Water and rubbing alcohol are common household substances, used for cleaning and disinfection. Understanding what happens when these two liquids combine reveals insights into their molecular structures and their interactions.
The Science of Mixing
Water and rubbing alcohol, which is typically isopropyl alcohol, readily mix due to their molecular characteristics. Both water (H₂O) and isopropyl alcohol (C₃H₈O) are polar molecules. Polarity means there is an uneven distribution of electrical charge, creating a slight positive charge on one part of the molecule and a slight negative charge on another. For instance, in water, the oxygen atom pulls electrons more strongly than the hydrogen atoms, giving oxygen a partial negative charge and hydrogen atoms partial positive charges.
Their polar nature allows them to form strong attractions called hydrogen bonds. Hydrogen bonding occurs when a hydrogen atom, bonded to a highly electronegative atom like oxygen, is attracted to another electronegative atom in a different molecule. When combined, hydrogen bonds form between water molecules, alcohol molecules, and crucially, between water and alcohol molecules. This extensive hydrogen bonding network allows the two liquids to mix completely, forming a homogeneous solution. This mixing is a physical change, meaning the chemical identities of water and isopropyl alcohol molecules remain intact; no new chemical compounds are formed.
Characteristics of the Combined Solution
When water and rubbing alcohol mix, the resulting solution exhibits altered properties. One notable phenomenon is volume contraction, where the total volume of the mixture is slightly less than the sum of the initial volumes. This occurs because smaller water molecules fit into spaces within the alcohol’s structure, leading to a more compact arrangement. Extensive hydrogen bonding between water and alcohol molecules also contributes to this tighter packing.
Mixing these liquids can also result in a slight temperature change, typically a small increase. This warming is due to the “enthalpy of mixing,” where the formation of new, stronger intermolecular bonds between water and alcohol molecules releases energy, causing the solution’s temperature to rise.
The combined solution’s properties differ significantly from pure alcohol, especially regarding disinfectant effectiveness, evaporation rate, and flammability. Rubbing alcohol is often sold at 70% concentration because this dilution enhances its disinfectant capabilities. While 100% alcohol denatures proteins too quickly, forming a protective layer, water slows evaporation and allows alcohol to penetrate bacterial cell walls, making it more efficient at killing microorganisms. The mixture’s evaporation rate is also influenced; the more volatile alcohol component evaporates first, gradually increasing the water concentration. Diluting rubbing alcohol with water also reduces flammability, raising its flash point—the lowest temperature at which its vapors can ignite.