Gasoline absolutely evaporates in a gas can. Gasoline is a complex blend of hundreds of different hydrocarbons, each with a distinct boiling point and volatility. Evaporation is the natural physical process where the liquid fuel transitions into a gaseous state (vapor), even below its boiling point. This inherent tendency drives the evaporation process inside any container.
The Science of Gasoline Volatility
The key scientific concept explaining this evaporation is volatility—the measure of how easily a liquid turns into a vapor. Gasoline is formulated to be highly volatile, which is necessary for it to mix with air and combust efficiently in an engine. A standardized measure called Reid Vapor Pressure (RVP) gauges this volatility by measuring the pressure exerted by the vapor in equilibrium with the liquid at a specific temperature.
Because gasoline is a blend, it contains lighter hydrocarbons, like butane and pentane, which have very low boiling points and a high vapor pressure. These lighter components are the first to escape into the air space inside the gas can, even at typical ambient temperatures. As these molecules gain enough kinetic energy, they break free from the liquid surface and become vapor, causing the total amount of liquid fuel in the can to decrease over time.
External Factors Affecting Evaporation Rate
The speed at which gasoline evaporates is significantly influenced by the surrounding environment. Higher ambient temperatures dramatically increase the kinetic energy of the fuel molecules, causing the vapor pressure to rise exponentially. Storing a gas can in a hot garage or direct sunlight will accelerate the evaporation of the lighter, more volatile fuel components. This is why refineries adjust gasoline formulas seasonally, creating less volatile blends for summer months to limit evaporative emissions.
The design and integrity of the gas can play a major role in vapor loss. Older, traditional gas cans often featured a simple vent that allowed vapors to escape freely. Modern, compliant gas cans (meeting CARB or EPA standards) use specialized seals and low-permeation materials to minimize vapor escape. However, modern cans can still lose vapor through a damaged seal or if the material exhibits porosity.
Impact on Fuel Quality and Storage Safety
Fuel Quality Degradation
Evaporation of the lightest hydrocarbons fundamentally alters the chemical composition of the remaining fuel, leading to quality degradation. The loss of these easily vaporized components leaves a “heavier” fuel mixture behind. This remaining fuel is less volatile and more difficult to ignite, often causing hard starting or poor performance in small engines. This process also lowers the fuel’s octane rating, which increases the risk of engine knock or detonation.
As volatile components escape, non-volatile residues and additives become more concentrated in the remaining liquid. This results in the formation of gummy deposits and a sticky, lacquer-like substance known as varnish. These residues can clog fine passages in carburetors, fuel injectors, and fuel lines, which is the primary cause of mechanical failure in equipment stored with old gasoline. Using a fuel stabilizer can help mitigate this chemical breakdown and slow the formation of these deposits.
Safety and Pressure Buildup
Evaporation creates a safety hazard by causing pressure to build up inside modern, tightly sealed gas cans. As the fuel vaporizes, the concentrated gas molecules increase the internal pressure, which can make the can swell and become difficult to open. Safety-rated cans are engineered with a pressure relief system, often a spring-loaded vent, designed to release this excess pressure when it reaches a certain threshold (typically between three and five pounds per square inch). Relieving this pressure slowly, sometimes called “burping” the can, is necessary before use to prevent a sudden, dangerous burst of pressurized, highly flammable vapor near the opening.