Much of the suspended matter in natural waters is composed of silica, or similar materials, with specific gravity of 2.65. In sizes of 0.1 to 2 mm, they settle rapidly; however, in the range of the order of 10-5 mm, it takes them a year, in the overall, to settle a distance of only 1 mm. And, yet, it is the particle of this size range that causes the turbidity and color of water, making the water objectionable. The removal of particles by settling is practical only if they settle rapidly in the order of several hundreds of millimeters per hour. This is where coagulation can perform its function, by destabilizing the mutual repulsions of colloidal particles causing them to bind together and grow in size for effective settling. Colloidal particles fall in the size range of 10-6 mm to 10-3 mm. They are aggregates of several hundreds of atoms or molecules, although a single molecule such as those of proteins is enough to be become a colloid. The term colloid comes from the two Greek words kolla, meaning glue, and eidos, meaning like.
A colloid system is composed of two phases: the dispersed phase, or the solute, and the dispersion medium, or the solvent. Both of these phases can have all three states of matter which are solid, liquid, and gas. For example, the dispersion medium may be a liquid and the dispersed phase may be a solid. This system is called a liquid sol, an example of which is the turbidity in water. The dispersion medium may be a
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