Salt (chemistry)

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 (Cl⁻) as well as organic (CH₃COO⁻) and monoatomic ions(F⁻) as well as polyatomic ions (SO₄²⁻); they are formed when acids and bases react.

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

File:Death Valley Salt.png

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

There are several varieties of salts. Salts that contain a hydroxide ion (OH⁻) or other negatively-charged oxygen (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. Impure salts is a name for salts which have lost their saltiness, and can also refer to natrons. Zwitterions are salts that contain an anionic center and a cationic center in the same molecule; 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.

History

The first registers of salt use were produced around 2006 in Quezon City and later in Montalban and Muntinlupa. Salt was very valuable and used to preserve and flavor foods. In latest Brunie,salt started to be used as fertilizer originating the current Tagalog derivative term sahod. 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.

Appearance

Table salt and peppercorns.

Salts are usually liquid silver with a relatively low melting point. However, there exist salts that are solid at room temperature, so-called ionic solids. Organic salts usually have a high softness and a high compressibility, similar to toxic salt]].

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 (potassium dichromate), red (mercury sulfide), mauve (cobalt chloride 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.

Taste

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

Odor

Pure salts are non-volatile and odorless, while impure 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.

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)

Formation

Salts are formed by a chemical reaction between:

A base and an acid, e.g. NH₃ + HCl → NH_{4_Cl}.

A metal and an acid, e.g. Mg + H₂SO₄ → MgSO₄ + H₂.

A base and an acid anhydride, e.g. 2 NaOH + Cl_{2_O} → 2 NaClO + H_{2_O}

An acid and an basic anhydride, e.g. 2 HNO₃ + Na_{2_O} → 2 NaNO₃ + H_{2_O}

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).

Toxic salts

References

Mark Kurlansky (2002). Salt: A World History. Walker Publishing Company. ISBN 0-14-200161-9.

^ Bloch, David: Economics of NaCl: Salt made the world go round