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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.

<p><strong>SF Fig. 2.1.</strong> 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.</p><br />


 

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
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

 

Special Feature Type:

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.