Can The Tip Of A Lit Cigarette Butt Ignite Gasoline?
This hub goes into detail about whether a lit cigarette butt can ignite gasoline. This is an extension of my hub about Common Mistakes In The Movies, which explores the many inconsistencies with real life that we see in the movies. The actual answer is a little long as I try to be rigorous, so I have supplied a short and long version of the answer.
Yes and no. The autoignition temperature of gasoline is lower than the temperature at the tip of a lit cigarette butt, so it is possible for the tip of a cigarette butt to ignite gasoline. But the tip of a cigarette does not contain a lot of heat, so it is also possible for the heat to be conducted away and the temperature at the tip to drop below the autoignition temperature before the gasoline ignites.
In other words, if you were to throw a cigarette butt into a can of gasoline, it might not ignite. But if you were to spill some gasoline onto the sidewalk and try to ignite the spill, there is a much greater chance that the gasoline will ignite.
Gasoline is a fuel used in the engines of cars. It is a colorless, pale brown or pink liquid, and is very flammable. It is produced from petroleum and is a mixture that typically contains more than 150 chemicals. The exact composition will depend on the source of the crude petroleum, the manufacturer, and the time of year. 1
As gasoline is a mixture, and its exact composition varies, it is not possible to determine an exact autoignition temperature for gasoline. I checked 2 sources on the web, US Motors2 and Wikipedia3, and the autoignition temperature of gasoline is given as 280 deg C (536 deg F) and 246-280 deg C (475 - 536 deg F) respectively.
The good folks over at PhysLink have kindly performed a series of experiments to determine the temperature of a lit cigarette. Depending on whether the point of measurement is at the middle or side of the lit portion, and whether the cigarette is being drawn on, the temperature varies from a low of 400 deg C (752 deg F) to a high of 700 deg C (1292 deg F)4.
So, as the temperature of a lit cigarette is higher that the autoignition temperature of gasoline, it is possible to ignite gasoline from a lit cigarette. However, the amount of heat contained in the lit portion is very small. And that is when things get a little more complicated.
When the lit portion of the cigarette comes into contact with gasoline, the temperature at the point of contact will rapidly decrease as heat is conducted into the gasoline. As the reservoir of heat is tiny, the temperature may drop to a point that is below the autoignition temperature of the gasoline.
Another way that a lit cigarette can ignite gasoline is when it comes into contact with gasoline vapors. As the flashpoint of gasoline is very low, at -40 deg C (-40 deg F),5 it is possible for a lit cigarette to ignite gasoline even when not in direct contact.
Factors that will determine whether ignition will take place include:
- The initial temperature of the gasoline. Obviously, the higher the initial temperature, the greater the chance of ignition.
- The volume of the gasoline in the immediate vicinity of the contact point will determine how much the temperature of the lit cigarette butt decreases on contact.
- The surroundings in which the gasoline is kept and how long the gasoline is allowed to sit. For example, if the gasoline is kept in a closed container for a while, gasoline vapors would be present in the container and might ignite when a lit cigarette is introduced.
Autoignition Temperature - A material's autoignition or ignition temperature is the temperature at which a material self-ignites without any obvious sources of ignition, such as a spark or flame.5
Flashpoint - The lowest temperature at which liquid can form a vapor which can be ignited.