Water evaporation is a fundamental natural process, central to the Earth’s water cycle and the basis for several methods of purification. Saltwater is a solution where water acts as the solvent and salt, primarily sodium chloride, is the solute. Evaporation is a simple phase change from liquid to gas, yet it results in a powerful separation that leaves the dissolved solids behind.
The Physics of Water Vaporization
Evaporation is a surface phenomenon where individual water molecules transition from the liquid state to the gaseous state, or water vapor. This phase change requires an input of heat energy, often supplied by sunlight or the ambient air temperature. This energy increases the kinetic energy, or motion, of the water molecules.
As the molecules move faster, they overcome the weak attractive forces holding them together, which are known as hydrogen bonds. Only the most energetic molecules near the surface of the liquid can escape into the atmosphere. The energy required to break these bonds and transform liquid water into a gas is called the heat of vaporization, a relatively high value for water. This continuous escape of water molecules is what causes the overall volume of the liquid to decrease over time.
The Chemistry of Salt Exclusion
The reason salt does not evaporate with the water is rooted in the vast difference between the chemical structure of water and salt. Water molecules are held together by relatively weak hydrogen bonds, which are easily broken by the kinetic energy of heat. Salt, typically sodium chloride, is an ionic compound held together by strong electrostatic forces between positively charged sodium ions and negatively charged chloride ions.
When salt dissolves in water, the ions become separated and are surrounded by water molecules in what are called hydration shells. The water molecules interact with the ions through ion-dipole forces. However, the energy required to break the ionic bonds in a salt crystal or to vaporize the individual ions is extremely high.
The boiling point of water is \(100^\circ\text{C}\) at sea level, but the melting point of sodium chloride is around \(801^\circ\text{C}\), and its boiling point is over \(1,400^\circ\text{C}\). Since evaporation occurs at temperatures far below the boiling point of salt, the immense energy needed to convert the dissolved ions into a gas is simply unavailable. The salt ions are non-volatile and remain trapped in the liquid as the water molecules selectively escape into the vapor phase.
Outcomes: Freshwater and Salt Deposits
Evaporation from saltwater results in two distinct products: nearly pure water vapor and a highly concentrated liquid residue. The water vapor produced is essentially distilled water, as the dissolved ions and other non-volatile impurities are left behind. This natural purification is the foundation of the Earth’s water cycle, where oceans evaporate to form freshwater clouds and precipitation.
As the water continues to evaporate, the remaining liquid becomes an increasingly concentrated solution known as brine. The concentration of salt, measured as salinity, rises steadily until the solution becomes saturated, meaning it can no longer hold any more dissolved salt. At this point, the salt ions begin to precipitate, or crystallize, out of the solution.
If all the water is eventually removed, a solid layer of salt deposits remains. This process is used commercially in salt evaporation ponds, which rely on solar energy to produce sea salt. Other minerals dissolved in the water, such as calcium sulfate and magnesium salts, will also precipitate out, often in a specific sequence depending on their solubility, creating complex mineral deposits known as evaporites.