A mixture combines two or more substances that do not chemically bond. Solutions, colloids, and suspensions are three distinct types of mixtures, categorized based on the physical arrangement and size of the particles dispersed throughout another substance. Understanding the differences among these systems is important because it dictates how each mixture behaves, appears, and how its components can be separated. The key distinction lies in the physical scale of the dispersed material and its effects on stability and light interaction.
Defining the Dispersed Phase: Particle Size
The fundamental distinction between these mixtures is the size of the dispersed phase particles relative to the continuous phase, which acts as the solvent. The dispersed phase is the substance distributed throughout the continuous phase, such as salt molecules in water or dust particles in air.
True solutions are homogeneous mixtures where the particle size is exceedingly small, typically less than one nanometer (nm) in diameter. At this scale, the particles are individual molecules or ions that are fully dissolved and are too small to be seen, even with a microscope. This results in a transparent mixture, such as saltwater or sugar dissolved in water, where components are distributed uniformly.
Colloids occupy an intermediate range, with particle diameters generally falling between 1 nm and 1000 nm. Particles in a colloid, such as fat globules in milk or droplets in fog, are larger than those in a solution but remain small enough that they do not settle out due to gravity. Although appearing uniform to the naked eye, colloids are technically heterogeneous because the dispersed particles are distinct from the continuous medium.
Suspensions contain the largest particles, greater than 1000 nm (or one micrometer) in diameter. These particles are often visible to the naked eye or with a low-power microscope, and they are significantly larger than the wavelengths of visible light. Classic examples include muddy water or sand mixed with water, where the dispersed phase is clearly distinguishable from the continuous medium.
Physical Stability and Separation Methods
The size of the dispersed particles directly determines the physical stability of the mixture, specifically its tendency to settle out over time. Both true solutions and colloids are stable mixtures because their particles remain permanently dispersed and do not separate upon standing. For example, salt dissolved in water will not settle, and the particles in milk remain mixed indefinitely.
In contrast, suspensions are inherently unstable. Their larger particles settle out of the continuous phase over time due to gravity. If left undisturbed, a suspension like muddy water will eventually separate into a layer of settled solid material and cleaner liquid, demonstrating its heterogeneity. This settling process is known as sedimentation, a primary characteristic of a suspension.
The different particle sizes also dictate the appropriate method for separating the components. Large particles in a suspension can be easily separated from the liquid by simple filtration using standard filter paper. Since the components of a true solution are dissolved on a molecular level, they cannot be separated by filtration, but they can be separated by methods like evaporation or distillation.
Colloids present a challenge because their particles are small enough to pass through standard filter paper, making simple filtration ineffective. To separate a colloidal system, specialized techniques are necessary. These include ultrafiltration, which uses extremely fine membranes, or centrifugation, which uses high-speed rotation to force the particles to settle. Alternatively, disrupting the colloid’s stability causes the particles to aggregate, a process called coagulation, allowing for separation by conventional methods.
Interaction with Light (The Tyndall Effect)
The Tyndall effect describes the scattering of light by dispersed particles, serving as a straightforward way to distinguish a true solution from a colloid. When a beam of light is passed through a true solution, the path is invisible because the solute particles are too small to deflect the light waves. The light passes straight through the transparent liquid without scattering.
In a colloid, the particles are within the size range (1 nm to 1000 nm) large enough to scatter the light, making the beam’s path clearly visible. This visible light beam is the Tyndall effect, named after the 19th-century physicist John Tyndall. Examples include seeing the beam of a car’s headlight in fog or the shaft of sunlight in a dusty room, both of which are common colloids.
While suspensions also contain large particles that scatter light, their instability and often opaque nature make the Tyndall effect less useful for identification. For practical purposes, the Tyndall effect is primarily used as a simple test to differentiate a transparent, non-scattering true solution from a translucent, light-scattering colloidal dispersion. The ability to scatter light highlights that colloids are not true solutions, despite their often uniform appearance.