When water and rubbing alcohol are introduced, the interaction is complex. Rubbing alcohol, typically a solution of isopropanol (\(text{C}_3text{H}_8text{O}\)), readily blends with water (\(text{H}_2text{O}\)) to form a single, homogenous liquid. This complete intermingling, known as miscibility, is governed by the forces acting between the individual molecules. The resulting solution exhibits characteristics distinct from either pure component, making the mixture widely used in household and industrial settings.
The Chemistry of Miscibility
The ability of water and isopropanol to dissolve into each other is rooted in the principle of “like dissolves like,” meaning substances with similar intermolecular forces will mix freely. Water is a highly polar molecule. Isopropanol, a short-chain alcohol, also possesses a polar hydroxyl (\(text{OH}\)) group attached to its three-carbon chain. This common structural feature facilitates the formation of strong attractions between the molecules.
The primary force driving this blending is hydrogen bonding, which is the attractive force between a hydrogen atom covalently bonded to a highly electronegative atom and another nearby electronegative atom. When the two liquids are mixed, the original hydrogen bonds are broken, but new, highly favorable hydrogen bonds immediately form between the water and isopropanol molecules.
The isopropanol molecule also contains a non-polar hydrocarbon section that disrupts the highly structured hydrogen-bond network of liquid water. While this non-polar section would typically resist mixing, the strong stabilizing interactions from the \(text{OH}\) group outweigh this resistance, allowing the molecules to integrate fully.
Observable Physical Changes
Mixing water and isopropanol produces two main macro-level changes: a slight decrease in volume and a release of heat. When two separate volumes of the liquids are combined, the resulting mixture’s total volume is actually less than the sum of the two original volumes. For example, combining 50 milliliters of water with 50 milliliters of isopropanol yields a total volume slightly less than 100 milliliters.
This phenomenon, known as volume contraction, occurs because the newly formed hydrogen bonds allow the molecules to pack more closely together than they could in their individual pure states. The smaller water molecules are able to fill in the spaces that existed between the larger isopropanol molecules. This closer-fitting arrangement results in a higher density for the final solution.
The process of mixing is also exothermic, meaning the container will feel warm to the touch as the liquids combine. The energy released when the new, stable water-alcohol bonds form is greater than the energy required to break the original separate bonds within the pure liquids. This difference is released into the solution as thermal energy, causing the temperature to rise.
Practical Applications and Effectiveness
The mixture of water and isopropanol forms the basis for common rubbing alcohol. While pure 99% isopropanol is available, a concentration of approximately 70% alcohol is more effective for killing germs. This effectiveness is dependent on the presence of water in the solution.
Water plays a dual role in enhancing the antimicrobial properties of the alcohol by slowing evaporation and aiding in penetration. Pure isopropanol evaporates very quickly, which limits its contact time with microorganisms on a surface. The water content in a 70% solution slows this evaporation, ensuring the alcohol remains on the surface long enough to denature the microbial proteins.
The water also facilitates the process of protein denaturation, which is how the alcohol kills pathogens. High concentrations of alcohol, such as 99%, cause the immediate coagulation of proteins on the outside of a bacterial cell. This instant coagulation creates a protective layer that shields the interior of the cell from further penetration. By contrast, the water in a 70% solution helps to carry the alcohol across the cell wall, allowing it to permeate the entire cell and coagulate all internal proteins, ensuring the microorganism’s death.