A solvent is a substance, most often a liquid, that can dissolve other substances. Here, the substance being dissolved is called a solute. When a solvent and a solute combine, they form a solution that, at the molecular level, is a homogenous mixture of both the solvent and the solute. Water is a universal biological solvent. Its polar structure allows it to dissolve many other polar compounds. The ability of water to dissolve is governed by a balance between water molecules binding to each other and binding to the solute.

A saturated solution contains the maximum amount of dissolvable solute. For example, salt (NaCl) is readily dissolved in water to create salt water, or saline. It dissolves because salt dissociates into its respective ions sodium (Na⁺) and chloride (Cl^-). Water is polar, so its oxygen atom, being slightly negative, is attracted to the positive sodium ions. Several water molecules can bind to a single sodium ion, creating a sphere of hydration. Likewise, water’s hydrogen atoms are slightly positive and are attracted to the negative chloride ions, again creating a sphere of hydration around the chloride ions. These hydration shells keep the solute particles separated and dispersed, creating a solution.

A saturated solution of salt water (at room temperature) contains about 26% sodium chloride. If more salt is added, the excess cannot be dissolved into the solution and becomes a precipitate at the bottom. The salt content of the Great Salt Lake in Utah (USA) ranges from 5-27%. The Dead Sea, which is bordered by Israel, Jordan, and the West Bank, has a salt content of ~34%. This is substantially higher than the saturation level of salt in water. The excess salt precipitates out, creating extraordinary salt crystal formations.

A solute’s solubility, or ability to dissolve in water, is crucial for biological functions. For example, proteins and amino acids must be dissolved to gain access to cells. Likewise, sodium, chloride, potassium, and calcium ions (among others) are necessary for cellular function. Proteins, ions and other nutrients are dissolved in the blood, which is ~79% water. The kidneys help maintain the proper levels of these dissolved solutes in the blood by removing or adding them during filtration, a process called osmoregulation.