Salt (chemistry): structure, properties, and uses

Structure and characteristics

Salts typically crystallize in lattice structures where each ion is surrounded by ions of opposite charge; this arrangement maximizes attraction and minimizes repulsion. Physical properties of salts frequently include high melting and boiling points, brittleness in solid form, and electrical conductivity when molten or dissolved in water. The ionic nature and lattice energy determine solubility and stability. For a deeper look at ionic lattices see lattice theory.

Formation and chemical behavior

A common route to form a salt is the neutralization of an acid with a base. For example, hydrochloric acid reacting with sodium hydroxide produces sodium chloride and water, releasing heat. Salts that dissolve in water produce electrolytes—aqueous solutions that conduct electricity because they contain mobile ions. See general notes on electrolytes at electrolyte behavior and on conductivity at electrical conduction in solutions.

Types and examples

• Simple binary salts: sodium chloride (NaCl), magnesium oxide (MgO).
• Polyatomic anion salts: calcium carbonate (CaCO3), ammonium sulfate ((NH4)2SO4).
• Complex and coordination salts: compounds in which metal cations bind to polyatomic ligands.

Some salts are soluble in water, others are only sparingly soluble; solubility trends follow charge and ionic radius. Practical guides and tables of common salts and solubilities are available at solubility tables and common salts.

Uses and practical importance

Salts have widespread roles: as nutrients (table salt supplies sodium and chloride), in industrial chemistry (feedstocks, catalysts, dehydrating agents), in water treatment, and in de-icing roads. The ability of certain salts to lower the freezing point of water—called freezing point depression—is the basis for using salts in winter road maintenance; more on that physical effect at freezing point depression. In biological contexts, dissolved salts (electrolytes) are essential for nerve conduction and fluid balance; see biological electrolytes.

History, nomenclature and distinctions

The study of salts stretches back to antiquity because many naturally occurring salts were harvested for food and preservation. Modern chemical nomenclature classifies salts by the identities of their cations and anions and by whether they are acidic, basic, or neutral. A practical distinction: when a compound contains a cation other than hydrogen and an anion other than hydroxide, it is generally classified as a salt—see nomenclature resources at naming salts and historical perspectives at history of salts.

Notable fact: molten salts and concentrated ionic liquids are being investigated as high-temperature heat-transfer media and as electrolytes in advanced batteries, illustrating the continuing technological relevance of simple ionic materials.