RLSH700

This is Bootcamp's post:

"Gas doesn't burn - the vapors do. When you throw a lit match into an open barrel of gas, the initial "blow up" is the "flash", or ignition of the concentrated gas vapors hovering over the liquid, and the subsequent expansion from the heat - all at once. However, gas - like any combustible - only "explodes" when under pressure. After lighting the gas and the initial "flash", the liquid in the barrel continues to evaporate (emit gas vapors) and feed the fire/reaction above the liquid until the gas has all evaporated/burned off.
Gunpowder is similar, but in granular form instead of liquid. If it's left in the open and ignited, it will initially "flash" and then burn until it's been expended. When you contain it or put it under pressure and ignite it, it will explode.
The initial "flash" from combustibles being ignited - while violent - isn't an explosion. It's an expansion from the heat created by the ignition, and continues until the fuel supply is exhausted.
An explosion is the rapid and violent ignition of an entire fuel source all at once - due to pressure - whether introduced before ignition, or as a result of the restricted expansion of the fuel by an enclosed container.

Hope that helps. "