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What is the physics behind Ash to forming?

  1. Oct 4, 2009 #1
    Hey everyone,

    I was thinking about the physical solid texture ash and how it form. The wood ash, fly ash, volcanic ash, and soot--types of Ash all look visually different and are cause by different thing; however, they all appear to have a similar texture that we associate with Ash and sometimes even dust.
    This made me wonder, what exactly in the process of burning causes ash to look the way it does? Is there a common thermodynamic property related to ash? Is are there conditions required for ash to form in exothermic reaction? What exactly is the physics behind ash and ash formation?
  2. jcsd
  3. Oct 4, 2009 #2


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    Gold Member

    Well, yeah, when a substance has a texture that we associate with ash we tend to call it 'ash'.

    If it doesn't have that texture, such as a solidified residue after plastic has been burned, then we don't call it 'ash'. If the residue is dark grey and powdery we call it ash. It doesn't even have to involve actual combustion; volcanic ash is tiny specks of lava that have solidified while up in the air, coming down as a dust.

    There is some physics to the color of ash, though.
    The blackest of ashes is soot. Soot is incompletely burned material. Under oxygen-rich circumstances, and with intense heat, all carbon in the starting material will oxydize to carbondioxyde. But with insufficient oxygen for complete combustion all kinds of molecules will be synthesized. Different molecules absorb different colors from the spectrum. The residue after incomplete combustion will usually absorb widely across the spectrum, so it tends to be black or at least grey.

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