Suspensions are mixtures of two or more undissolved materials, one of which is a liquid. The preparer of a suspension may distribute the two materials by mechanical means, such as stirring, but they separate over time. Suspensions appear cloudy because the suspended particles are visible due to the Tyndall effect.
Typically, suspensions include a solid and a liquid. However, some suspensions, such as oil and vinegar salad dressing include tiny droplets of one liquid in a second liquid. Suspensions in water might include sand, organic material and bacteria.
Particles in a suspension often vary in size and are larger than the particles in a solution. In a solution, all the particles are atoms, molecules or ions, having a particle size of 1 nanometer. In suspensions, some particles are large enough to see with the naked eye.
Filtration helps to remove many particles from suspension. Municipalities use screening and sedimentation of water to remove these large particles before pumping it to the distribution system. However, filtration is not effective at removing dissolved contaminants.
In order to remove dissolved contaminants from water, treatment facility workers add other chemicals that react with the contaminant to form a precipitant. Precipitants are insoluble solids that bind the undesirable ions from the water. The process converts the solution to a suspension, allowing physical separation of the dissolved contaminants.