Rubbing alcohol is a mixture, not a single chemical substance. This common household product, widely used for its disinfectant properties, is a blend of different ingredients. The substance sold commercially is a classic example of a solution, which is a uniform blend of components.
Defining Mixtures and Pure Substances
In chemistry, matter is categorized as either a pure substance or a mixture. A pure substance consists only of one type of element or compound, maintaining a constant, uniform composition throughout. Examples include elements like gold, or compounds such as pure water, which are chemically bonded in a fixed ratio.
A mixture is a physical combination of two or more pure substances that are not chemically joined. The ratio of ingredients can vary, and individual parts retain their original properties. Mixtures are classified as either homogeneous or heterogeneous.
A heterogeneous mixture, such as sand mixed with water, is not uniform, and its components are visibly distinct. Rubbing alcohol is a homogeneous mixture, also known as a solution, because it has a perfectly uniform composition at the molecular level.
The Components of Rubbing Alcohol
Commercial rubbing alcohol is a clear illustration of a homogeneous mixture. The primary active component is typically isopropyl alcohol, also known as isopropanol, which is dissolved in water.
Rubbing alcohol uses water as the solvent to dissolve the isopropyl alcohol solute. Water is the component present in the largest amount. The combination of these two substances creates the uniform liquid sold commercially.
In addition to the alcohol and water, commercial rubbing alcohol often contains stabilizers and denaturants. Denaturants, such as denatonium benzoate, are bitter-tasting additives included to prevent accidental consumption, as the alcohol is toxic. The presence of these multiple components confirms the product’s classification as a mixture.
Why Concentration Matters
Rubbing alcohol is commonly sold in various concentrations, most notably 70% and 91% by volume. This difference in concentration, which is a key characteristic of a mixture, significantly impacts the product’s effectiveness as a disinfectant. Counterintuitively, the 70% solution is generally more effective for killing germs than the higher 91% concentration.
The water content in the 70% solution is not merely a diluent but a necessary component for optimal microbial destruction. Water slows the evaporation rate of the highly volatile alcohol, allowing the solution to remain on a surface longer. This increased contact time is crucial for the alcohol to fully penetrate and destroy microbial cells.
The water also acts as a catalyst that facilitates the denaturation of proteins within the cell membrane and cytoplasm of bacteria and viruses. Highly concentrated alcohol, such as 91%, can cause proteins on the outer surface of a microbe to coagulate instantly. This rapid coagulation forms a protective layer, sealing the cell and preventing the alcohol from penetrating the interior, thus rendering the disinfection process incomplete.