I currently stay with a chemistry grad student. He occasionally starts a fire pit in the back. Somehow, we got to pontificating on why fire looks the way it does. I thought to synthesize his hunches because: 1. they sounded right 2. i have no reason to doubt a phd chemist 3. it sounded cool
Basically wood is a solid fuel. It's packed densely with cellulose (amongst other sugar chains and other long chain molecules). Heat liberates these sugars (along with other molecules). As the heat increases, the sugars themselves decompose into alcohols. The liberated decomposed sugars and other molecules become a gas. What we observe as the flames of the fire is oxidation of this heated gas. The escaping gas ignites under heat and oxygen, giving the fire its characteristic flicker. The light and heat we feel from a fire is actually the radiation from this oxidation reaction. Some of liberated radiation in the visual spectrum (the light of the fire). A lot of the heat we feel from the fire is radiation released in the IR spectrum.
The flame persists for a while because the fire is a self-catalyzing reaction. The radiation released from the oxidation reaction becomes the heat that's used to liberate more molecules from the wood. This creates more gas. New oxygen diffuses in and reacts with the heated gas -- releasing even more energy that's again used to liberate more gas. This is why fanning flames sometimes help fires catch. When you fan, you blow away the gas and replace it with a bunch of oxygen. So when you stop fanning, there's a lot of oxygen for the gas to react with. But it's also important not to fan too much. You run the risk of decreasing the temperature of the reaction zone above the wood.
The embers of charcoal that are left glowing is actually extremely hot carbon remnants of the wood. The heat and light we see from charcoal is actually blackbody radiation. You'll notice if you pick up what's left from a fire, the wood is considerably less dense. Basically, the solid sugar fuel for the fire burned up, leaving the carbon shell of what held the fuel. This carbon shell itself glows when its hot. The charcoal you buy in a store is basically this stuff ground down to a dust and pressed together with clay.
Basically wood is a solid fuel. It's packed densely with cellulose (amongst other sugar chains and other long chain molecules). Heat liberates these sugars (along with other molecules). As the heat increases, the sugars themselves decompose into alcohols. The liberated decomposed sugars and other molecules become a gas. What we observe as the flames of the fire is oxidation of this heated gas. The escaping gas ignites under heat and oxygen, giving the fire its characteristic flicker. The light and heat we feel from a fire is actually the radiation from this oxidation reaction. Some of liberated radiation in the visual spectrum (the light of the fire). A lot of the heat we feel from the fire is radiation released in the IR spectrum.
The flame persists for a while because the fire is a self-catalyzing reaction. The radiation released from the oxidation reaction becomes the heat that's used to liberate more molecules from the wood. This creates more gas. New oxygen diffuses in and reacts with the heated gas -- releasing even more energy that's again used to liberate more gas. This is why fanning flames sometimes help fires catch. When you fan, you blow away the gas and replace it with a bunch of oxygen. So when you stop fanning, there's a lot of oxygen for the gas to react with. But it's also important not to fan too much. You run the risk of decreasing the temperature of the reaction zone above the wood.
The embers of charcoal that are left glowing is actually extremely hot carbon remnants of the wood. The heat and light we see from charcoal is actually blackbody radiation. You'll notice if you pick up what's left from a fire, the wood is considerably less dense. Basically, the solid sugar fuel for the fire burned up, leaving the carbon shell of what held the fuel. This carbon shell itself glows when its hot. The charcoal you buy in a store is basically this stuff ground down to a dust and pressed together with clay.
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