Why does burning produce more smoke after the fire has been put out?



Why does burning produce more smoke after the fire has been put out?

In: Physics

You generally don’t burn solid or liquid materials, you burn the gases given off by solid or liquid materials. Once you snuff the flame, your fuel source is still hot enough to give off flammable gases for a while, which is what you’re seeing.

Those unburnt gases are typically more visible than the smoke from a flame because burning them would typically convert most of the material into CO2 and a little steam.

In addition to what tsuuga said, it’s also because the flame burns less cleanly (burns up the fuel less completely) as it goes out. The reason why flames produce light isn’t much from the gases burning – if you’ve ever used a gas stovetop, camp stove or Bunsen burner, you’ll have seen they their flames are blue and quite faint – that’s because they fuel source, the gas, is pure. Things like candles and wood produce light when they are burned because they initial oxidation produces tiny particles of soot and ash. When these particles enter the outer part of the flame where the gases are burning, they are heated so that they glow as they burn up – that’s what produces the light. When you put a fire out, you usually can’t put it out instantly – there’s embers that remain for a couple seconds. Because there’s so much less heat being produced, they let a lot more of those particles of unburned fuel escape – they cooler flame isn’t hot enough to burn the fuel very cleanly at first, and then there’s no flame to get rid of the particles that are produced. Because of that, all those extra tiny bits of soot and ash that are being created and escaping rise in the smoke, making it very visible as the fire goes out.