Wikipedia:Reference desk/Archives/Science/2021 June 15

= June 15 =

Why do hot bodies emit smokes sometimes but won't emit smoke ?
I went to industry, where they produced hot rebar coils. I only see mirage effect above coils but not smoke. Why do hot bodies emit smokes sometimes but won't emit smoke ?Rizosome (talk) 04:05, 15 June 2021 (UTC)
 * Smoke is just soot, and usually carbon specifically. A substance has to oxidize to produce soot. If there was nothing in the rebar that would oxidize (burn) at the temps experienced, there wont be any smoke. --50.234.188.27 (talk) 04:57, 15 June 2021 (UTC)
 * Smoke is caused by incomplete combustion. Which is to say, it contains things that didn't oxidize.  If it had oxidized it would be invisible gases for the most part.  For example, carbon will burn to make carbon dioxide, when you see smoke, it contains particulates that didn't burn, either because they didn't (not enough oxygen) or they couldn't (not oxidizable).  Lots of things burn "clean".
 * To get at the OP's main misunderstanding is that combustion (burning) is not the same thing as "being hot". Combustion generally means that something is oxidized exothermically.  While that does by definition make things hot, just being hot is not burning.  Glowing hot metal, as described by the OP, is produced by blackbody radiation.  Any object which has a temperature above absolute zero (which is to say, every object, including you, the walls of the room you are in, etc. etc.) produces light as thermal radiation.  You are glowing right now; the deal is that at temperatures comfortable to humans (about 300 Kelvin), you are glowing in what we call the infrared range; essentially no visible light is radiated at these temperatures.  Colder things radiate at lower energy (longer wavelength) light; while hotter things radiate higher energy light; that's why when you heat up rebar to hot enough temperatures, it starts to glow red; at that temperature, there is enough visible light radiating for your eyes to see it.  The color of a flame is a complex matter; assuming that it is a flame where complete combustion is taking place (i.e. one that does not produce smoke or soot), then much of the color is produced by the emission spectrum of the substances in the flame; it is not thermal radiation in this case but light produced by electrons releasing energy as they relax from higher energy states to lower ones.  If you see an orangy-red flame, that is blackbody radiation produced by solid soot; it is the sign of a sooty flame.  Complete combustion of pure hydrocarbon is almost invisible, because the gas in the flame is too diffuse to produce enough light by blackbody radiation to see much.  -- Jayron 32 11:44, 15 June 2021 (UTC)
 * Wikipedia has an article on smoke.--Shantavira|feed me 07:53, 15 June 2021 (UTC)