Turbidity is the cloudiness or haziness in a water supply caused by individual particles or suspended solids that are generally invisible to the naked eye, similar to smoke in air. The measurement of turbidity is a key test of water quality. The water can contain suspended solid matter consisting of particles of many different sizes. Some suspended material will be large enough and heavy enough to settle rapidly to the bottom. If a liquid sample is left to stand (the settleable solids), very small particles will settle only very slowly or not at all if the sample is regularly agitated or the particles are colloidal.
