What is fire? - Elizabeth Cox
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0:07 - 0:10Sitting around a campfire,
you can feel its heat, -
0:10 - 0:13smell the woody smoke,
and hear it crackle. -
0:13 - 0:14If you get too close,
-
0:14 - 0:17it burns your eyes
and stings your nostrils. -
0:17 - 0:20You could stare at the bright
flames forever -
0:20 - 0:24as they twist and flicker
in endless incarnations. -
0:24 - 0:26But what exactly are you looking at?
-
0:26 - 0:29The flames are obviously not solid,
-
0:29 - 0:30nor are they liquid.
-
0:30 - 0:33Mingling with the air, they’re
more like a gas, -
0:33 - 0:35but more visible--and more fleeting.
-
0:35 - 0:39And on a scientific level,
fire differs from gas -
0:39 - 0:43because gases can exist in
the same state indefinitely -
0:43 - 0:47while fires always burn out eventually.
-
0:47 - 0:51One misconception is that
fire is a plasma, -
0:51 - 0:53the fourth state of matter in which atoms
-
0:53 - 0:56are stripped of their electrons.
-
0:56 - 0:59Like fire and unlike the
other kinds of matter, -
0:59 - 1:03plasmas don’t exist in a stable
state on earth. -
1:03 - 1:09They only form when gas is exposed
to an electric field or superheated -
1:09 - 1:13to temperatures of thousands
or tens of thousands of degrees. -
1:13 - 1:17By contrast, fuels like wood
and paper burn -
1:17 - 1:19at a few hundred degrees —far below the
-
1:19 - 1:24threshold of what's usually
considered a plasma. -
1:24 - 1:27So if fire isn’t a solid, liquid, gas,
-
1:27 - 1:30or a plasma, what does that leave?
-
1:30 - 1:34It turns out fire isn’t actually
matter at all. -
1:34 - 1:40Instead, it’s our sensory experience of a
chemical reaction called combustion. -
1:40 - 1:44In a way, fire is like the leaves
changing color in fall, -
1:44 - 1:46the smell of fruit as it ripens,
-
1:46 - 1:49or a firefly’s blinking light.
-
1:49 - 1:51All of these are sensory clues that a
-
1:51 - 1:54chemical reaction is taking place.
-
1:54 - 1:57What differs about fire is that
it engages a lot of -
1:57 - 2:01our senses at the same time,
creating the kind of vivid -
2:01 - 2:06experience we expect to come
from a physical thing. -
2:06 - 2:08Combustion creates that sensory experience
-
2:08 - 2:12using fuel, heat, and oxygen.
-
2:12 - 2:16In a campfire, when the logs are
heated to their ignition temperature, -
2:16 - 2:18the walls of their cells decompose,
-
2:18 - 2:22releasing sugars and other
molecules into the air. -
2:22 - 2:25These molecules then react
with airborne oxygen -
2:25 - 2:28to create carbon dioxide and water.
-
2:28 - 2:32At the same time, any trapped
water in the logs -
2:32 - 2:36vaporizes, expands, ruptures
the wood around it, -
2:36 - 2:39and escapes with a satisfying crackle.
-
2:39 - 2:43As the fire heats up, the carbon
dioxide and water vapor -
2:43 - 2:46created by combustion expand.
-
2:46 - 2:51Now that they’re less dense,
they rise in a thinning column. -
2:51 - 2:54Gravity causes this expansion and rising,
which gives -
2:54 - 2:56flames their characteristic taper.
-
2:56 - 3:00Without gravity, molecules don’t separate
-
3:00 - 3:04by density and the flames
have a totally different shape. -
3:04 - 3:07We can see all of this because combustion
-
3:07 - 3:08also generates light.
-
3:08 - 3:11Molecules emit light when heated,
-
3:11 - 3:13and the color of the light depends
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3:13 - 3:15on the temperature of the molecules.
-
3:15 - 3:18The hottest flames are white or blue.
-
3:18 - 3:20The type of molecules in a fire can
-
3:20 - 3:22also influence flame color.
-
3:22 - 3:26For instance, any unreacted
carbon atoms from the logs -
3:26 - 3:29form little clumps of soot that rise
-
3:29 - 3:32into the flames and emit the yellow-orange
-
3:32 - 3:35light we associate with a campfire.
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3:35 - 3:38Substances like copper, calcium chloride,
-
3:38 - 3:40and potassium chloride can add their
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3:40 - 3:44own characteristic hues to the mix.
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3:44 - 3:46Besides colorful flames,
-
3:46 - 3:49fire also continues to generate heat
as it burns. -
3:49 - 3:52This heat sustains the flames by keeping
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3:52 - 3:56the fuel at or above ignition temperature.
-
3:56 - 3:59Eventually, though, even the hottest fires
-
3:59 - 4:01run out of fuel or oxygen.
-
4:01 - 4:04Then, those twisting flames
give a final hiss -
4:04 - 4:07and disappear with a wisp of smoke
-
4:07 - 4:09as if they were never there at all.
- Title:
- What is fire? - Elizabeth Cox
- Speaker:
- Elizabeth Cox
- Description:
-
View full lesson: https://ed.ted.com/lessons/is-fire-a-solid-a-liquid-or-a-gas-elizabeth-cox
Sitting around a campfire, you can feel its heat, smell the woody smoke, and hear it crackle. If you get too close, it burns your eyes and stings your nostrils. You could stare at the bright flames forever as they twist and flicker in endless incarnations… But what exactly are you looking at? Elizabeth Cox illuminates the science behind fire.
Lesson by Elizabeth Cox, directed by Héloïse Dorsan Rachet.
- Video Language:
- English
- Team:
- closed TED
- Project:
- TED-Ed
- Duration:
- 04:25
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