Why can't you use your phone in petrol stations?

It's common knowledge that you shouldn't use your phone in a petrol station as it can cause a fire to break out. Why does this happen?

While it is very rare, there is chance that any electronic devices, not just mobile phones can produce sparks. Even a minor spark can be lethal, igniting flammable fuel vapours, causing a fire or risk of explosion.

Photo by Joshua Newton on Unsplash

But how do these sparks even occur? Sometimes it can be due to faulty wiring, battery issues or damage to your device. More commonly, your phone can generate static electricity when it is being used, especially in cold or dry conditions. This static electricity doesn't always come from using mobile phones. Static electricity can build up on a person from contact with certain materials or even touching the car door. Don't worry, the chances of this happening are very slim. One way of discharging this static electricity is by touching a metallic part of your vehicle called grounding.

However, the idea that mobile phones can ignite petrol fumes is a myth. To date, there has not been any petrol station fire or explosion confirmed to have been caused by phone use. It is much more likely that it may've been a source of static electricity which led to ignition. Despite this, many safety agencies advise against the use of mobile phones in petrol stations due to the theoretical risk that it could ignite petrol fumes.

A more common cause of petrol station fires is smoking or lighting a match near one. Very hot car engines and exhaust systems can ignite petrol vapours, especially if there is a fuel spill or leak nearby. Other times, like a lot of other accidents, a fire can be ignited by human error. Overfilling your tank or spilling fuel can increase the risk of fire.

In the case that a spark does occur, it creates a very high temperature often over 1000°C which provides enough activation energy for the volatile petrol fumes to combust. As the heat breaks the covalent bonds in the fuel hydrocarbons, it produces highly reactive free radicals. Free radicals are atoms or molecules with one or more unpaired electrons in its valence shell.

There are three parts to the reaction of free radicals. First, there is initiation where bonds in the hydrocarbon and oxygen molecules are broken to produce these radicals. Next is propagation where radicals go on to react with non-radicals, creating new radicals in the process. A self-sustaining chain reaction is triggered. These reactions are exothermic, releasing a large amount of heat energy that vapourises more petrol for the reaction. Finally, there is termination, where two radicals react to form a non-radical molecule. The chain reaction stops as concentration of reactants decrease or the temperature drops below the activation threshold.

This mechanism can also be applied to other combustion reactions such as that of coal, wood, or in a lighter. Next time you light a fire, you can appreciate the free radicals keeping that flame alive!