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Weird Science: Types of Salts in Seawater

NGSS Disciplinary Core Ideas

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.


Image caption

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 copyright and source

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.


SF Table 2.1. Solubility of salt compounds in seawater.
Salt Compound Picture Solubility Taste
(calcium carbonate)
Image copyright and source

Image courtesy of Ondrej Mangl

Insoluble at 50% evaporation
(≈70 ppt)


Image copyright and source

Image courtesy of Wilson44691

Insoluble at 80% evaporation
(≈100 ppt)
(sodium chloride or halite)
Image copyright and source

Image courtesy of NASA

Insoluble at 90% evaporation
(≈130 ppt)
KCl and MgCl2
(potassium and magnesium salts)
Image copyright and source

Image courtesy of Romain Behar

Insoluble at >95% evaporation
(>150 ppt)


Exploring Our Fluid Earth, a product of the Curriculum Research & Development Group (CRDG), College of Education. University of Hawaii, 2011. This document may be freely reproduced and distributed for non-profit educational purposes.