Liquor evaporates because it is a natural consequence of the liquid state. Evaporation is the process where molecules at the surface of a liquid gain enough energy to transition into a gaseous state, or vapor, even below the liquid’s boiling point. This phenomenon occurs continuously in any bottle of spirits, turning a small amount of liquid into an invisible gas. The rate of evaporation is dictated by the chemical makeup of the liquor and the conditions of its environment.
The Volatility of Ethanol
The primary reason liquor evaporates is due to the inherent chemical properties of ethanol, the alcohol component. Liquor is a mixture of water and ethanol, and ethanol is far more volatile than water, meaning it turns into a vapor more easily.
This difference in volatility is rooted in the boiling points of the two liquids. Ethanol boils at approximately \(78^\circ\text{C}\) (\(173^\circ\text{F}\)), which is significantly lower than water’s \(100^\circ\text{C}\) (\(212^\circ\text{F}\)). A lower boiling point means the molecules require less energy to break free and escape into the air.
The molecular structure explains this behavior because ethanol has weaker intermolecular forces holding its molecules together than water does. Water molecules form more hydrogen bonds, creating a stronger, more cohesive liquid structure. Ethanol molecules form fewer and weaker hydrogen bonds, making it easier for them to overcome these attractive forces and vaporize at room temperature. Because ethanol escapes more readily than water, the overall percentage of alcohol in an opened bottle will subtly decrease over many years.
Key Variables Affecting the Rate of Loss
The rate at which liquor is lost to the atmosphere is modulated by several external factors. Temperature is a significant variable, as providing more heat directly translates to increased molecular energy. A warmer environment gives more liquid molecules the necessary kinetic energy to overcome surface tension and escape as vapor, accelerating the evaporation process.
The integrity of the seal on the bottle is a major determinant of loss. A tight seal, such as a well-fitted screw cap or a properly maintained cork, helps the vapor inside the bottle reach a state of equilibrium. In this saturated state, the rate of molecules escaping the liquid is balanced by the rate of vapor molecules returning to the liquid, dramatically slowing further net loss. A poor or damaged seal constantly allows the alcohol vapor to escape, preventing equilibrium from being established and allowing evaporation to continue unchecked.
The concentration, or proof, of the spirit also plays a role in the rate of loss. Spirits with a higher alcohol content contain a greater proportion of the more volatile ethanol component. A higher concentration of the volatile liquid means a higher potential for evaporation, especially if the bottle is stored improperly.
Proper Storage to Minimize Evaporation
To minimize the loss of volume and flavor from evaporation, the storage environment should be controlled. Bottles should always be stored upright, especially those sealed with natural corks. This positioning prevents the high-proof alcohol from being in constant contact with the cork, which can cause it to degrade, while still allowing the vapor to keep the underside of the cork moist.
Maintaining a cool and consistent temperature is important for preserving the liquid. The ideal range is often cited as \(13^\circ\text{C}\) to \(16^\circ\text{C}\) (\(55^\circ\text{F}\) to \(60^\circ\text{F}\)), as this temperature reduces the vapor pressure of the ethanol. Bottles should also be kept away from direct sunlight, which introduces heat and accelerates the evaporation rate.
Checking that all caps and corks are secured tightly after each use is the simplest step to control evaporation. For bottles that are nearly empty, some collectors transfer the remaining spirit into a smaller glass container with a good seal. This reduces the headspace inside the bottle and limits the volume of vapor that can build up, preserving the spirit’s original character.