A mixture is a combination of two or more substances that are not chemically bonded, categorized based on the size of their internal components. True solutions, like salt dissolved in water, appear perfectly uniform, while other mixtures clearly show their different parts. Falling between these two extremes is a specific class of materials known as colloids. These mixtures are present in many everyday items, from food to the air we breathe, possessing unique properties that set them apart from other combinations of matter.
Defining Colloids
A colloid is a type of mixture where one substance is evenly dispersed throughout another, but not fully dissolved. This mixture is formally referred to as a colloidal dispersion. The substance that is spread out is called the dispersed phase, and the surrounding substance it is mixed into is the continuous phase, or dispersion medium. Colloids are characterized by a specific particle size range, larger than the individual molecules in a true solution but smaller than the clumps found in a suspension.
The particles in the dispersed phase are sized between 1 nanometer and 1,000 nanometers (or 1 micrometer). If the particles were smaller, the mixture would be a solution; if they were larger, it would be a suspension. Because of this intermediate size, the particles remain permanently suspended and evenly distributed throughout the continuous phase. This arrangement gives colloids their distinct physical properties and appearance.
Distinguishing Colloids from Other Mixtures
Colloids differ from true solutions and suspensions primarily due to their particle size and stability. In a true solution, components are mixed at the molecular level, resulting in a clear, homogeneous mixture. The particles are too small to be affected by gravity or interact with light.
A suspension contains particles larger than 1,000 nanometers, making it a heterogeneous mixture where components are visible. These larger particles eventually settle out of the dispersion medium upon standing, which is why some liquid medicines require shaking before use. Colloids maintain their dispersed state indefinitely and will not settle, sharing this stability with true solutions.
A way to distinguish a colloid from a true solution is through the Tyndall effect, which involves the scattering of light. When a beam of light is passed through a true solution, the path is invisible because the particles are too small to deflect it. However, when the light passes through a colloid, the intermediate-sized particles scatter the light, making the beam’s path visible. This visible light scattering confirms the presence of a colloidal system.
Common Examples and Classification of Colloids
Colloids are classified into eight categories based on the physical state (solid, liquid, or gas) of both the dispersed phase and the dispersion medium. The term “colloid” includes familiar mixtures like emulsions, foams, sols, and aerosols. Since gas-in-gas mixtures always form true solutions, only eight of the nine possible combinations of states can form a colloid.
Milk is an example of an emulsion, which is a liquid dispersed in another liquid. Milk contains tiny globules of fat (the dispersed phase) suspended in a water-based solution (the dispersion medium). Mayonnaise is another common emulsion, where oil droplets are dispersed in vinegar.
Foams are colloids where a gas is dispersed throughout a liquid or a solid. Whipped cream is a liquid foam, consisting of gas bubbles trapped within a liquid cream. A solid foam example is pumice stone or memory foam, where gas is dispersed in a solid.
Aerosols involve either a liquid or a solid dispersed in a gas. Fog is a liquid aerosol, where tiny water droplets are suspended in air. Smoke is a solid aerosol, consisting of small solid particles suspended in the air.
Another common type is a sol, which is a solid dispersed in a liquid. Paint is an example of a sol, containing solid pigment particles suspended throughout a liquid solvent. Similarly, gels are colloids where a liquid is dispersed in a solid, such as gelatin or jelly, which has a liquid trapped within a solid network. Finally, colored glass and gemstones are examples of a solid sol, where one solid is dispersed in another solid.