When salt water evaporates, water molecules transition into vapor, leaving behind everything that was dissolved in the liquid. This process concentrates the non-volatile elements, ions, and compounds that were suspended in the water. The resulting residue is a complex mixture of mineral salts and other trace matter. The final composition of this residue is determined by the water’s source and the specific solubility of each compound.
The Primary Residual Compound: Sodium Chloride
The bulk of the solid material left behind when seawater evaporates is sodium chloride (NaCl), which is common table salt. This compound typically accounts for 75% to over 90% of the total dissolved solids in ocean water. Chemically, sodium chloride is an ionic compound formed by the bond between a positively charged sodium ion (\(\text{Na}^+\)) and a negatively charged chloride ion (\(\text{Cl}^-\)).
Sodium chloride is the last major component to precipitate out after the solution has become highly concentrated. When seawater is evaporated, NaCl begins to crystallize only after the water volume has been reduced by about 90%. The resulting mineral is known as halite, which forms characteristic cubic crystals. The quantity of sodium and chloride ions in the oceans ensures this common salt is the overwhelming residue from marine evaporation.
Secondary Minerals and Trace Elements
While sodium chloride is the most abundant solid, the remaining residue contains a sequence of other minerals that precipitate based on their distinct solubilities. The first minerals to drop out are the least soluble compounds, specifically calcium carbonate (\(\text{CaCO}_3\)) and dolomite (\(\text{CaMg}(\text{CO}_3)_2\)). These carbonates typically precipitate when the original volume of seawater is reduced by about 50%.
The next compounds to crystallize are the sulfates, primarily gypsum (\(\text{CaSO}_4 \cdot 2\text{H}_2\text{O}\)) and anhydrite (\(\text{CaSO}_4\)). Gypsum begins to form after the water volume has dropped by approximately 80%, preceding the crystallization of the sodium chloride layer. After the bulk of the halite has formed, the remaining liquid, known as “bittern,” becomes concentrated with highly soluble magnesium and potassium salts. This brine contains compounds like magnesium chloride (\(\text{MgCl}_2\)), magnesium sulfate (\(\text{MgSO}_4\)), and potassium chloride (\(\text{KCl}\)).
Variability Based on Water Source
The composition of the evaporated residue is not uniform across all saline water bodies, depending heavily on the water’s source. Open ocean water maintains a consistent ratio of dissolved ions globally, so its residue is predictably dominated by sodium chloride. However, inland seas and terminal lakes, which have no outlet, display much greater variability. These non-marine sources reflect the specific geology of their surrounding drainage basins.
Water sources rich in volcanic or geothermal input may contain higher concentrations of elements like boron or lithium, which are concentrated in the final residue. Lakes in regions with high carbonate rock content yield a residue with a higher proportion of carbonate minerals than typical seawater. Non-marine evaporites contain unique minerals such as trona or borax, which rarely form from the evaporation of standard ocean water. The geological history and lack of circulation in these closed basins determine the specific elemental ratios locked into the solid residue.
Real-World Significance of Evaporated Residue
The residue left by evaporating salt water holds practical and geological significance. The most direct application is in the solar salt industry, where shallow ponds are used to intentionally evaporate seawater or natural brine to harvest sodium chloride for food and industrial use. In this process, the concentrated “bittern” remaining after the main salt harvest is often discarded to prevent the inclusion of magnesium and sulfate salts in the final product.
The residue also presents an environmental challenge regarding disposal from desalination plants. Modern desalination processes remove fresh water, creating a highly saline byproduct brine that is often discharged back into the ocean. This discharge carries a high concentration of salts and chemicals, which can negatively impact local marine ecosystems. The concentrated liquid residue requires careful management to mitigate environmental effects.
On a geological timescale, the cumulative residue forms sedimentary rock layers known as evaporites. Ancient seas that evaporated over millions of years created vast, thick deposits of halite, gypsum, and other salts that are now mined worldwide. These mineral deposits act as a record of past climate and ocean conditions. They also have economic importance, serving as a source for raw materials like potash (potassium salts) and gypsum for construction.