The temperature at which alcohol disappears involves two distinct physical processes: boiling and evaporation. The alcohol most frequently discussed, whether in beverages or disinfectants, is ethanol. Like all liquids, ethanol is a simple organic compound that transitions from a liquid to a gaseous state. Understanding this transition requires differentiating between boiling, the rapid, bulk phase change, and evaporation, the slower, surface-level change.
The Specific Boiling Point of Ethanol
Boiling is a rapid phase change that occurs only at a specific temperature for a given pressure. For pure ethanol, this temperature is \(78.37^\circ\text{C}\) (\(173.1^\circ\text{F}\)) under standard atmospheric pressure. This is significantly lower than the boiling point of water, which is \(100^\circ\text{C}\) (\(212^\circ\text{F}\)).
The boiling point is reached when the liquid’s vapor pressure equals the surrounding atmospheric pressure. At this point, the liquid turns to gas, forming bubbles throughout its entire volume. This difference in boiling points makes ethanol more volatile than water. In mixtures like distilled spirits, the alcohol vaporizes rapidly well before the water reaches its boiling point.
Evaporation Versus Boiling
While boiling occurs at a fixed temperature and is a bulk phenomenon, evaporation is a surface phenomenon that can occur at any temperature. Evaporation happens because liquid molecules are in constant motion, possessing a range of kinetic energies. Only molecules at the surface with sufficient kinetic energy can overcome intermolecular forces and escape into the air as a gas.
Since ethanol has weaker intermolecular forces than water, its molecules require less energy to escape the surface, allowing it to evaporate more readily at room temperature. The rate of this slow phase change is influenced by several external factors. Increasing the temperature of the liquid or air provides more molecules with the kinetic energy required to transition into vapor.
A greater exposed surface area also speeds up the process by providing more escape routes for the molecules. Airflow and lower humidity increase the rate of evaporation by sweeping away vapor molecules from the liquid’s surface. This explains why a spill of rubbing alcohol disappears much faster than a spill of water at the same temperature.
Alcohol in Cooking
Boiling and evaporation are relevant when using alcohol in cooking. The common belief that all alcohol instantly disappears when heated is a simplification. While ethanol vaporizes quickly due to its lower boiling point, it does not all flash away immediately.
Studies show that the final amount of alcohol remaining depends on the cooking time, temperature, and preparation method. For example, a dish baked or simmered for 15 minutes may retain about 40% of the initial alcohol content. Even after prolonged simmering for two and a half hours, approximately 5% of the alcohol can still remain.
Methods like flambéing, where alcohol is ignited, are often less effective than prolonged simmering, with some flamed dishes retaining about 75% of the alcohol. The continuous, slower evaporation process throughout the cooking duration gradually reduces the alcohol content. Because alcohol molecules can bond with other ingredients, a small residual amount will nearly always remain, even after extensive heating.