Butane is a colorless, highly flammable hydrocarbon gas primarily used as a fuel source, a propellant in aerosols, and a refrigerant. It is one of the four main components of Liquefied Petroleum Gas (LPG) and is stored in canisters as a liquid under pressure. This article clarifies the scientific temperature required for butane to change into a solid state and distinguishes it from the temperature at which it stops working effectively as a fuel.
The Temperature of Solidification
Butane does not freeze under normal cold-weather conditions encountered outside of specialized laboratories or industrial processes. The temperature required for butane to change from a liquid to a solid, known as its freezing point, is extremely low. Butane exists in two primary structural forms, called isomers: normal butane (n-butane) and isobutane (i-butane or 2-methylpropane).
These two isomers have distinct freezing points due to their different molecular structures. Normal butane, with its straight-chain structure, solidifies at approximately \(-138.3^\circ \text{C}\) (\(-216.9^\circ \text{F}\)). Isobutane, which has a branched-chain structure, solidifies at about \(-159.4^\circ \text{C}\) (\(-255.0^\circ \text{F}\)).
These temperatures are far below any natural ambient temperature on Earth. This confirms that canister failure is not due to the fuel freezing into a solid block.
The Transition to Vapor
The common confusion about butane “freezing” relates to its inability to transition from a liquid to a gas, which is the process of vaporization. Butane is stored as a liquid in a sealed canister, and the vaporized gas is burned for fuel. This phase transition is governed by the boiling point, which is significantly higher than the freezing point.
Normal butane boils at approximately \(-0.5^\circ \text{C}\) (\(31.1^\circ \text{F}\)), while isobutane boils at about \(-11.7^\circ \text{C}\) (\(10.9^\circ \text{F}\)). When the liquid temperature inside the canister drops below its specific boiling point, the liquid cannot vaporize sufficiently. This prevents the internal pressure needed for a steady flame, causing the device to fail.
Evaporative Cooling
As the liquid butane vaporizes, it draws energy from the surrounding liquid and the canister itself, a process called evaporative cooling. This energy loss causes the temperature of the remaining liquid to drop rapidly. This chilling effect further reduces the vaporization rate and causes the flame to sputter or extinguish.
Butane in Everyday Applications
The relatively high boiling point of butane is its main drawback for cold-weather applications, such as camping stoves and outdoor lighters. Pure butane fuel is virtually unusable outdoors once the ambient temperature approaches the freezing point of water. Fuel manufacturers often use blends of liquefied petroleum gases to improve performance in colder climates.
A common solution involves mixing butane with propane, creating a “winter mix” or blended fuel. Propane has a much lower boiling point of approximately \(-42^\circ \text{C}\) (\(-44^\circ \text{F}\)). This allows it to continue vaporizing and maintain adequate pressure in far colder conditions.
Propane acts as the driving force in these blends, generating the necessary vapor pressure to push the remaining butane out. The presence of isobutane is also advantageous because its lower boiling point gives it an operational edge over normal butane. For users relying on butane appliances in cold weather, a practical strategy is to warm the canister before use, perhaps by storing it in a sleeping bag or coat pocket.