Weird Science: Types of Salts in Seawater

Sea salt is composed of many different kinds of salts. These salts have different chemical compositions and properties. If seawater evaporates from a surface with a slight curve, the different types of salt appear as distinct rings due to their different solubilities.

As water evaporates from seawater, the salinity of the remaining solution increases. Different salts become insoluble at different salinities. When a salt becomes insoluble, it precipitates (falls out of) solution and forms crystals. Figure 1 identifies some of the salt rings formed when seawater was evaporated from a watch glass. The outer ring, which precipitated out of solution first, is primarily made up of calcium carbonate (CaCO₃). Carbonates are the least soluble salts in seawater. The inner ring is primarily made up of potassium (KCl) and magnesium (MgCl₂) salts, which are very soluble.

Table 1 details the solubility of each of the main salts that precipitate out of seawater and are shown in the rings of Fig. 1. The tastes of these salts vary based on their chemical composition.

Table 1. Solubility of salt compounds in seawater.

Salt Compound	Picture	Solubility	Taste
CaCO3 (calcium carbonate)		Insoluble at 50% evaporation (≈70 ppt)	Chalky
CaSO4·H2O (gypsum)		Insoluble at 80% evaporation (≈100 ppt)	Chalky
NaCl (sodium chloride or halite)		Insoluble at 90% evaporation (≈130 ppt)	Salty
KCl and MgCl2 (potassium and magnesium salts)		Insoluble at >95% evaporation (>150 ppt)	Salty