What is Fire?

Gretchen K. asks, "Dr. Volt, can you please explain fire?"

Well, Gretchen, fire is actually a complex series of chemical reactions termed exothermic, which means that the reaction emits energy as heat. The burning of a fuel then releases latent energy present in the fuel, transforming it from chemical potential energy into more useful forms: in this case heat and light. The color of fire depends both on the heat of the fire (bright yellow flames are hotter than orange flames) and the fuel source used (for example, the blue flame that results from burning natural gas).

Fire requires three elements: fuel, oxygen, and heat. Once a fire starts it becomes a chain reaction that will continue as long as its three requirements remain present in sufficient quantities to sustain the reaction.

Byproducts of the fire include carbon dioxide (CO2), which results from the combination of oxygen in the air with carbon in the fuel source. Soot is carbon that remains unburned; ash is also remnants of the fuel source that haven't burned. A very efficient fire (such as results from the burning of flash paper) will leave little to no soot or ash behind. Other byproducts of the fire will depend on the chemicals present in the fuel source.

The shape of flames is affected by gravity – NASA has learned that in low or zero gravity environments flames becomes spherical, although these flames can quickly go out because the carbon dioxide generated does not disperse well and can smother the flame.

Thank you, Gretchen, for such a scintillating question!