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.
SF Fig. 2.1. Salt rings formed by evaporation of seawater on watch glass. The blue ring is the outermost, least soluble salt. The orange salt ring is the most soluble salt.
Image by Joanna Philippoff and Brittany Supnet
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. SF Fig. 2.1 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.
SF Table 2.1 details the solubility of each of the main salts that precipitate out of seawater and are shown in the rings of SF Fig. 2.1. The tastes of these salts vary based on their chemical composition.
| Salt Compound | Picture | Solubility | Taste |
|---|---|---|---|
| CaCO3 (calcium carbonate) |
Image
Image copyright and source
Image courtesy of Ondrej Mangl |
Insoluble at 50% evaporation (≈70 ppt) |
Chalky |
|
CaSO4·H2O |
Image
Image copyright and source
Image courtesy of Wilson44691 |
Insoluble at 80% evaporation (≈100 ppt) |
Chalky |
| NaCl (sodium chloride or halite) |
Image
Image copyright and source
Image courtesy of NASA |
Insoluble at 90% evaporation (≈130 ppt) |
Salty |
| KCl and MgCl2 (potassium and magnesium salts) |
Image
Image copyright and source
Image courtesy of Romain Behar |
Insoluble at >95% evaporation (>150 ppt) |
Salty |
