Mixtures are classified into categories based on their behavior. A core distinction is made between homogeneous mixtures, which appear uniform throughout, and heterogeneous mixtures, which have visibly separate components. Both suspensions and colloids are types of heterogeneous mixtures, but they differ based on particle size and stability. This classification system allows scientists to predict how a mixture will behave, whether it is a pharmaceutical drug or an industrial paint.
The Critical Factor Particle Size
The classification of a heterogeneous mixture as a suspension or a colloid is determined solely by the diameter of the dispersed particles. These mixtures consist of a dispersed phase (the small particles) and a dispersion medium (the surrounding substance). The physical size of the dispersed phase dictates the mixture’s properties and category.
The nanometer (nm) is the unit of measure used for particle diameter in this classification system. True solutions have particle diameters smaller than one nanometer. Colloids range in size from one nanometer up to 1,000 nanometers (one micrometer). Any mixture with dispersed particles greater than 1,000 nanometers is categorized as a suspension.
Defining the Suspension
Suspensions are characterized by large particles, which leads to specific physical behaviors. Since these particles are larger than 1,000 nanometers, they are often visible to the naked eye or under a standard microscope. A suspension typically appears cloudy or opaque because the large particles scatter light extensively. Common examples include muddy water or sand stirred into water.
The large particle size makes the mixture inherently unstable. Gravitational force overcomes the forces keeping the particles dispersed. If left undisturbed, the dispersed phase quickly settles out of the medium, a process called sedimentation. This instability allows the components to be easily separated using simple physical methods. The particles are too large to pass through standard filter paper, allowing separation by simple filtration.
Defining the Colloid
Colloids represent an intermediate state where the dispersed particles are large enough to be more than just individual molecules but small enough to remain stable within the medium. The particle size, between one and 1,000 nanometers, prevents settling under gravity, providing long-term stability. Although a colloid may appear translucent, the particles are too small to be seen without specialized instruments.
Brownian Motion
A defining mechanical feature of colloids is Brownian motion, the continuous, random, zigzag movement of the dispersed particles. This motion is caused by the constant, unbalanced bombardment of the colloidal particles by the smaller molecules of the dispersion medium. This agitated movement is a primary reason why colloidal particles resist settling and remain dispersed.
Surface Area and Categorization
The intermediate size of colloidal particles results in a high surface area relative to their volume, enhancing their interfacial properties. This large surface area leads to unique chemical and electrical interactions, such as the adsorption of ions onto the particle surfaces. Colloids are further categorized based on the physical state of the dispersed phase and the dispersion medium, including emulsions (liquid in liquid, like milk), gels (liquid in solid, like jelly), and aerosols (liquid or solid in gas, like fog).
Observing the Distinction
Practical testing methods allow for clear differentiation between a true solution, a colloid, and a suspension, moving beyond simple observation of particle settling. The most recognized test is the Tyndall effect, which involves passing a beam of light through the mixture. Colloidal particles are precisely the right size to scatter the light beam, making the path of the light visible. True solutions do not scatter the light, while suspensions scatter light but often settle quickly, ending the effect.
Filtration and Centrifugation
Filtration is a straightforward method used to test particle size. Suspensions can be separated by pouring the mixture through standard filter paper because their particles are too large to pass through. Colloidal particles, however, are small enough to pass through the pores of regular filter paper, meaning they cannot be separated by simple filtration.
While gravity causes the rapid separation of suspensions, separating the stable particles in a colloid requires specialized methods. This typically involves the use of a centrifuge, which applies a powerful rotational force to induce sedimentation.