What is the role of an external source of ignition?

The role of an external source of ignition is to provide the necessary energy for combustion to begin. Combustion occurs when a fuel reacts with an oxidizing agent (usually oxygen) at a high enough temperature. The external source of ignition, such as a spark, flame, or heat, supplies the initial energy that brings the fuel to its ignition temperature, thus starting the combustion process.

In scenarios involving flammable materials, understanding the need for an ignition source is crucial for safety and prevention measures. Once sufficient energy is provided to overcome the activation energy barrier, the chemical reactions that lead to fire can proceed. This highlights the importance of controlling sources of ignition in environments where flammable substances are present, in order to prevent accidents and fires.

The other options do not accurately describe the function of an ignition source. Cooling flammable materials might reduce the risk of fire but does not initiate combustion. Altering the chemical structure of fuel doesn't directly relate to ignition and is not a characteristic of ignition sources. Likewise, increasing the amount of available oxygen might support combustion but does not serve as an ignition source itself.