A colloid is a type of mixture, representing a unique category that sits between a simple solution and a coarse suspension. They are known as colloidal dispersions because one substance is finely dispersed throughout another without dissolving or settling out. Understanding the properties of a colloid requires examining the physical characteristics that place it in this intermediate class of mixtures.
Defining Colloids and Their Components
A colloid is a heterogeneous mixture that often appears uniform to the naked eye. This mixture is composed of two primary parts: the dispersed phase and the dispersion medium. The dispersed phase is the substance that is spread out, analogous to the solute in a solution, while the dispersion medium is the continuous substance, similar to the solvent.
The defining feature of any colloid is the size of the particles in the dispersed phase. These particles are much larger than the individual molecules or ions found in a true solution, yet they are too small to be seen easily or to settle out under gravity. The diameter of colloidal particles typically ranges from 1 nanometer (nm) to 1,000 nanometers (or 1 micrometer). This specific size range gives colloids their distinct physical properties.
The particles in the dispersed phase can be solid, liquid, or gas, and the dispersion medium can also exist as any of these three states of matter. For instance, milk is an example where liquid fat globules are dispersed within a liquid water medium. This combination of physical states results in many common colloids, including fog, jelly, and paint.
How Colloids Compare to Solutions and Suspensions
The differences between solutions, colloids, and suspensions are primarily determined by the size of the dispersed particles. In a true solution, such as saltwater, the particles are less than 1 nm in diameter and are individual molecules or ions. A suspension, like muddy water, contains particles larger than 1,000 nm.
Solutions are classified as homogeneous mixtures because the dispersed particles are fully dissolved and the mixture is uniform throughout. Suspensions are clearly heterogeneous because the large particles are visible and unevenly distributed. Colloids occupy a middle ground; they are technically considered heterogeneous, but they appear homogeneous because the particles are not visible to the naked eye.
The stability of these mixtures also varies greatly with particle size. Solutions are completely stable, and their particles will never separate or settle out. Suspensions are unstable, and their large particles will separate from the medium and settle at the bottom over time due to gravity. Colloids are stable because the particles are small enough to remain suspended, often due to constant, random movement called Brownian motion, which prevents them from settling.
Recognizing Colloids Through Physical Properties
One of the most reliable ways to distinguish a colloid from a true solution is by observing how it interacts with light, a phenomenon known as the Tyndall effect. This effect occurs when a beam of light is passed through the mixture and the path of the light becomes visible. The visibility is caused by the colloidal particles scattering the light in all directions.
The intermediate size of colloidal particles allows them to effectively scatter visible light, making the light beam’s path traceable. In contrast, the particles in a true solution are too small to scatter light, so the beam passes through invisibly. This difference provides a simple visual test to identify a colloid.
Many familiar substances demonstrate the Tyndall effect, confirming their classification as colloids. Common examples include dust particles visible in a sunbeam (an aerosol), milk (an emulsion of liquid fat in liquid water), and fog (an aerosol of liquid water droplets in air).