A suspension is a fundamental type of heterogeneous mixture in chemistry. It is defined by solid particles physically dispersed throughout a fluid medium, which can be a liquid or a gas. The components do not chemically combine or dissolve into one another. Instead, the solid material remains as separate fragments floating within the continuous medium.
Defining Characteristics of a Suspension
A suspension is characterized by its non-uniform composition, classifying it as a heterogeneous mixture. The solid particles distributed throughout the fluid are large enough to be readily visible, often to the naked eye or under a low-power microscope. This visibility is due to the size of the dispersed particles, which are typically greater than 1,000 nanometers (1 micrometer) in diameter.
These large particles give a fresh suspension its cloudy or opaque appearance. Since the solid component does not dissolve, it maintains its distinct physical and chemical identity within the mixture. This lack of true dissolution is the primary feature setting suspensions apart from other mixtures. The mixture remains a two-phase system where the solid is kept afloat by mechanical action or the fluid’s viscosity.
The Spectrum of Mixtures
Mixtures in chemistry are categorized based on the uniformity and size of their constituent particles. Unlike a suspension, a true solution is a homogeneous mixture where the dissolved substance exists as individual ions or molecules, with particles smaller than 1 nanometer. These microscopic particles are invisible, do not settle out, and pass through most filters.
A colloid sits between a solution and a suspension, with particle sizes ranging from 1 to 1,000 nanometers. Colloidal particles are too small to be seen and do not settle out over time because they are constantly bombarded by the medium’s molecules. Colloids can be distinguished from solutions because their intermediate-sized particles are large enough to scatter light, a phenomenon known as the Tyndall effect. While suspensions may also scatter light, their particles are so large they often block the light beam, and their key difference is the inevitable settling due to gravity.
Separation and Instability
The defining physical behavior of a suspension is its inherent instability; the dispersed particles will eventually separate from the fluid medium. This separation occurs because the mass of the large particles is sufficient for gravity to overcome the forces keeping them suspended. The process where the solid particles fall out of the mixture and accumulate at the bottom is called sedimentation.
To re-form the suspension, a mechanical force like shaking or stirring must be applied to temporarily re-disperse the settled particles. When faster separation is needed, filtration is used, where the mixture is passed through a porous barrier like filter paper. The large solid particles are trapped on the filter’s surface while the fluid passes through. Alternatively, centrifugation can accelerate the natural sedimentation process by using high-speed rotation to apply an artificial gravitational force.
Common Applications of Suspensions
Suspensions are common in manufactured products and natural environments, often recognizable by instructions to “shake well before use.” In the pharmaceutical industry, many liquid medications, such as certain antacids and oral antibiotics, are formulated as suspensions. This design allows for the administration of an active drug ingredient that is insoluble in water, ensuring a consistent dose after shaking.
Industrial applications utilize suspensions for their functional properties, such as in paints, where pigment particles are suspended in a liquid vehicle. The pigments settle over time, necessitating stirring before use to ensure even color application. Naturally occurring examples include muddy water, where silt and clay particles are temporarily suspended in a river or lake, and food items like orange juice with pulp.