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Concept
Salt (chemistry)
Summary
In chemistry, a salt is a chemical compound consisting of an ionic assembly of positively charged cations and negatively charged anions, which results in a compound with no net electric charge. A common example is table salt, with positively charged sodium ions and negatively charged chloride ions.
The component ions in a salt compound can be either inorganic, such as chloride (Cl−), or organic, such as acetate (CH3COO−). Each ion can be either monatomic, such as fluoride (F−), or polyatomic, such as sulfate (SO42−).
Salts can be classified in a variety of ways. Salts that produce hydroxide ions when dissolved in water are called alkali salts and salts that produce hydrogen ions when dissolved in water are called acid salts. Neutral salts are those salts that are neither acidic nor alkaline. Zwitterions contain an anionic and a cationic centre in the same molecule, but are not considered salts. Examples of zwitterions are amino acids, many metabolites, peptides, and proteins.
Solid salts tend to be transparent, as illustrated by sodium chloride. In many cases, the apparent opacity or transparency are only related to the difference in size of the individual monocrystals. Since light reflects from the grain boundaries (boundaries between crystallites), larger crystals tend to be transparent, while the polycrystalline aggregates look like opaque powders or masses.
Salts exist in many different colors, which arise either from their constituent anions, cations or solvates. For example:
sodium chromate is made yellow by the chromate ion
potassium dichromate is made orange by the dichromate ion
cobalt nitrate is made red by the chromophore of hydrated cobalt(II) ([Co(H2O)6]2+).
copper sulfate is made blue by the copper(II) chromophore
potassium permanganate is made violet by the permanganate anion.
nickel chloride is typically made green by the hydrated nickel(II) chloride [NiCl2(H2O)4]
sodium chloride, magnesium sulfate heptahydrate appear colorless or white because the constituent cations and anions do not absorb light in the part of the spectrum that is visible to humans.
Official source
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