The common household product known as rubbing alcohol has a freezing temperature significantly lower than that of water, which is why it remains liquid in a standard freezer. This low-temperature behavior is a direct result of the solution’s chemical composition. The temperature at which this substance turns solid is far below the \(0^\circ\text{C}\) (\(32^\circ\text{F}\)) freezing point of pure water.
Defining Rubbing Alcohol and its Chemical Composition
The term “rubbing alcohol” is a general name for a solution that is typically a mixture of an alcohol and water. The most common active ingredient is isopropyl alcohol, also known as isopropanol, which is the simplest example of a secondary alcohol. Sometimes, the product may be made with denatured ethyl alcohol, or ethanol, which is the same alcohol found in alcoholic beverages but has been rendered toxic and undrinkable for safety.
The concentration of the alcohol is the most important factor determining the solution’s freezing point. Rubbing alcohol is most frequently sold in two main concentrations: \(70\%\) and \(91\%\) alcohol by volume, with the remainder being mostly water. These solutions are primarily used as topical disinfectants. Because water makes up a portion of the mixture, the solution’s freezing point will be higher than that of the pure chemical.
The Specific Freezing Points
The temperature at which rubbing alcohol freezes varies greatly depending on its concentration. Pure isopropyl alcohol, without any water added, has an extremely low freezing point of approximately \(-89.5^\circ\text{C}\) (\(-129.1^\circ\text{F}\)). This temperature is cold enough that the pure chemical will remain a liquid even in the coldest natural environments on Earth.
The most common commercial concentrations have a substantially higher freezing point. A \(91\%\) isopropyl alcohol solution freezes at about \(-75.5^\circ\text{C}\) (\(-103.9^\circ\text{F}\)). The more diluted \(70\%\) isopropyl alcohol solution freezes at a somewhat warmer temperature, around \(-61.7^\circ\text{C}\) (\(-79.1^\circ\text{F}\)).
Why the Freezing Point is Depressed
The phenomenon responsible for the low freezing temperature of the alcohol-water mix is called freezing-point depression, a concept rooted in the study of colligative properties. Colligative properties depend on the number of solute particles dissolved in a solvent, not on the nature of the solute itself. In this case, the alcohol is the solute and water is the solvent.
When water cools, its molecules naturally try to organize themselves into the highly ordered, open crystalline structure of ice. The presence of alcohol molecules dissolved throughout the liquid physically interferes with this process, blocking the water molecules from locking into their lattice formation. This disruption means that a lower temperature is required to remove enough energy from the water molecules for them to overcome the obstruction and solidify.
This same principle is demonstrated when salt is added to roads during winter to prevent ice formation. The dissolved salt particles interfere with the water’s ability to freeze, lowering the temperature required for ice to form. Rubbing alcohol works similarly, as the alcohol molecules act as foreign particles that prevent the water from establishing a stable crystal structure until the temperature drops much lower.
Practical Applications of the Low Freezing Point
The significantly lowered freezing point of rubbing alcohol has several practical uses, especially in cold weather situations. One common application is in homemade de-icing solutions for car windshields and frozen door locks. A mixture of water and rubbing alcohol can quickly melt ice by lowering its freezing point below the current ambient temperature.
The low freezing temperature makes alcohol a suitable ingredient in certain engine coolants and antifreeze formulations. This property ensures that the fluid circulating in a car’s engine block will not solidify, preventing costly damage in extremely cold climates. The addition of isopropyl alcohol is also used in “gas dryer” fuel additives to help prevent water condensation in the fuel tank from freezing and blocking fuel lines during winter.
The same principle is employed when making certain reusable cold packs designed for medical use or injury treatment. By including alcohol in the liquid mixture, the pack remains a thick, slushy consistency instead of freezing solid like a block of ice in a typical home freezer. This allows the cold pack to conform to the body’s shape, providing more comfortable and effective cooling.