A hypertonic solution is one with a high concentration of solutes when compared to another solution which is separated from it by a semipermeable membrane. The property of tonicity is often used to illustrate the biology of the body, with the solute concentration of cells and surrounding fluids being used as an example. Tonicity is related to osmosis, in which fluids flow back and forth across a semipermeable membrane; osmolarity differs from tonicity in that it considers the concentration of solutes that penetrate the membrane and those that do not, while tonicity only considers those that do not penetrate.
If a solution is hypertonic, it means that fluid will flow across the membrane and into the hypertonic solution until an isotonic state is reached. In an isotonic state, the solutions on either side of the membrane have the same distribution of solutes. Conversely, with a hypotonic solution, the concentration of solutes is lower than that of a solution on the other side of a membrane, which means that water will be drawn out of the hypotonic solution and into a hypertonic solution.
Only solutes which cannot pass through the membrane are considered when evaluating tonicity. Solutes small enough to move through the membrane will naturally distribute themselves as needed in an attempt to even the solution out. In the classic example of hypertonicity, if salts in body fluids rise too much, they cause cells to lose water, shriveling up. This can happen with dehydration, in which water lost by the body is not replaced, leading to an increase in salts. Conversely, people who have consumed too much water too quickly can develop hypotonic body fluids, forcing water into the cells and potentially causing them to explode.
The hypertonic solution has applications outside the body. Osmosis is used in tasks like water purification, preparation of scientific samples, and scientific research. By manipulating the tonicity of a solution to make it a hypertonic solution, the flow of fluid across a membrane can be controlled, taking advantage of the fact that water moves from areas of low solute concentration to areas of high solute concentration in an attempt to even out the osmotic pressure.
Illustrations of tonicity are commonly given in basic science classes with the assistance of dyed fluids which can be clearly tracked as they move back and forth across a semipermeable membrane. Understanding tonicity is also important for tasks which may not immediately lead people to think of science. Cooking, for example, can involve tonicity when fluids are absorbed or expressed. For example, many cooks use the trick of sprinkling eggplant slices with salt to force them to express water before cooking; they have essentially created a hypertonic environment which leads fluid to flow out of the eggplant in an attempt to equalize tonicity.