Everyday physics · show the check

The Fourth State That Isn't

You have almost certainly been told that fire is a plasma — the fourth state of matter, atoms torn into ions and free electrons. It's a tidy story, and for an ordinary flame it is wrong. Drag the temperature below and watch the Saha equation compute a flame's ionization from scratch. A candle is about 0.00001% ionized — a million times short of the plasma line — and even those few ions come from chemistry, not heat. So why does it glow? For two reasons, and neither of them is plasma.

A plasma is a gas ionized enough that its charged particles call the tune — enough free electrons and ions that electric forces, not ordinary collisions, govern how it behaves. Lightning is a plasma. A neon sign, a welding arc, the Sun, the inside of a fluorescent tube: all plasma. The usual working line is drawn around 1% ionized; below that a gas is at most weakly ionized.¹ The question is simply where a flame falls. So let's compute it.

Instrument 1 / the questionHow ionized is a flame, really?

Pick what's being heated, then drag the temperature. The Saha equation gives the fraction of atoms that thermal energy alone can strip an electron from, in equilibrium. The curve is the thermal prediction; the shaded stripe is what a real flame is measured to have.

Even at a candle's temperature, the air it burns in is ionized by a factor around 10⁻²⁶ — that is not a typo, it is essentially zero. Switch to caesium, the easiest ordinary atom to ionize, and heat barely manages 10⁻⁷. To drag thermal ionization up to the 1% plasma line you need thousands of degrees more than any candle, stove, or campfire reaches — the realm of arcs and lightning. By heat alone, a flame is not remotely a plasma.

And yet a flame does contain a whisper of ions — you can bend one with a charged rod, and a smoke detector's cousin, the flame-ionization detector, earns its living off them. Those ions are real, but they don't come from heat. They come from chemi-ionization: a specific reaction in the burning itself, CH + O → CHO⁺ + e⁻, whose product hands its charge on to become H₃O⁺, the flame's most common ion.² It tops out at a fraction near 10⁻⁷ — the stripe on the plot. That's a hundred-million-fold more than heat could manage, and still ten thousand times below the plasma line. A flame is a gas with a chemical trace of ions in it. That trace is not a fourth state of matter.

Instrument 2 / the yellowThen why does it glow?

If the light isn't ions recombining, what is it? Two different things. The familiar yellow is the boring, beautiful answer: incandescence. A flame that can't get enough oxygen cracks its fuel into microscopic flecks of soot — carbon — and heats them white-hot. Any hot object radiates a black-body spectrum set only by its temperature; soot at a candle's ~1200 °C glows exactly the way a poker or a lightbulb filament does. Drag the temperature and watch the true colour and spectrum.

Notice the peak of the spectrum sits far off the right edge, deep in the infrared: at 1200 °C a black body radiates most of its energy as heat you feel but can't see, and the visible glow is only the thin blue-ward tail of that curve. That's why a cooler flame looks dim orange and a hotter one whitens — you're watching the tail brighten and even out. (A single 1200 °C soot temperature is really a deep amber; a candle looks yellower because the brightest flecks near the tip run hotter, nearer 1700 K.) None of this needs ionization. It is the same glow as everything hot.

Instrument 3 / the blueThe cold light at the base

Look at the base of a candle, or a well-mixed gas ring: a delicate blue. That colour is not the soot glow — a clean blue flame is barely making soot at all. It's chemiluminescence: light released the instant certain molecules form in an electronically excited state and drop back down. These are specific, named radicals with specific colours — not a smooth thermal rainbow but a handful of sharp bands, the fingerprint combustion scientists read to see inside a flame. Toggle the two lights:

The sharp lines are OH* at 309 nm (in the ultraviolet — you never see it), CH* at about 431 nm (the blue-violet that tints a flame's base), and C₂*, the Swan bands around 473 and 516 nm (the blue-green named after William Swan, who mapped them in 1856).³ That's the whole picture. A flame's light is a hot solid glowing (the yellow) plus a few excited molecules flashing (the blue) — incandescence and chemiluminescence. Recombining plasma is nowhere in it.

So — is fire a plasma?

For an ordinary flame, no — not by the working definition anyone uses. It is about 10⁻⁵ percent ionized, a hundred thousand times shy of the line, and that trace is chemistry, not heat. The honest caveat, named plainly: “plasma” has no single legislated cutoff. A physicist who defines plasma as any gas with free charges showing collective behaviour can call a flame a vanishingly weak plasma, and won't be lying. But the claim people mean — that fire is the fourth state of matter, kin to the Sun and lightning — fails by five or more orders of magnitude. Very hot, heavily sooting, or arc-driven flames genuinely do ionize into plasma; a candle on a table does not. Its light was never the light of torn atoms. It was a hot speck of soot, and a molecule catching fire.