Mixtures are combinations of two or more substances that are not chemically joined. Solutions, colloids, and suspensions represent three distinct categories of mixtures, each exhibiting unique physical characteristics. Understanding these differences depends on the size of the particles dispersed throughout the primary substance. Particle dimensions determine the behavior of light and the long-term stability of the mixture.
Defining Mixtures by Particle Size
The fundamental distinction between these three types of mixtures is the size of the dispersed particles. Solutions have the smallest particles, typically individual molecules or ions, measuring less than 1 nanometer (nm) in diameter. These minuscule components are completely and uniformly distributed throughout the solvent, making the mixture appear entirely homogeneous.
Colloids occupy an intermediate range, with dispersed particles generally between 1 nm and 1,000 nm (or 1 micrometer). These particles are significantly larger than those in a solution, but they are still too small to be seen individually with the naked eye. This intermediate size allows them to share some properties with both solutions and suspensions.
Suspensions contain the largest particles, with diameters exceeding 1,000 nm. The particles in a suspension are large enough to be visible and are not truly dissolved in the surrounding medium. This substantial size difference is the single factor that governs all other observable behaviors of the mixture.
How Light Reveals the Difference
The interaction of a mixture with a beam of light provides a simple visual method for classification, known as the Tyndall effect. When a beam of light passes through a true solution, the path of the light remains invisible. This occurs because the dissolved particles are far smaller than the wavelength of visible light and cannot scatter the light rays.
Colloids exhibit the Tyndall effect because their intermediate-sized particles are large enough to scatter light. When a light beam shines through a colloid, the path of the beam becomes visibly illuminated as the dispersed particles reflect the light. This phenomenon is commonly observed when sunlight streams through a dusty room or through fog.
Suspensions may appear cloudy or opaque, but they do not typically show a clearly defined beam path like colloids do. Their large particles tend to block or absorb the light rather than scattering it in a way that defines the beam’s path. The light-scattering property in a suspension is often temporary because the large particles quickly settle out of the mixture.
Stability and Separation
The stability of a mixture describes its ability to remain uniformly mixed over time without external interference. True solutions are inherently stable and will remain uniformly mixed indefinitely, as their particles are too small to be affected by gravity. Similarly, colloids are relatively stable, with their particles remaining dispersed due to constant, random movement called Brownian motion, which counteracts the force of gravity.
Suspensions are inherently unstable mixtures, and their large particles will quickly settle out of the medium when left undisturbed. This tendency to separate is a defining characteristic of a suspension. For instance, if muddy water is allowed to sit, the particles of soil will fall to the bottom of the container.
The differences in particle size dictate the methods required for separation. The components of a solution cannot be separated by simple filtration because the dissolved particles pass easily through the filter’s pores. Suspensions are easily separated using standard filtration methods because the large particles are physically trapped by the filter paper. Separating colloids requires more intensive techniques, such as centrifugation or the use of semi-permeable membranes.
Common Examples in Daily Life
Many everyday substances can be categorized based on their behavior as solutions, colloids, or suspensions. Common examples of solutions include saltwater and the air we breathe, which is a gaseous mixture of nitrogen, oxygen, and other gases. Clear vinegar, a mixture of acetic acid and water, is another common solution.
Colloids are common because of their stable, uniform appearance. Milk is an emulsion, a type of colloid where tiny liquid fat droplets are dispersed in water. Other examples include mayonnaise, an emulsion stabilized by egg yolk, and fog, which consists of microscopic water droplets suspended in the air.
Examples of suspensions require shaking before use due to their instability. Muddy water is a classic suspension where soil particles settle out over time. Certain liquid medicines, such as antacids, must be shaken vigorously to re-disperse the active ingredients before being taken.