A mixture is a combination of two or more substances physically combined without a chemical reaction, allowing each component to retain its unique properties. Mixtures are categorized based on the uniformity of their composition, leading to distinct types like solutions, colloids, and suspensions. The primary distinction is the size of the dispersed component and how that size affects the mixture’s appearance and behavior. This article explores the differences between a true solution and a colloid.
The Fundamental Difference: Particle Size
The most significant physical distinction between a solution and a colloid lies in the size of the dispersed particles. In a true solution, the dispersed component (solute) exists as individual molecules or ions. These particles measure less than 1 nanometer (nm) in diameter, making the solution a homogeneous mixture that appears completely uniform throughout.
In contrast, a colloid contains particles much larger than those in a solution, ranging from approximately 1 nm to 1,000 nm. This intermediate size means that while the colloidal mixture may appear homogeneous to the naked eye, it is technically classified as heterogeneous. The components of a colloid are the dispersed phase and the dispersion medium.
Visual Distinction: The Tyndall Effect
The difference in particle size directly influences how each mixture interacts with light, providing a simple visual test for differentiation. When a beam of light passes through a true solution, the small particles are unable to deflect the light waves. The light passes straight through, and the beam remains invisible when viewed from the side.
Colloids exhibit the Tyndall effect, named after John Tyndall. Because colloidal particles are large enough, they scatter the light directed at them. This scattering makes the path of the light beam visible as it travels through the colloid, often appearing as a bright cone. The Tyndall effect is the definitive visual characteristic used to distinguish a colloid from a true solution.
Stability and Separation
Both true solutions and colloids are considered stable mixtures because their dispersed particles do not settle out over time due to gravity. The stability of a colloid, despite its larger particles, is maintained by the constant, random movement of the particles, known as Brownian motion. This motion results from the dispersed particles colliding with the molecules of the surrounding medium, preventing them from settling.
The difference in particle size dictates the methods needed for separation. Since solution particles are at the molecular or ionic level, they cannot be separated by simple physical methods like filtration or centrifugation. They can only be separated by methods that exploit differences in chemical properties, such as distillation or evaporation.
Colloidal particles are too small to be trapped by standard filter paper, meaning simple filtration is ineffective. However, due to their larger size compared to solutions, colloids can often be separated using high-speed centrifugation, which applies a strong force to accelerate the settling of the dispersed phase. Additionally, specialized processes like dialysis, which utilizes semipermeable membranes, can be used to separate colloidal particles from smaller dissolved ions.
Common Examples and Context
The distinction between these two types of mixtures is apparent in many everyday substances. Common examples of true solutions include saltwater, sugar dissolved in water, and clear air. Colloids are generally translucent or cloudy and include substances like milk, fog, gelatin, and paint. Milk is an emulsion where tiny fat globules are dispersed throughout a water-based medium, and fog is an aerosol where microscopic liquid water droplets are dispersed in air.
Suspensions, such as muddy water, contain particles larger than 1,000 nm. These particles are unstable, visible to the naked eye, and settle out over time, completing the spectrum of mixture types.