A mixture is a combination of two or more substances that are not chemically bonded. Chemists classify these mixtures based on the size of their constituent particles, creating a spectrum from fully dissolved components to those that visibly separate. Colloids represent a distinct, intermediate class within this spectrum, falling between true solutions and coarse suspensions. These unique mixtures are far more common than many people realize, forming the basis of countless materials encountered in daily life.
Defining the Colloid
A colloid is a two-phase mixture where the dispersed phase is finely distributed throughout a second substance, called the dispersion medium. The defining characteristic is the size of the dispersed particles, which must measure between 1 nanometer and 1,000 nanometers in diameter. These particles are larger than individual molecules but small enough to remain suspended indefinitely. The dispersed phase can be a solid, liquid, or gas, while the dispersion medium is the continuous substance. Because the particles are not truly dissolved at the molecular level, the mixture is considered heterogeneous, despite often appearing uniform to the naked eye.
Distinguishing Colloids from Solutions and Suspensions
The primary factor separating colloids from other mixtures is the size of the dispersed material. A true solution, such as salt dissolved in water, contains particles smaller than 1 nanometer, which are fully integrated into the solvent and pass easily through most filters. In contrast, a suspension, like muddy water, contains particles larger than 1,000 nanometers. These large particles are visible, unstable, and settle out over time due to gravity. Colloidal particles, positioned between 1 and 1,000 nanometers, are too large to dissolve but too small to be affected by gravity, allowing them to remain stable and dispersed without settling.
Observable Physical Behaviors
The unique size of colloidal particles results in two distinct, observable physical phenomena. One is the Tyndall effect, which describes the scattering of light by the dispersed particles. When a beam of light passes through a colloid, the particles deflect the light, making the path of the beam visible. This effect is not seen in true solutions because their particles are too small to scatter light. Another characteristic is Brownian motion, the continuous, random movement of colloidal particles within the dispersion medium. This movement is caused by the bombardment of the dispersed particles by the molecules of the surrounding medium, which prevents them from aggregating and settling out.
Colloids in Everyday Materials
A vast array of everyday materials are classified as colloids.
Common Types of Colloids
Emulsions: These involve a liquid dispersed in another liquid. Milk is an example where fat droplets are dispersed in water, while butter is the reverse, with water dispersed in fat.
Aerosols: These feature a liquid or solid dispersed in a gas, such as fog and clouds.
Foams: These are gas dispersed in a liquid, represented by shaving cream and whipped cream.
Sols: These involve solid components distributed in a liquid. Blood, paint, and ink are examples of sols.