When a liquid is heated, its molecules gain energy, increasing their movement. This increased activity can lead to a phase transition, where the liquid transforms into a gas or vapor. Alcohol, specifically ethanol, is an organic compound characterized by a hydroxyl (-OH) group attached to a carbon atom.
The Science of Alcohol Evaporation
Boiling is a process where a liquid rapidly changes into a gas or vapor, forming bubbles throughout its volume when its vapor pressure equals the surrounding atmospheric pressure. Ethanol, the alcohol found in beverages, boils at approximately 78.3°C (173.1°F) at standard atmospheric pressure, significantly lower than water’s boiling point of 100°C (212°F).
Ethanol has a lower boiling point than water because the intermolecular forces holding its molecules together are not as strong. Water molecules form more extensive hydrogen bonds, which require more energy to break during the phase change. Consequently, ethanol molecules can escape into the gas phase at a lower temperature, and this difference also means ethanol has a higher vapor pressure than water at the same temperature.
When alcohol boils, heat energy breaks these intermolecular bonds, allowing ethanol molecules to transition from liquid to gas. This is a physical change of state, not a chemical alteration, meaning the individual ethanol molecules remain intact.
Practical Considerations and Safety
Boiling alcohol, such as in cooking, decreases its alcohol content; the liquid does not become stronger. Despite the common belief that all alcohol “burns off” during cooking, this is rarely the case in typical home scenarios.
Studies show a significant amount of alcohol can remain in food even after extended cooking times. For example, dishes cooked for 15 minutes may retain about 40% of their initial alcohol content, and even after two and a half hours, approximately 5% can still be present. Factors like cooking time, temperature, vessel surface area, and other ingredients influence how much alcohol evaporates.
A primary safety concern when boiling alcohol is the flammability of its vapor. Ethanol vapor is highly flammable and can ignite readily from an ignition source like an open flame or spark. Ethanol has a flash point as low as 14°C (57°F), the temperature at which it produces enough vapor to ignite. Alcohol vapors are also heavier than air and can travel along surfaces, potentially reaching a distant ignition source and flashing back. Adequate ventilation is crucial to prevent the buildup of these flammable vapors in confined spaces.