Lighter fluid does evaporate. The term refers to a class of highly volatile, petroleum-derived substances. This fuel is a mixture of hydrocarbon molecules, and its tendency to vaporize is the fundamental property that allows it to ignite and burn. The specific rate and mechanism of this process depend on whether the fluid is a liquid fuel, like the kind used for grills, or a pressurized gas, such as the fuel in disposable lighters.
The Science of Volatility
Evaporation is a surface phenomenon where molecules in a liquid gain enough kinetic energy to break free from the attractive forces holding them together and escape into the surrounding air. The tendency of a liquid to evaporate is quantified by its vapor pressure, which is the pressure exerted by the gas molecules above the liquid in a closed container. Substances with a high vapor pressure are highly volatile because their molecules escape easily, leading to rapid evaporation. The boiling point is the temperature at which the vapor pressure equals the surrounding atmospheric pressure. Lighter fluids are specifically engineered to contain light hydrocarbons that possess high vapor pressures at normal temperatures, ensuring they form a combustible vapor cloud upon exposure.
How Liquid Grill Fuels Evaporate
Liquid lighter fluid, such as the naphtha-based fuel used in wick lighters or the kerosene-based fluid used for charcoal grills, is a blend of petroleum distillates. These fuels are composed of relatively large hydrocarbon molecules, generally ranging from six to fourteen carbon atoms per chain (C6–C14). This molecular structure gives them a lower volatility. Charcoal starter fluid is designed with a higher molecular weight, making its evaporation slower than wick-lighter fluid. This slower rate is intentional, allowing the fluid to soak into charcoal briquettes before the flammable vapors fully dissipate. The rate of evaporation for these liquids increases dramatically with temperature and surface area. Spilling a thin layer on a hot surface will cause it to disappear much faster than leaving a cap off the bottle.
The Behavior of Pressurized Lighter Gases
The fuel in most disposable lighters is a gas like butane or isobutane that is stored as a liquid under pressure. Butane’s boiling point is approximately -0.5 degrees Celsius, which is far below standard room temperature. The lighter’s container maintains enough pressure to keep the butane in its denser liquid state. When the lighter valve is opened, the pressure on the liquid fuel instantly drops to atmospheric pressure. Because the liquid’s temperature is much higher than its boiling point, it undergoes an immediate and rapid phase change, known as flash vaporization, turning into a gas. This is a fundamentally different and far faster process than the slow, surface-level evaporation seen with liquid fuels.
Storage and Safety Implications
Because all forms of lighter fuel are volatile, they require careful storage and use. The high vapor pressure means that even sealed containers can build up pressure if exposed to heat. Storing these products away from direct sunlight and any source of ignition prevents dangerous pressure increases and the risk of fire. The vapors released by evaporating lighter fluid are heavier than air and can travel along the floor to an ignition source some distance away, causing a flash-back fire. Using the fluids only in well-ventilated areas minimizes the concentration of these flammable vapors in the air. Containers must be kept tightly sealed when not in use to prevent the fuel from evaporating into the environment.