Mixtures are fundamental to chemistry and are classified based on the size and distribution of their components. Two common liquid mixtures are the solution and the colloid, which often appear similar to the casual observer. A solution, like saltwater, is a truly homogeneous mixture, meaning its composition is uniform throughout. A colloid, such as milk or fog, may look uniform but possesses different underlying properties. The distinction hinges on their internal structure and how the dispersed substance is integrated into the surrounding medium.
Defining Differences: Particle Size and Structure
The primary contrast between a solution and a colloid lies in the size of the dispersed particles. In a true solution, the solute is dissolved at the molecular or ionic level, meaning particles are typically less than 1 nanometer (nm) in diameter. When sugar is dissolved in water, the individual molecules are seamlessly integrated into the solvent, creating a single-phase system. This minuscule size ensures the mixture remains perfectly clear and transparent.
Colloids, in contrast, have particle sizes ranging from 1 nm up to 1000 nm (1 micrometer). These larger particles are not dissolved but are microscopically dispersed throughout a continuous medium, making the mixture technically heterogeneous. Milk is a classic example, where microscopic fat globules are distributed throughout an aqueous medium. This intermediate size range means colloids represent a state between a true solution and a coarse suspension, where particles are large enough to eventually settle out.
Structurally, a solution is a single phase because the solute and solvent are indistinguishable. A colloid, while appearing uniform, consists of two separate phases: the dispersed substance and the continuous dispersing medium. The dispersed colloidal particles often consist of thousands of molecules clumped together. This difference explains why simple table salt disappears entirely in water, forming a solution, while the larger components in milk remain suspended, forming a colloid.
Visible Distinctions: The Tyndall Effect
The Tyndall effect provides a clear visual distinction between solutions and colloids. This test involves shining a beam of light through the mixture and observing whether the light path becomes visible. Solutions do not exhibit the Tyndall effect because their particles are too small to effectively scatter light. A laser beam passed through salt water, for example, remains invisible from the side.
Colloidal particles are large enough to intercept and scatter light in all directions, making the path of the light beam distinctly visible. This is readily observable when car headlights illuminate fog, an aerosol colloid. Colloidal particles are also subject to Brownian motion, a constant, erratic movement caused by the bombardment of molecules in the surrounding medium. This movement helps prevent the colloidal particles from settling out under gravity.
Stability and Separation Techniques
Solutions exhibit complete long-term stability, meaning the dissolved solute particles will never settle out. Separating the components typically requires a phase change, such as boiling the solvent away in a process like distillation or evaporation. Simple mechanical separation methods, like pouring the mixture through standard filter paper, are ineffective because the molecular-sized solute particles pass through the filter pores easily.
Colloids are also stable and do not separate into layers quickly like a coarse suspension. Their stability is maintained partly by the electrical charges on the particle surfaces, which cause them to repel each other and resist aggregation. Separating the dispersed phase usually requires specialized techniques that overcome this stability. These include ultracentrifugation, which applies immense centrifugal force to accelerate settling, and coagulation, which involves adding an electrolyte to neutralize particle charges, causing them to clump and precipitate.