When different substances are combined, they create a physical blend known as a mixture, where the components retain their individual chemical identities. Mixtures are categorized based on how uniformly the components are distributed. Solutions and suspensions are two common types, distinguished primarily by the size of the dispersed particles. This exploration will define these two mixture types and explain why their internal structure causes them to behave so differently.
The Nature of Solutions
A solution is a homogeneous mixture, meaning the components are so thoroughly mixed they appear as a single substance with a uniform composition throughout. This uniformity is achieved when the solute (the substance being dissolved) completely breaks down into individual molecules or ions within the solvent (the substance doing the dissolving). The particles within a true solution are incredibly small, typically less than 1 nanometer (nm) in diameter.
Because of their molecular size, these dispersed particles cannot be seen, even under high magnification, and they do not significantly scatter light. This results in a transparent or clear solution, allowing light to pass directly through it. Solutions are highly stable because the tiny particles are fully surrounded by solvent molecules, preventing them from ever settling out. Common examples include salt dissolved in water, sugar dissolved in tea, and air.
The Characteristics of Suspensions
In sharp contrast, a suspension is a heterogeneous mixture where the dispersed particles are much larger and are temporarily held within the medium. The particles are not dissolved; instead, they are simply dispersed and are large enough to be easily seen. The particle size in a suspension is typically greater than 1000 nanometers (1 micrometer), a size range thousands of times larger than particles in a solution.
Because of this large particle size, suspensions are often opaque or cloudy, as the particles interfere with and scatter transmitted light. This makes them appear non-uniform, allowing the different components to be visibly distinguished. Common examples include muddy water, sand mixed in water, and certain liquid medications that require shaking to re-disperse the settled particles.
Comparing Stability and Separation Methods
The difference in particle size between solutions and suspensions dictates their behavior, particularly regarding stability. Solutions exhibit permanent stability because their molecularly small particles remain indefinitely dispersed and will not settle out under the force of gravity. A salt solution, for example, remains a perfect mixture even if left undisturbed for years.
Suspensions, however, are inherently unstable because the dispersed particles are large enough for gravity to act upon them. If a suspension is left to stand, the heavy particles will gradually settle out of the medium, a process known as sedimentation. This is why sand and water separates into distinct layers over a short period.
Separation Methods
This difference in particle size also determines the methods required to separate the components. Since the solute particles in a solution are molecularly small, they pass straight through the pores of filter paper. Separating a solution requires methods like evaporation or distillation, which change the state of the solvent. Conversely, the large, undissolved particles in a suspension are easily trapped by filter paper or simpler methods like decanting, which involves pouring off the liquid after the solids have settled.