%
This all happens in one step
as the reaction is progressing.

00:02:05.060 --> 00:02:06.723 align:middle line:90%
Here's what's happening.

00:02:06.723 --> 00:02:25.470 align:middle line:90%


00:02:25.470 --> 00:02:29.270 align:middle line:84%
This is the energy level of
our reactants, A and B. When

00:02:29.270 --> 00:02:34.920 align:middle line:84%
they react, they form C
and D in its excited state.

00:02:34.920 --> 00:02:37.690 align:middle line:84%
Now to relax back down
to where it needs to be,

00:02:37.690 --> 00:02:40.204 align:middle line:90%
D emits energy as light.

00:02:40.204 --> 00:02:41.870 align:middle line:84%
Now there's a different
amount of energy

00:02:41.870 --> 00:02:43.850 align:middle line:84%
in our reactants
than our products,

00:02:43.850 --> 00:02:48.050 align:middle line:84%
so that's why these two
energy levels are different.

00:02:48.050 --> 00:02:50.120 align:middle line:84%
All right, so in Dr.
Dolhun's experiment,

00:02:50.120 --> 00:02:53.510 align:middle line:84%
each volunteer has a cup
containing a different chemical

00:02:53.510 --> 00:02:56.720 align:middle line:84%
and adds hydrogen
peroxide to each cup.

00:02:56.720 --> 00:03:01.880 align:middle line:84%
And each cup glows a different
color, red, blue, or green.

00:03:01.880 --> 00:03:03.830 align:middle line:84%
This happens because
the hydrogen peroxide

00:03:03.830 --> 00:03:07.080 align:middle line:84%
is oxidizing each of
the different chemicals.

00:03:07.080 --> 00:03:09.980 align:middle line:84%
So oxidation is the chemical
reaction that will ultimately

00:03:09.980 --> 00:03:12.650 align:middle line:90%
cause the chemiluminescence.

00:03:12.650 --> 00:03:14.330 align:middle line:84%
So why did each
of these chemicals

00:03:14.330 --> 00:03:15.830 align:middle line:90%
glow a different color?

00:03:15.830 --> 00:03:17.450 align:middle line:84%
Well it's because
they each emitted

00:03:17.450 --> 00:03:19.100 align:middle line:90%
a different amount of energy.

00:03:19.100 --> 00:03:21.224 align:middle line:90%
And energy equals color.

00:03:21.224 --> 00:03:23.390 align:middle line:84%
So let's think of it in
terms of a rainbow spectrum.

00:03:23.390 --> 00:03:33.870 align:middle line:90%


00:03:33.870 --> 00:03:35.420 align:middle line:84%
So the amount of
energy increases

00:03:35.420 --> 00:03:36.990 align:middle line:90%
as you move up the spectrum.

00:03:36.990 --> 00:03:39.890 align:middle line:84%
So red is lower
energy than blue.

00:03:39.890 --> 00:03:42.680 align:middle line:84%
So if the difference between
the ground state and the excited

00:03:42.680 --> 00:03:45.200 align:middle line:84%
state is small, there
is a smaller amount

00:03:45.200 --> 00:03:46.850 align:middle line:84%
of energy released,
and the light

00:03:46.850 --> 00:03:49.220 align:middle line:84%
is on the redder
side of the spectrum.

00:03:49.220 --> 00:03:51.200 align:middle line:84%
Conversely, if
that gap is larger,

00:03:51.200 --> 00:03:52.890 align:middle line:90%
there's more energy released.

00:03:52.890 --> 00:03:55.940 align:middle line:84%
So the light is on the
bluer side of the spectrum.

00:03:55.940 --> 00:03:57.650 align:middle line:84%
And this chemiluminescence
chemistry

00:03:57.650 --> 00:04:00.110 align:middle line:84%
has practical uses
beyond glowsticks

00:04:00.110 --> 00:04:02.030 align:middle line:90%
that light up a good party.

00:04:02.030 --> 00:04:04.160 align:middle line:84%
We also use reactions
like these to create

00:04:04.160 --> 00:04:08.120 align:middle line:84%
flares that don't get too hot
in use of emergency situations.

00:04:08.120 --> 00:04:10.160 align:middle line:84%
And nature uses
chemiluminescence too.

00:04:10.160 --> 00:04:12.440 align:middle line:84%
[WHOOSH] If you've ever
seen a firefly light up

00:04:12.440 --> 00:04:16.010 align:middle line:84%
at night or an [WHOOSH] Angler
fish hunt for prey, you've seen

00:04:16.010 --> 00:04:18.350 align:middle line:84%
bioluminescence which
is chemiluminescence

00:04:18.350 --> 00:04:20.329 align:middle line:90%
in a biological system.

00:04:20.329 --> 00:04:23.030 align:middle line:84%
It's the exact same principle
as the chemical reactions

00:04:23.030 --> 00:04:24.759 align:middle line:90%
that Dr. Dolhun showed us.

00:04:24.759 --> 00:04:26.550 align:middle line:84%
[MUSIC PLAYING] Hope
you enjoyed the video.

00:04:26.550 --> 00:04:28.510 align:middle line:90%
I'll see you next time.

00:04:28.510 --> 00:04:59.536 align:middle line:90%