Suspensions

Suspensions

In addition to the species that are dissolved in water, a number of particles may be found to be suspended in the water. A suspension is a system in which the dispersed particles (typically solids, but may also be liquid droplets) are larger than ordinary molecules and ions, but are still small enough to remain dispersed (suspended) in the solvent. Because the suspended particles remain in the solid state (or as droplets), the particles are not said to be dissolved, and the system is not homogenous. The differentiation between a suspension and a solution is based on particle size (the suspended particles typically have diameters in the range of 1-100 nm); however, the distinction between a suspension and a solution on the basis of particle size is not well-defined. Experimentally, the differenece between the two types of mixtures is seen using the Tyndall effect, in which light is scattered by the suspended particles in a suspension (i.e., the solution appears cloudy), but is not scattered in a homogenous solution. Examples of suspended particles in water samples may include:

bacteria
floating debris (e.g., twigs, bits of leaves, and trash)
sand and dirt

The types and amounts of these species in a given water sample depend on the source of the sample, and what sort of treatment the water has undergone. Some of these dissolved species, such as O₂ gas, are unavoidable and do not detract from our ability to use the water. Other species, such as F^-, are desirable and are often added during the water-treatment process. Many of the species dissolved in water, however, are harmful for humans to drink, or otherwise render the water unsuitable for human use (e.g., Ca²⁺ is responsible for "hard water", which can leave insoluble scum on appliances and industrial boilers). These products must be removed via water-treatment facilities.