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  • 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?

  • The flames are obviously not solid,

  • nor are they liquid.

  • Mingling with the air, they're more like a gas,

  • but more visible--and more fleeting.

  • And on a scientific level, fire differs from gas

  • because gases can exist in the same state indefinitely

  • while fires always burn out eventually.

  • One misconception is that fire is a plasma,

  • the fourth state of matter in which atoms

  • are stripped of their electrons.

  • Like fire and unlike the other kinds of matter,

  • plasmas don't exist in a stable state on earth.

  • They only form when gas is exposed to an electric field or superheated

  • to temperatures of thousands or tens of thousands of degrees.

  • By contrast, fuels like wood and paper burn

  • at a few hundred degreesfar below the

  • threshold of what's usually considered a plasma.

  • So if fire isn't a solid, liquid, gas,

  • or a plasma, what does that leave?

  • It turns out fire isn't actually matter at all.

  • Instead, it's our sensory experience of a

  • chemical reaction called combustion.

  • In a way, fire is like the leaves changing color in fall,

  • the smell of fruit as it ripens,

  • or a firefly's blinking light.

  • All of these are sensory clues that a

  • chemical reaction is taking place.

  • What differs about fire is that it engages a lot of

  • our senses at the same time, creating the kind of vivid

  • experience we expect to come from a physical thing.

  • Combustion creates that sensory experience

  • using fuel, heat, and oxygen.

  • In a campfire, when the logs are heated to their ignition temperature,

  • the walls of their cells decompose,

  • releasing sugars and other molecules into the air.

  • These molecules then react with airborne oxygen

  • to create carbon dioxide and water.

  • At the same time, any trapped water in the logs

  • vaporizes, expands, ruptures the wood around it,

  • and escapes with a satisfying crackle.

  • As the fire heats up, the carbon dioxide and water vapor

  • created by combustion expand.

  • Now that they're less dense, they rise in a thinning column.

  • Gravity causes this expansion and rising, which gives

  • flames their characteristic taper.

  • Without gravity, molecules don't separate

  • by density and the flames have a totally different shape.

  • We can see all of this because combustion

  • also generates light.

  • Molecules emit light when heated,

  • and the color of the light depends

  • on the temperature of the molecules.

  • The hottest flames are white or blue.

  • The type of molecules in a fire can

  • also influence flame color.

  • For instance, any unreacted carbon atoms from the logs

  • form little clumps of soot that rise

  • into the flames and emit the yellow-orange

  • light we associate with a campfire.

  • Substances like copper, calcium chloride,

  • and potassium chloride can add their

  • own characteristic hues to the mix.

  • Besides colorful flames,

  • fire also continues to generate heat as it burns.

  • This heat sustains the flames by keeping

  • the fuel at or above ignition temperature.

  • Eventually, though, even the hottest fires

  • run out of fuel or oxygen.

  • Then, those twisting flames give a final hiss

  • and disappear with a wisp of smoke

  • as if they were never there at all.

Sitting around a campfire, you can feel its heat,

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B2 US TED-Ed fire combustion gas sensory plasma

Is fire a solid, a liquid, or a gas? - Elizabeth Cox

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    Samuel posted on 2018/11/10
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