Mixtures are combinations of two or more substances that are physically combined. These combinations can be classified based on how uniformly their components are distributed. A heterogeneous mixture, for instance, has a composition that is not uniform throughout, meaning its different parts can be visibly distinguished. Within the broad category of heterogeneous mixtures, suspensions and colloids represent distinct types, each with unique characteristics.
What is a Suspension?
A suspension is a heterogeneous mixture where solid particles are dispersed within a liquid or gas. These particles are large enough to be seen with the naked eye and do not dissolve in the medium. If left undisturbed, the dispersed particles will eventually settle out due to gravity, indicating the suspension’s inherent instability.
Suspensions typically appear cloudy or opaque because their large particles block or significantly scatter light. The particles are also large enough to be separated from the medium by simple physical methods, such as filtration. Common examples include muddy water or sand mixed in water, where the components are clearly distinguishable and will separate over time.
What is a Colloid?
A colloid is a heterogeneous mixture with particles much smaller than those found in suspensions. These particles are larger than individual molecules but too small to be seen without specialized equipment. Colloidal particles remain evenly dispersed throughout their medium and generally do not settle out over time, giving them an appearance of stability.
Colloids scatter light, a phenomenon known as the Tyndall effect, making the path of a light beam visible when a beam of light passes through. Unlike suspensions, their dispersed particles are small enough to pass through standard filter paper, making simple filtration ineffective for separation. Colloids can appear translucent or cloudy, depending on the concentration of the dispersed particles.
Key Differences in Properties
The most significant distinctions between suspensions and colloids lie in their particle size, stability, interaction with light, and separation methods.
Particles in a suspension are relatively large, typically greater than 100 nanometers (nm) or 1 micrometer (µm) in diameter, and are visible to the naked eye. In contrast, colloidal particles fall within an intermediate size range, generally between 1 and 1000 nm, and are too small to be individually seen.
Regarding stability, suspensions are inherently unstable, with their larger particles settling out over time due to gravity. Colloids, however, are considered stable systems because their dispersed particles remain suspended indefinitely and do not settle out.
The appearance of these mixtures also varies significantly. Suspensions are typically opaque, meaning light cannot pass through them easily. Colloids, while heterogeneous, can appear translucent and readily exhibit the Tyndall effect, scattering light beams and making their path visible. While suspensions may scatter light, their larger particles often block it entirely, leading to opaqueness.
Finally, their separation methods differ. Particles in a suspension can be separated by simple filtration, whereas colloidal particles pass through ordinary filters, making simple filtration ineffective for their separation.
Common Examples
Numerous everyday substances serve as practical examples of suspensions and colloids.
For suspensions, familiar instances include muddy water, where soil particles eventually settle. Liquid medications that require shaking before use, such as antacid suspensions, are also common examples, as their active ingredients settle over time. Sand mixed in water is another clear example, where the sand grains are visible and will separate.
Colloids are also prevalent in daily life. Milk, for example, is an emulsion where tiny fat globules are dispersed in water but do not settle. Fog and smoke are atmospheric colloids, consisting of liquid droplets or solid particles dispersed in air, which can make light beams visible, demonstrating the Tyndall effect. Paint, gelatin, and mayonnaise are additional examples of colloids, showcasing their stable, dispersed nature.