Colloids are mixtures found widely in our environment and daily lives. These unique systems bridge the gap between simple solutions and coarse suspensions. They consist of one substance finely dispersed throughout another, creating a mixture that often appears uniform to the naked eye. What sets colloids apart is the specific size of their internal particles.
The Defining Feature of Colloids
The defining characteristic of a colloid is the size range of its dispersed particles. These particles typically measure between 1 nanometer (nm) and 1,000 nm. This intermediate size allows them to remain suspended within a continuous medium without settling out over time, unlike larger particles found in suspensions. While colloids may visually appear homogeneous, they are fundamentally heterogeneous mixtures because the dispersed particles are distinct from the surrounding medium. The dispersed substance is known as the “dispersed phase,” and the substance it is suspended in is called the “continuous phase” or “dispersion medium.”
How Colloids Differ from Other Mixtures
Colloids occupy a middle ground compared to true solutions and suspensions, primarily due to their particle size. In true solutions, particles are individual molecules or ions, typically smaller than 1 nm. They are completely dissolved, forming a homogeneous mixture that is transparent and does not scatter light. These particles never settle out.
In contrast, suspensions contain much larger particles, generally exceeding 1,000 nm, which are visible and will settle over time due to gravity. Suspensions are clearly heterogeneous and can often be separated by filtration. Colloids, with their intermediate particle size, do not settle out upon standing like suspensions, yet their particles are large enough to interact with light.
This interaction with light is a key differentiator: colloids can scatter light, a phenomenon known as the Tyndall effect, while true solutions do not. Colloidal particles scatter light, making the path of a light beam visible through the mixture. This allows for a clear distinction between a transparent solution and a seemingly clear colloid.
Everyday Colloids and Their Observable Traits
Many common substances we encounter daily are colloids, showcasing their unique properties. Milk, for instance, is an emulsion, a type of colloid where tiny liquid fat globules are dispersed in water. Fog and smoke are aerosols, with fog being liquid water droplets dispersed in gas, and smoke consisting of solid particles dispersed in gas. Paints and jellies are also colloidal systems, illustrating the diverse forms colloids can take.
One of the most recognizable observable traits of colloids is the Tyndall effect, the scattering of light by the dispersed particles. This effect makes a beam of light visible as it passes through a colloid, such as headlights cutting through fog or sunlight revealing dust particles in a room. The particles in a true solution are too small to scatter light, so the beam remains invisible.
Another trait is Brownian motion, the erratic, random movement of colloidal particles. This motion is caused by the incessant collisions between the tiny, dispersed colloidal particles and the much smaller molecules of the dispersion medium. Observing these random movements under a microscope confirms a mixture’s colloidal nature.