Alcohol is not a single chemical compound but a family of organic molecules characterized by a hydroxyl (OH) functional group. This family includes compounds like ethanol, methanol, and isopropanol, and each one has a distinct freezing temperature. Their freezing points are drastically lower than that of water, which freezes at \(0^\circ \text{C}\) (\(32^\circ \text{F}\)).
Freezing Points of Pure Alcohols
The three most common industrial and consumer alcohols each have a unique temperature at which they solidify. Ethanol, the alcohol found in beverages, has the lowest freezing point, settling around \(-114^\circ \text{C}\) (or \(-173^\circ \text{F}\)). Methanol, sometimes called wood alcohol, solidifies at approximately \(-97.6^\circ \text{C}\) (or \(-143.7^\circ \text{F}\)). Isopropyl alcohol, widely known as rubbing alcohol, has the highest freezing point of the trio, typically around \(-89.5^\circ \text{C}\) (or \(-129.1^\circ \text{F}\)). The molecular structure of each compound dictates its specific phase change point.
How Concentration Affects Freezing Temperature
When alcohol is mixed with water, the freezing temperature of the resulting solution increases significantly from that of pure alcohol. This phenomenon is known as freezing point depression, a colligative property where the ethanol solute interferes with the ability of water molecules to form their typical solid crystal lattice.
The concentration of alcohol is typically measured by Alcohol By Volume (ABV) or proof. Higher proof spirits, containing \(40\%\) ethanol, have a freezing point of approximately \(-27^\circ \text{C}\) (or \(-17^\circ \text{F}\)).
Conversely, beverages with a lower alcohol content, like wine (around \(12\%\) ABV) or beer (around \(5\%\) ABV), contain a much higher proportion of water. Because water freezes at a relatively high temperature, these drinks will easily freeze and turn slushy in a standard freezer. Beer often solidifies near \(-2^\circ \text{C}\) (\(28^\circ \text{F}\)). The higher the water content, the closer the solution’s freezing temperature will be to that of pure water.
The Science Behind Low Freezing Points
The extremely low freezing points of pure alcohols are rooted in the physics of how their molecules interact. For a liquid to freeze, its molecules must slow down enough to be locked into a stable, rigid, and repeating three-dimensional structure called a crystalline lattice. The temperature required to achieve this lock depends on the strength of the attractive forces between the molecules.
Water molecules are very small and can form strong, efficient hydrogen bonds with four other water molecules, allowing them to rapidly snap into a highly ordered lattice at \(0^\circ \text{C}\). Alcohol molecules, such as ethanol, also possess the ability to form hydrogen bonds due to the hydroxyl (OH) group, but they are far less efficient at forming a solid.
The reason lies in the non-polar hydrocarbon chain—the “tail” of the alcohol molecule—which is bulky and lacks the ability to form strong bonds with neighboring molecules. This large, non-polar section physically hinders the alcohol molecules from aligning correctly and compactly to form the stable crystalline structure. Because the intermolecular forces are weaker, a significantly greater reduction in molecular kinetic energy is required to force them into a solid arrangement. This translates directly into the ultra-low freezing temperatures observed for pure alcohols.