The answer to whether colloids have smaller particles than solutions is no; true solutions contain the smallest particles of all common mixtures. Mixtures are classified into three main types—solutions, colloids, and suspensions—based on the size of the dispersed particles they contain. This size difference dictates the physical properties and behavior of the mixture, including its appearance and interaction with light. The size spectrum begins with the molecular scale of solutions and progresses to the larger aggregates found in suspensions.
The Defining Characteristics of True Solutions
True solutions are homogeneous mixtures where the dissolved substance, or solute, is dispersed at the molecular or ionic level. The particle size is less than one nanometer (nm) in diameter. This minimal size means the solute particles are essentially the same size as the solvent molecules, such as sugar or salt dissolved in water. Because the particles are so small, they integrate completely with the solvent, making the resulting mixture transparent.
The small size of these dispersed particles prevents them from being separated by physical means, such as standard filtration. Both the solvent and the solute particles pass through the pores of a filter paper. Furthermore, the solute particles never settle out of the solution, even when left undisturbed. This stability and uniformity result directly from the particles having molecular dimensions.
Colloidal Systems and Intermediate Particle Size
Colloidal systems, often called colloidal dispersions, are an intermediate class of mixtures between true solutions and suspensions. The particles in a colloid are significantly larger than those in a true solution, typically falling within the range of 1 nm to 1000 nm (or 1 micrometer) in diameter. This intermediate size means colloidal particles are much larger than simple molecules but remain too small to be seen with the naked eye.
Although colloids may appear uniform, they are technically heterogeneous mixtures because the dispersed substance is not dissolved at the molecular level. Common examples include milk, fog, and paint. The particles are kept dispersed and stable, preventing them from settling out due to gravity. This stability arises from factors like Brownian motion and surface charges on the particles.
The size range of colloidal particles is responsible for their unique appearance, which is often translucent or opaque. Unlike the clearness of a true solution, a colloid’s appearance reflects the interaction of light with the larger dispersed particles. The particles are small enough to pass through ordinary filter paper, but they are generally retained by specialized semi-permeable membranes.
How Particle Size Determines Mixture Behavior
The differences in particle size between the three mixture types dictate their observable physical behaviors, providing a way to distinguish them. True solutions, with particles smaller than 1 nm, do not scatter light. This is because the particles are too small to interact with the wavelengths of visible light. Consequently, a beam of light passed through a true solution is invisible.
Colloids exhibit the Tyndall effect, which is the scattering of light by the dispersed particles. This effect makes the path of a light beam clearly visible when shone through a colloidal dispersion like milk or fog. The particles are in the size range of roughly 40 to 900 nm, allowing them to effectively deflect visible light.
Suspensions contain the largest particles, exceeding 1000 nm in diameter, such as mud mixed in water. These large particles make the mixture visibly cloudy or opaque. Suspensions are the only type of mixture where the particles will settle out naturally under the influence of gravity. Their large size also means they can be easily separated from the liquid medium using simple filtration.