Suspension is a mixture in which the particles of a substance separate from a liquid or gas slowly. Each of the particles consists of many atoms or molecules, and so a suspension can be visually recognized as a mixture of two different substances.
There are several types of suspensions. They include (1) a solid in a gas, such as dust and smoke; (2) a liquid in a gas, such as fog and aerosols; (3) a solid in a liquid, such as muddy or soapy water; (4) a gas in a liquid, such as foam; and (5) a liquid in a liquid, such as latex or water-based paints. A suspension that contains extremely small particles is called a colloid. The particles in many colloids can only be seen with the aid of a microscope. Homogenized milk with its tiny particles of suspended fat is a common colloid. See Colloid .
The molecules of a liquid or gas in a suspension move rapidly and collide with the suspended particles. The buffeting effect of these collisions is important in resisting the natural tendency of the particles to settle because of gravity. The rapid, random motion of the suspended particles that results from the collisions is called Brownian motion.
A suspension has certain other basic properties that distinguish it from another type of mixture called a solution. When a beam of light is shone through a colloidal suspension, such as smoke or dust-filled air, its path becomes clearly visible. This phenomenon, called the Tyndall effect, occurs because the suspended particles reflect and scatter light. A solution shows no such effect because its particles are too small to scatter light. Also, a suspension can be separated into its component parts by filtration, but a solution cannot. Particle size is again the determining factor.
