Many years ago after we did our O levels at school we had a bunch of organisations come in to school doing talks and demonstrations. One such talk was by the Fire Brigade who demonstrated the dangers of chip pan fires. I remember the fireman extinguishing a chip pan fire and then shortly after, him pointing his finger near it, and the fire reigniting. I think I remember him saying it was the heat from his finger but recounting this story tonight, my friends don’t believe me, or at least would like to know how this can happen. Can anyone help explain please?
In: Chemistry
What is a “chip pan fire”?
Edit: Got it– pan of oil fire.
First, [chip pan](https://en.wikipedia.org/wiki/Chip_pan) is not a term I heard before. I’d probably call it a deep fry pan instead. I can’t say for sure about this specific instance, but a classic magic trick is similar.
Take a lighter that has a wick (Zippo brand as an example). Bend the wick so that it is curving, mostly horizontal but slightly down. Ignite the lighter, and then put most of the flame on the wick out by running a finger along the top of the wick (there will still be some invisible flames on the bottom of the wick). Snap your fingers while lightly shaking the lighter; the invisible flame leaks out from under the wick, and it appears as if your snap caused the flame to reignite.
Now, with this in my mind, chances are what actually happened is that when the pan was extinguished, it was by suffocating the flame. This type of extinguishing doesn’t remove any heat; if the oil was hot enough to combust before it was extinguished, it will still be hot enough when it is uncovered. When air is mixed in (though any number of things, such as the firefighter gesturing or breathing or whatever), the remaining heat can reignite the oil.
But no, the heat of the finger would make no more of a difference than the heat of the air. Heat difference is what is important, and 400° vs 40° (oil fire temps vs body temp) is almost the same as 400° vs 20° (oil fire vs room temperature).
Chip pans during a fire get really hot and remain so for long after the fire has been put out by starving it of oxygen, so reintroducing oxygen to the pan by lifting the lid can cause that stored heat to reignite the fire without a direct flame.
No way that the heat from his finger reignited it. Assuming that you remember his trick correctly, what he did was move enough air around the pan to introduce fresh air to the hot oil. Poof, instant fire.
Edit: the full trick is covering the pan with a metal lid which snuffs the fire because it runs out of oxygen. The oil is still hot enough to ignite, but it needs more oxygen to burn.
Now he removes the lid carefully which leaves a layer of smoke overtop of the oil. That smoke contains no free oxygen because it’s already all been burnt off. Then he waves his hand near the pan, that moves a bit of air that disturbs the blanket of smoke and allows the fire to start.
The fire was absolutely going to start again on its own as soon as the smoke started to dissipate. All he did was set the timing. This is why grease fires are so dangerous. You put them out by smothering them, but that doesn’t remove the heat
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