If the pressure is very low, free radicals are likely to collide with the walls of the container before reacting with other molecules, and no explosion occurs. If the pressure is high, the free radicals are likely to collide with each other, terminating a reaction chain, and no explosion occurs. Only when free radicals collide with molecules and generate other free radicals does the chain reaction continue, producing an explosion. Thus, chain reaction explosions are likely to occur at intermediate pressures. The figure shows how the explosiveness of a hydrogen-oxygen mixture depends on temperature and pressure.
Explosion occurs in the shaded area.
At 800K and low pressure, no explosion occurs. As the pressure is raised, the mixture enters the chain reaction explosion region. As the pressure is raised further, the mixture is no longer explosive. When the pressure is increased even more, the mixture enters the thermal explosion region.