Salt

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This article is about the term salt as referred to in chemistry. For the everyday meaning, see edible salt or its main ingredient, sodium chloride. For other meanings of the word salt, see salt (disambiguation).

A magnified crystal of a salt (halite/sodium chloride)

Salt covering the floor of Bad Water in Death Valley, CA, the lowest point in the US.

A salt, in chemistry, is any ionic compound composed of cations (positively charged ions) and anions (negative ions) so that the product is neutral (without a net charge). These component ions can be inorganic such as chloride (Cl⁻), as well as organic such as acetate (CH₃COO⁻) and monoatomic ions such as fluoride (F⁻), as well as polyatomic ions such as sulfate (SO₄²⁻). Salts are formed when acids and bases react with one another.

There are several varieties of salts. Salts that contain a hydroxide ion (OH⁻) or some other negatively-charged oxygen containing ions (such as carbonate and phosphate) are basic salts and salts that contain a hydrogen ion (H⁺) are acid salts. Normal salts are those that are neither acid nor basic salts. Zwitterions contain an anionic center and a cationic center in the same molecule but are not considered to be salts. Examples include amino acids, many metabolites, peptides and proteins.

When salts are dissolved in water, they are called electrolytes, and are able to conduct electricity, a property that is shared with molten salts. 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.

Salts can be dehydrating to the human body if consumed in excess.

1 History
2 Appearance
3 Nomenclature
4 Formation
5 References
6 See also

[edit] History

Main article: History of salt

It has been suggested that this article or section be merged into Edible salt#History of edible salt. (Discuss)

The first registers of salt use were produced around 4000 B.C. in Egypt, and later in Greece and Rome. Salt was very valuable and used to preserve and flavor foods. In Ancient Rome, salt was used as a currency originating the Latin-derived term salary. Unfortunately for those paid with salt, it was easily ruined by rain and other factors. Payments to Roman workers were made in salt.^[1] Salt was also given to the parents of the groom in marriage until the 8th century.

From the Phoenicians dates the evidence of harvesting solid salt from the sea. They also exported it to other civilizations. As a result of the increased salt supply from the sea, the value of salt depreciated. The harvest method used was flooding plains of land with seawater, then leaving the plains to dry. After the water dried, the salt which was left was collected and sold.

[edit] Appearance

Table salt and peppercorns.

[edit] Color

Salts can appear to be clear and transparent (sodium chloride), opaque, and even metallic and lustrous (iron disulfide). 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 phase boundaries, larger crystals tend to be transparent, while poly-crystalline aggregates look like white powders. Of course, some salts are inherently opaque.

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

[edit] Taste

Different salts can elicit all five basic tastes, e.g. salty (sodium chloride), sweet (lead diacetate Warning: lead diacetate is extremely toxic and therefore, should not be ingested), sour (potassium bitartrate), bitter (magnesium sulfate), and umami or savory (monosodium glutamate).

[edit] Odor

Salts of strong acids and strong bases ("strong salts") are non-volatile and odorless, while salts of either weak acids or weak bases ("weak salts") may smell after the conjugate acid (e.g. acetates like acetic acid (vinegar) and cyanides like hydrogen cyanide (almonds)) or the conjugate base (e.g. ammonium salts like ammonia) of the component ions. That slow, partial decomposition is usually accelerated by presence of water, since hydrolysis is the other half of the reversible reaction equation of formation of weak salts.

[edit] Nomenclature

Various salt minerals

Chemical composition of sea salt

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 include:

ammonium NH₄⁺
calcium Ca²⁺
iron Fe²⁺ and Fe ³⁺
magnesium Mg²⁺
potassium K⁺
pyridinium C₅H₅NH⁺
quaternary ammonium NR₄⁺
sodium Na⁺

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

acetate CH₃COO⁻ (acetic acid)
carbonate CO₃²⁻ (carbonic acid)
chloride Cl⁻ (hydrochloric acid)
citrate HOC(COO⁻)(CH₂COO⁻)₂ (citric acid)
cyanide C≡N⁻ (hydrogen cyanide)
hydroxide OH⁻ (water)
nitrate NO₃⁻ (nitric acid)
nitrite NO₂⁻ (nitrous acid)
oxide O²⁻ (water)
phosphate PO₄³⁻ (phosphoric acid)
sulfate SO₄²⁻ (sulfuric acid)

[edit] 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.

A base and an acid anhydride, e.g. 2 NaOH + Cl₂O → 2 NaClO + H₂O

An acid and an basic anhydride, e.g. 2 HNO₃ + Na₂O → 2 NaNO₃ + H₂O

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).