What Is Fire?

    In this post I am going to talk about what is fire? More specifically I am going to discuss two things, what is fire and what are the various components of a flame.

What Is Fire?

    Simply put fire is a process. It is a chemical reaction in which you need three things: fuel, an oxidizer and heat.

    The chemical reaction between the fuel and the oxidizer will convert the original two chemicals into new chemicals that have lower total internal energy. The energy that is lost appears as heat and/or electromagnetic radiation (light).

    The simplest fire is hydrogen burning in an oxygen atmosphere. A couple of hydrogen molecules (2 x H₂) react with a single oxygen molecule (O₂) to create a couple of water molecules (2 x H₂O).

    The water molecule has a lower internal binding energy than the internal binding energies of the original hydrogen and oxygen molecules. That energy difference appears as added velocity for the water molecule which we sense as heat.

    Other fuels and oxidizers produce different compounds and release more or less heat but it is all the same idea.

What Are The Components Of A Flame?

    A flame is basically very hot gas (and possibly a plasma right in the reaction zone). In the photo the blue region (area (1)) is the chemical reaction. The chemical reaction in this case is emitting blue light as electrons are dropping down into lower energy orbitals (i.e. part of the chemical process).

    The orange/yellow region of the flame (area (2)) is very hot soot. When hydrocarbons burn, the process is not always perfect and soot particles will usually be created. This is especially true for a wood fire.

    Imperfect combustion will occur if the supply of air or oxygen is poor. The soot is hot and will emit thermal radiation corresponding to its temperature (see black-body radiation ).

This is the same sort of glow you see when a piece of metal is heated in a furnace.

Above the flame is very hot gas but it is not emitting visible black-body radiation. It is still emitting black-body radiation but your eyes cannot detect it.

So a flame is a hot gas, a partial plasma and very hot soot.

This post in Ask a Mathematician tells us:

"Fire is a genuine plasma. Maybe not the best plasma, or the most ionized plasma, but it does alright."

Thank you for taking the time to read my post.