A mixture is a material made up of two or more different substances that are physically combined but not chemically bonded. Mixtures are categorized based on the uniformity of their composition: homogeneous or heterogeneous.
A homogeneous system, or solution, has components uniformly distributed at a molecular level. The minor component, called the solute, is completely dissolved in the major component, known as the solvent, which is often water or alcohol in liquid medications. When solute particles are fully dispersed, the resulting liquid appears clear and its composition is identical throughout the entire volume.
Understanding Mixtures and Solutions
In a solution, components are perfectly uniform; a sample taken from the top has the exact same concentration as one taken from the bottom. Solute particles are dispersed at the molecular level, meaning they are too small to be seen, cannot be separated by filtration, and will never settle out. This dispersion ensures a precise and consistent amount of the medicinal component.
A heterogeneous mixture, by contrast, is characterized by a non-uniform composition, meaning its components are not evenly distributed. Distinct phases or components can often be visually identified. If left undisturbed, the components of a heterogeneous mixture will typically separate over time due to differences in density.
Key Differences Between Homogeneous and Heterogeneous
The classification of a mixture depends on the size of the dispersed particles. A homogeneous solution features particle sizes less than one nanometer in diameter, allowing them to remain suspended indefinitely. These tiny particles cannot scatter light, meaning a beam of light passed through the solution will not be visible (absence of the Tyndall effect).
Heterogeneous mixtures contain larger particles that prevent a uniform distribution. They are divided into two sub-types: colloids and suspensions.
Colloids
Colloids contain particles ranging from 1 to 1,000 nanometers. These particles are large enough to scatter light, making the mixture appear cloudy or opaque, but they are still small enough that they will not settle out quickly.
Suspensions
Suspensions involve particles larger than 1,000 nanometers. These larger particles can be seen and will settle out of the liquid phase over time due to gravity. This settling creates visible layers and results in an inconsistent concentration, requiring physical agitation to temporarily redistribute the components.
Applying the Classification to Cough Syrup
Most commercial cough syrups are classified as heterogeneous mixtures, specifically suspensions. This classification is primarily due to the composition of the active pharmaceutical ingredients (APIs), such as the cough suppressant dextromethorphan or the expectorant guaifenesin. These active compounds are often solid crystalline powders that are insoluble or only poorly soluble in the syrup’s liquid base, which is usually a mixture of water, sugar, flavorings, and thickeners.
Because the APIs do not fully dissolve, they exist as fine solid particles dispersed throughout the liquid. Over time, gravity causes these dispersed solid particles to slowly fall to the bottom of the bottle, a process known as sedimentation. This settling creates a layer of concentrated medicine at the bottom and a less potent liquid at the top.
The manufacturer’s instruction to “shake well before using” is the clearest practical evidence of the cough syrup’s heterogeneous nature. Shaking is required to temporarily re-suspend the settled solid particles, ensuring the active ingredients are evenly distributed throughout the liquid immediately before dosing. This action is necessary to guarantee that the patient receives a consistent amount of medication, confirming that the mixture is not stable and uniform on its own.