Fire, ions, magnets and welding

Cecil's Columns/Staff Reports
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Cecil writes:
Can fire be ionized? Is it affected by magnetism? Not so’s you’d notice. You’re thinking of plasma, which is ionized (and thus electromagnetically reactive) gas, often described as the fourth state of matter. You see it in welding arcs, lightning bolts, and the sun. Ordinary fire isn’t plasma.

Fire is certainly not plasma, but there are some important points to make here…

For example, there are detectors in most gas cromatographs (GCs, analytical instruments used in chemistry) that are called FIDs, Flame Ionization Detectors. In a FID, the electrical resistance over a flame is measured. When something combustible enters the flame, ionziation occurs and current is lead more easily over the flame as more ions are present for the electrons to “jump” on. See for example the Quadrex web site. So even if a flame is not ionized enough to be a plasma (Frank Waelbroeck agrees with Cecil here) it doesn’t mean that it is not ionized.

A flame also contains lots of free radicals that simply put are atoms that have their original number of electrons but don’t feel so happy about it (while ions are atoms or molecules that have gotten or lost electrons and feel happy about that). Halon fire extinguishers and flame retardants use components that stops the free radicals and ions from reacting so fast, thereby quenching the fire. So the electron transferring is really important in fire. (This comes as no surprise to chemists, who consider almost all reactions, mild or wild, to involve electron transfer.)

And just because something is ionized, it doesn’t mean that it is magnetic. Think about saturated salt water. It contains about 300 gram ions per liter and does interact with magnets at all except for them rust faster…

The average welding arcs are also not usually plasma. Plasma is a highly ionized matter of state (with the same number of positive ions and free electrons). To achive this in a welding arc, a gas stream is passed through a electric arc. This is only done in larger machines and not usually in your every-day DIY home-welding equipment. (www.designsite.dk)

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Because the glowing part of a flame is ionized, it is opaque and responds to electric fields. Get a piezoelectric igniter. They’re used to light gas grills and cost around $10. Attach a wire to each electrode so the free ends are separated by an inch or so. Put a candle flame in the gap. Squeeze the trigger. The flame will momentarily shoot to one wire. Release the trigger and the flame will shoot to the opposite wire. This assumes that your igniter creates a voltage upon release opposite to that upon squeezing the trigger. Some igniters have a mechanism that just gives the piezo element a whack when you squeeze the trigger, resulting in a +/- impulse and a less visible response from the flame. I forget whether the flame is attriacted to the electrode with a positive charge or with a negative charge.

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Minor nitpick: Water is diamagnetic, so it is repelled by magnets. Doesn’t have anything to do with ions, but saying that magnets have no interaction with water is inaccurate.