Salt

From Wikipedia, the free encyclopedia.

This article is about the general chemical term salt. For the everyday meaning, see edible salt or its main ingredient, sodium chloride. For other meanings of the word salt, see salt (disambiguation).

In chemistry, salt is a term used for ionic compounds composed of positively charged cations and negatively charged anions, so that the product is neutral and without a net charge. These ions can be inorganic (Cl^-) as well as organic (CH₃-COO^-) and monoatomic (F^-) as well as polyatomic ions (SO₄^2-).

Solutions of salts in water are called electrolytes. Electrolytes as well as molten salts conduct electricity.

Zwitterions are salts that contain an anionic center and a cationic center in the same molecule, examples are the amino acids, many metabolites, peptides, and proteins.

Mixtures of many different ions in solution like in the cytoplasm of cells, in blood, urine, plant saps, and mineral waters usually do not form defined salts after evaporation of the water. Therefore their salt content is given for the respective ions.

Impure salt is a name for salt which has lost its saltiness. It can also refer to natron.

1 Appearance
2 Nomenclature
3 Formation
4 References
5 See also

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Appearance

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Consistency

Salts are usually solid crystals with a relatively high melting point. However, there exist salts that are liquid at room temperature, so-called ionic liquids. Inorganic salts usually have a low hardness and a low compressibility, similar to edible salt.

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Color

Salts can be clear and transparent (sodium chloride), opaque (titanium dioxide), and even metallic and lustrous (iron disulfide).

Salts exist in all different colors, e.g. yellow (sodium chromate), orange (sodium dichromate), red (mercury sulfide), mauve (cobalt dichloride hexahydrate), blue (copper sulfate pentahydrate, ferric hexacyanoferrate), green (nickel oxide), colorless (magnesium sulfate), white (titanium dioxide), and black (manganese dioxide). Most minerals and inorganic pigments as well as many synthetic organic dyes are salts.

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Taste

Different salts can elicit all five basic tastes, i.e. salty (sodium chloride), sweet (lead diacetate), sour (potassium bitartrate), bitter (magnesium sulfate), and umami or savory (monosodium glutamate).

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Odor

Pure salts are odorless, while impure salts may smell after the acid (e.g. acetates like acetic acid (vinegar), cyanides like hydrogen cyanide (almonds)) or the base (e.g. ammonium salts like ammonia).

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Nomenclature

The name of a salt starts with the name of the cation (e.g. sodium or ammonium) followed by the name of the anion (e.g. chloride or acetate). Salts are often referred to only by the name of the cation (e.g. sodium salt or ammonium salt) or by the name of the anion (e.g. chloride or acetate).

Common salt-forming cations are:

ammonium NH₄⁺
calcium Ca²⁺
iron Fe²⁺ and Fe³⁺
magnesium Mg²⁺
potassium K⁺
pyridinium C₅H₅NH⁺
quaternary ammonium NR₄⁺
sodium Na⁺

Common salt-forming anions (and the name of the parent acids in parentheses) are:

acetate CH₃-COO^- (acetic acid)
carbonate CO₃^2- (carbonic acid)
chloride Cl^- (hydrochloric acid)
citrate HO-C(COO^-)(CH₂-COO^-)₂ (citric acid)
cyanide C≡N^- (hydrogen cyanide)
hydroxide OH^- (water)
nitrate NO₃^- (nitric acid)
nitrite NO₂^- (nitrous acid)
oxide O^2- (water)
phosphate PO₄^3- (phosphoric acid)
sulfate SO₄^2- (sulfuric acid)

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Formation

Salts are formed by a chemical reaction between:

a base and an acid, e.g. NH₃ + HCl → NH₄Cl
a metal and an acid, e.g. Mg + H₂SO₄ → MgSO₄ + H₂

Salts can also form if solutions of different salts are mixed, their ions recombine, and the new salt is insoluble and precipitates (see: Solubility equilibrium).

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