Children at play often make suspensions such as mixtures of mud and water, flour and water, or a suspension of solid pigments in water known as tempera paint. These suspensions are heterogeneous mixtures composed of relatively large particles visible to the naked eye or seen with a magnifying glass. They are cloudy, and the suspended particles settle out after mixing. The suspended particles in a suspension settle out after some time of mixing. The separation of particles from a suspension is called sedimentation. A blood test to determine the rate of sedimentation measures how quickly red blood cells in a test tube settle out of the watery portion of blood (known as plasma) over a set time. Rapid sedimentation of blood cells does not usually happen in a healthy body. Still, aspects of certain diseases can cause blood cells to clump together, and these heavy clumps of blood cells settle to the bottom of the test tube more quickly than normal blood cells.
Another class of mixtures called colloids (or colloidal dispersions) exhibit properties intermediate between those of suspensions and solutions. The particles in a colloid are larger than most simple molecules; however, colloidal particles are small enough that they do not settle out upon standing. The particles in a colloid are large enough to scatter light; a phenomenon called the Tyndall effect; which is why colloidal mixtures appear cloudy or opaque, such as searchlight beams. Clouds are also colloidal mixtures. They are composed of water droplets much larger than molecules but small enough not to settle out. Some colloids have the property to undergo sol-gel transformations, which can transform reversibly from a fluid to a solid (gel) state. For example, gelatin products solidify to form a gel when refrigerated, which can be liquefied again when heated. The sol-gel property of cytosol, the fluid of living cells, is essential for many cellular activities, such as cell division or change in cell shape. The particulate component of a colloidal solution, present in a relatively minor amount, is called the dispersed phase. The substance or solution throughout which the particulate is distributed is called the dispersion medium.
This text is adapted from Openstax, Anatomy and Physiology 2e, Section 2.4: Inorganic Compounds Essential to Human Functioning and Openstax, Chemistry 2e, Section 11.5: Colloids.
