Colloids are a widespread type of mixture found throughout our daily lives, often without us realizing their presence. From the foods we eat to the air we breathe, these unique substances play a role in many natural and manufactured products. This article explores their fundamental characteristics, composition, and classification, distinguishing them from other common mixtures and highlighting familiar examples.
Defining a Colloid
A colloid is a specific type of mixture where microscopically dispersed insoluble particles are suspended throughout another substance. The defining characteristic lies in the size of these dispersed particles, which typically range from 1 nanometer (nm) to 1000 nm (or 1 micrometer) in diameter. This intermediate size distinguishes colloids from other mixtures; their particles are larger than those in solutions, yet smaller than those in suspensions that visibly settle.
Despite their microscopic nature, these particles are large enough to remain evenly distributed within the mixture without settling due to gravity, contributing to their stability. Unlike suspensions, colloids generally do not separate into distinct layers over time unless external conditions like temperature or chemical composition are significantly altered. While a colloid may appear uniform and clear to the naked eye, giving the impression of a homogeneous mixture, it is in fact heterogeneous at a microscopic level. This unique appearance is due to the light-scattering properties of the dispersed particles.
Every colloid consists of two primary components: the dispersed phase (the substance distributed as particles) and the dispersion medium (the continuous substance in which these particles are spread). The dispersed phase is present in a relatively minor amount, while the dispersion medium is the substance present in a larger quantity, analogous to a solute and solvent in a solution.
Distinguishing Colloids from Other Mixtures
To understand colloids, it is helpful to compare them with the other two main types of mixtures: true solutions and suspensions. The fundamental difference among these three mixture types lies in the size of their constituent particles.
In a true solution, such as sugar dissolved in water, particles are individual atoms, ions, or very small molecules, typically less than 1 nm in diameter. These particles are invisible and do not scatter light, appearing completely transparent.
In contrast, suspensions, like sand in water, contain particles larger than 1000 nm (or 1 micrometer). These particles are visible and, due to their size and density, will eventually settle out of the mixture. Suspensions can also be separated by simple filtration because the particles are too large to pass through filter paper.
Colloids occupy an intermediate position, with particle sizes between 1 nm and 1000 nm. This prevents colloidal particles from settling out like suspensions or being separated by standard filtration methods. A distinguishing phenomenon for colloids is the Tyndall effect, where dispersed particles are large enough to scatter a beam of light, making its path visible through the mixture. True solutions do not exhibit the Tyndall effect, while suspensions may scatter light but are often opaque or settle quickly.
Common Examples of Colloids
Colloids are abundant in our everyday environment, demonstrating diverse forms and applications. Milk, for instance, is a common colloid classified as an emulsion, where tiny liquid fat globules are dispersed within a liquid water medium. Fog and mist are aerosols, consisting of microscopic liquid water droplets (dispersed phase) suspended in gaseous air (dispersion medium). Smoke is another aerosol type, with solid particles dispersed in a gaseous medium.
Paint is a “sol” type of colloid, with solid pigment particles (dispersed phase) evenly distributed throughout a liquid medium, such as water or oil. Jelly is a gel, formed by a liquid dispersed phase within a solid dispersion medium, giving it a semi-solid consistency. Whipped cream exemplifies a foam, with gas (air) bubbles as the dispersed phase spread throughout a liquid cream dispersion medium, creating its light texture. These examples illustrate how different states of matter interact at a specific particle size range to create unique colloidal properties.