Rubbing alcohol, primarily isopropyl alcohol mixed with water, does not freeze under typical household or natural winter conditions. Common solutions remain liquid at temperatures far below the freezing point of pure water, making it a reliable liquid even when stored inside a standard home freezer. The temperature required to solidify rubbing alcohol is significantly colder than most people ever experience.
Freezing Points by Concentration
The exact temperature at which rubbing alcohol will freeze depends on its concentration. The two most widely available concentrations found on store shelves are 70% and 91% isopropyl alcohol, which refer to the volume of alcohol present in the solution.
The 70% solution, which contains more water, freezes at approximately -61.7°C (-79.1°F). The 91% solution, being purer, has a significantly lower freezing point of about -75.5°C (-103.9°F). For comparison, the average home freezer maintains a temperature between -18°C and -23°C (0°F and -10°F), meaning even the coldest places on Earth rarely sustain temperatures low enough to freeze these common solutions.
The Science of Freezing Point Depression
The reason rubbing alcohol resists freezing is explained by a phenomenon in chemistry called freezing point depression. This occurs when a solute, the isopropyl alcohol molecules, is dissolved into the water solvent. The alcohol interferes with the water’s natural process of solidification.
Pure water molecules arrange themselves into a rigid, highly ordered crystal structure when they freeze, forming ice. The alcohol molecules disrupt this process by acting as physical obstacles. They prevent the water molecules from aligning properly to form the stable ice lattice.
To force the water molecules to solidify despite the alcohol’s presence, the mixture requires a much greater reduction in thermal energy. This need for a substantially lower temperature overcomes the molecular interference and depresses the freezing point. The more alcohol molecules present, the greater the interference with crystal formation, resulting in a lower freezing point.
Practical Applications
This resistance to freezing makes rubbing alcohol useful in several practical applications. A common homemade solution uses rubbing alcohol to create a reusable cold pack that remains flexible. By mixing alcohol with water or dish soap in a sealed bag, the freezing point is lowered, resulting in a slush rather than a solid block of ice when placed in a freezer.
The low freezing point also makes it a functional, low-cost antifreeze agent. It is often used in do-it-yourself de-icing sprays for car windshields. A mixture containing rubbing alcohol rapidly melts existing frost and prevents water from refreezing immediately. This principle is utilized in commercial windshield washer fluids to prevent them from solidifying inside the reservoir during cold weather.