Mixtures are combinations of two or more substances that are physically combined but not chemically bonded. Among these, colloids represent a distinct category, positioned between the uniformity of true solutions and the visible separation of suspensions.
What Defines a Colloid
A colloid is a mixture where one substance, the dispersed phase, is evenly distributed throughout another, the dispersion medium. The particles within the dispersed phase are microscopically dispersed. These particles have a specific size range, generally between 1 and 1000 nanometers (or 1 micrometer) in at least one dimension. This particle size is too small to be seen with the naked eye, yet larger than individual molecules or ions found in true solutions.
Despite appearing uniform, colloids are heterogeneous mixtures. A defining characteristic of colloids is the Tyndall effect, where the dispersed particles scatter light, making the path of a light beam visible as it passes through the mixture. This scattering occurs because the particles are large enough to interact with light waves.
Brownian motion is another property of colloids, referring to the random, zigzag movement of the dispersed particles. This continuous motion results from the particles being unevenly bombarded by the molecules of the dispersion medium. Brownian motion prevents the colloidal particles from settling out over time, contributing to their stability.
How Colloids Differ from Other Mixtures
Colloids occupy a unique position among mixtures, distinguishing themselves from true solutions and suspensions primarily by particle size, appearance, and behavior. True solutions, like sugar dissolved in water, consist of particles smaller than 1 nanometer. These particles are fully dissolved and uniformly distributed, making solutions appear transparent and homogeneous. Solutions are also highly stable, with their solute particles never settling out.
In contrast, suspensions, such as sand in water, contain much larger particles, typically exceeding 1000 nanometers (1 micrometer). These particles are visible to the naked eye and cause the mixture to appear cloudy or opaque. Suspensions are unstable, and their particles will eventually settle out due to gravity if left undisturbed.
Colloids bridge this gap with particles ranging from 1 to 1000 nanometers. While appearing uniform or translucent, colloids are heterogeneous at a microscopic level. Unlike true solutions, colloids exhibit the Tyndall effect. True solutions pass easily through ordinary filter paper, and colloids also pass through standard filters but can be retained by specialized ultrafilters or semipermeable membranes. Suspensions, with their larger particles, can be separated by simple filtration.
Colloids in Everyday Life
Colloids are ubiquitous, appearing in numerous products and natural phenomena. Emulsions are a type of colloid where a liquid is dispersed in another immiscible liquid. Familiar examples include milk (fat dispersed in water) and mayonnaise (oil in vinegar stabilized by egg yolk). Many cosmetic lotions and creams are also emulsions.
Sols are colloids where solid particles are dispersed in a liquid. Paint, for instance, contains pigment particles suspended in a liquid medium. Ink is another common sol, with colored particles dispersed in a liquid. Blood also functions as a sol, with various cells and proteins dispersed throughout plasma.
Gels involve liquid particles dispersed within a solid medium. Jelly is a prime example, where liquid molecules are dispersed within a solid network, giving it a semi-solid texture. Cheese and butter are also classified as gels.
Aerosols are colloids with either liquid or solid particles dispersed in a gas. Fog is a liquid aerosol (water droplets in air), while smoke is a solid aerosol (solid particles in air).
Foams are where gas bubbles are dispersed in a liquid or solid. Examples include whipped cream (gas in liquid) and shaving cream.