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  • Fire is the visible effect of the process of combustion – a special type of chemical reaction. It occurs between oxygen in the air and some sort of fuel. The products from the chemical reaction are completely different from the starting material.

    The fuel must be heated to its ignition temperature for combustion to occur. The reaction will keep going as long as there is enough heat, fuel and oxygen. This is known as the fire triangle.

    Combustion is when fuel reacts with oxygen to release heat energy. Combustion can be slow or fast depending on the amount of oxygen available. Combustion that results in a flame is very fast and is called burning. Combustion can only occur between gases.

    Chemical reaction in the combustion process

    Fuels can be solids, liquids or gases. During the chemical reaction that produces fire, fuel is heated to such an extent that (if not already a gas) it releases gases from its surface.

    Only gases can react in combustion. Gases are made up of molecules (groups of atoms). When these gases are hot enough, the molecules in the gases break apart and fragments of molecules rejoin with oxygen from the air to make new product molecules – water molecules (H2O) and carbon dioxide molecules (CO2) – and other products if burning is not complete.

    The heat generated by the reaction is what sustains the fire. The heat of the flame will keep remaining fuel at ignition temperature. The flame ignites gases being emitted, and the fire spreads. As long as there is enough fuel and oxygen, the fire keeps burning.

    Fuel + oxygen (from the air) = combustion products (mainly CO2 + H2O) + heat energy.

    Complete combustion

    In complete combustion, the burning fuel will produce only water and carbon dioxide (no smoke or other products). The flame is typically blue. For this to happen, there needs to be enough oxygen to combine completely with the fuel gas.

    Many of us use methane gas (CH4), commonly known as natural gas, at home for cooking. When the gas is heated (by a flame or spark) and if there is enough oxygen in the atmosphere, the molecules will break apart and reform totally as water and carbon dioxide.

    CH4(g) + 2O2(g) + heat CO2(g) + 2H2O(g) + heat
    methane + oxygen + heat carbon dioxide + water + heat

    Incomplete combustion

    If there is not enough oxygen available during a chemical reaction, incomplete combustion occurs, and products such as carbon (C) and carbon monoxide (CO) as well as water and carbon dioxide are produced. Less heat energy is released during incomplete combustion than complete combustion.

    In incomplete combustion, the burning flame is typically yellow or orange and there is smoke.

    Related content

    Explore some more of the big science ideas and concepts related to fire:

    • What is smoke? explains incomplete combustion and the dangers that smoke presents.
    • Heat energy looks at heat transfer – convection, conduction and radiation.
    • Fire behaviour explores the variables that influence fire.

    Wildfires – what are they? What causes them? How do wildfires impact us and are they affected by the climate?

    Activity ideas

    Drama in the microworld – using drama to model atoms, molecules, heat transfer and combustion.

    Light a candle – observe and investigate a candle flame and the process of burning.

    Putting out the fire – investigate ways of extinguishing fire, using knowledge of the fire triangle and fire chemistry.

    Exploding flour – observing combustion.

      Published 19 November 2009 Referencing Hub articles
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