A mixture is composed of two or more materials that are physically combined but not chemically bonded. Mixtures are sorted based on how uniformly their components are distributed. The classification of colloids, a specific type of mixture, as homogeneous or heterogeneous is often confusing. Colloids appear uniform to the naked eye, leading many to assume they are homogeneous, but the science of particle distribution reveals a more complex reality.
Classifying Chemical Mixtures
Mixtures are defined by the uniformity of their composition at a microscopic level. Homogeneous mixtures, or true solutions, exhibit a uniform composition throughout, making components indistinguishable even under a microscope. For example, dissolved saltwater has salt ions evenly dispersed among water molecules, existing as a single phase.
Heterogeneous mixtures have a non-uniform composition, where individual components remain distinct and can be physically separated. A mixture of sand and water is an example, as the sand particles remain separate and settle out over time. These mixtures often exist in multiple phases, such as solid sand dispersed in liquid water.
The appearance of colloids causes confusion during classification. While a true solution is clear and transparent, a colloid often appears opaque or cloudy yet remains stable. This observed uniformity is deceptive, as the unique characteristics of colloids challenge the simplicity of the two-part classification system.
The Defining Characteristic of Colloids: Particle Size
Colloids are defined by the size of their dispersed particles, which fall into a specific intermediate range. Colloidal particles typically measure between 1 nanometer (nm) and 1,000 nm (1 micrometer) in diameter. This size range is distinctly larger than the individual molecules or ions found in true solutions, which are generally smaller than 1 nm.
The particles are large enough that they are not truly dissolved into the dispersion medium, meaning they are not chemically bonded to the medium’s molecules. Because the particles exist as separate, distinct phases, the mixture is technically non-uniform at the microscopic level, classifying the colloid as a heterogeneous system. The particles are small enough to remain suspended and resist settling due to gravity, making the mixture appear stable and uniform.
Colloidal particles are significantly smaller than those in a coarse suspension, which are larger than 1,000 nm. These larger suspension particles settle rapidly under gravity, requiring continuous stirring to prevent separation. The intermediate size of colloidal particles allows them to bridge the gap between true solutions and suspensions.
Observational Evidence of Heterogeneity
The physical proof that a colloid is heterogeneous is the Tyndall effect, which demonstrates how it interacts with light. When a beam of light passes through a true solution, the path remains invisible because the dissolved particles are too small to scatter the light.
When the same light beam passes through a colloid, the path becomes clearly visible from the side. This visibility is caused by the light scattering off the larger colloidal particles, which are big enough to deflect light waves. The scattering confirms the presence of distinct, separate particles, proving the mixture is heterogeneous and non-uniform at the particle level.
Another evidence point relates to separation. Although colloidal particles do not settle naturally like a suspension, they can be separated from the dispersion medium using specialized techniques. Ultracentrifugation or dialysis can isolate the dispersed phase, which would be impossible in a true homogeneous solution. The ability to separate the components confirms they exist as two distinct phases.
Colloids in Daily Life
Colloids are pervasive in daily life, existing in many familiar products that appear perfectly uniform. Milk is a classic example of an emulsion, a liquid-liquid colloid where tiny fat droplets are dispersed throughout an aqueous medium. The smooth appearance of milk is deceptive, as the fat and water are not truly dissolved.
Fog and clouds are liquid aerosols, which are colloids of tiny water droplets dispersed in air. The visible beam of car headlights in fog illustrates the Tyndall effect, as light scatters off the water droplets.
Common Colloids
Common colloids include:
- Paint, which is a sol of solid pigment particles dispersed in a liquid.
- Whipped cream, a foam of gas dispersed in a liquid.
- Mayonnaise, an emulsion.
- Gelatin, a gel.
These everyday items look smooth and uniform, reinforcing the difficulty in classifying colloids. Despite their deceptive appearance, the distinct, non-dissolved nature of their dispersed particles makes all these substances technically heterogeneous mixtures.