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chemistry Chemistry is the scientific study of the properties and behavior of matter. It is a natural science that covers the elements that make up matter to the compounds made of atoms, molecules and ions: their composition, structure, proper ...
, 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 Electric charge is the physical property of matter that causes charged matter to experience a force when placed in an electromagnetic field. Electric charge can be ''positive'' or ''negative'' (commonly carried by protons and electrons res ...
. A common example is table salt, with positively charged
sodium Sodium is a chemical element with the symbol Na (from Latin ''natrium'') and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 of the periodic table. Its only stable ...
ions and negatively charged chloride ions. The component ions in a salt compound can be either inorganic, such as chloride (Cl), or
organic Organic may refer to: * Organic, of or relating to an organism, a living entity * Organic, of or relating to an anatomical organ Chemistry * Organic matter, matter that has come from a once-living organism, is capable of decay or is the product ...
, such as acetate (). Each ion can be either monatomic, such as fluoride (F), or polyatomic, such as sulfate ().


Types of salt

Salts can be classified in a variety of ways. Salts that produce
hydroxide Hydroxide is a diatomic anion with chemical formula OH−. It consists of an oxygen and hydrogen atom held together by a single covalent bond, and carries a negative electric charge. It is an important but usually minor constituent of water. ...
ions when dissolved in
water Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as ...
are called '' alkali salts'' and salts that produce
hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-to ...
ions when dissolved in
water Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as ...
are called '' acid salts''. ''Neutral salts'' are those salts that are neither acidic nor basic. Zwitterions contain an anionic and a cationic centre in the same
molecule A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and b ...
, but are not considered salts. Examples of zwitterions are
amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha ...
s, many metabolites,
peptide Peptides (, ) are short chains of amino acids linked by peptide bonds. Long chains of amino acids are called proteins. Chains of fewer than twenty amino acids are called oligopeptides, and include dipeptides, tripeptides, and tetrapeptides. ...
s, and
protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...
s.


Properties


Color

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 Sodium chromate is the inorganic compound with the formula Na2CrO4. It exists as a yellow hygroscopic solid, which can form tetra-, hexa-, and decahydrates. It is an intermediate in the extraction of chromium from its ores. Production and react ...
is made yellow by the
chromate ion Chromate salts contain the chromate anion, . Dichromate salts contain the dichromate anion, . They are oxyanions of chromium in the +6 oxidation state and are moderately strong oxidizing agents. In an aqueous solution, chromate and dichromate ...
* potassium dichromate is made orange by the dichromate ion *
cobalt nitrate Cobalt nitrate can refer to: *Cobalt(II) nitrate Cobalt is a chemical element with the symbol Co and atomic number 27. As with nickel, cobalt is found in the Earth's crust only in a chemically combined form, save for small deposits found in allo ...
is made red by the chromophore of hydrated cobalt(II) ( o(H2O)6sup>2+). * copper sulfate is made blue by the copper(II) chromophore * potassium permanganate is made violet by the
permanganate A permanganate () is a chemical compound containing the manganate(VII) ion, , the conjugate base of permanganic acid. Because the manganese atom is in the +7 oxidation state, the permanganate(VII) ion is a strong oxidizing agent. The ion is a ...
anion. *
nickel chloride Nickel(II) chloride (or just nickel chloride) is the chemical compound NiCl2. The anhydrous salt is yellow, but the more familiar hydrate NiCl2·6H2O is green. Nickel(II) chloride, in various forms, is the most important source of nickel for che ...
is typically made green by the hydrated nickel(II) chloride iCl2(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. Few
minerals In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed ...
are salts, because they would be solubilized by water. Similarly, inorganic pigments tend not to be salts, because insolubility is required for fastness. Some organic
dye A dye is a colored substance that chemically bonds to the substrate to which it is being applied. This distinguishes dyes from pigments which do not chemically bind to the material they color. Dye is generally applied in an aqueous solution and ...
s are salts, but they are virtually insoluble in water.


Taste

Different salts can elicit all five
basic taste The gustatory system or sense of taste is the sensory system that is partially responsible for the perception of taste (flavor). Taste is the perception produced or stimulated when a substance in the mouth reacts chemically with taste receptor ...
s, e.g., salty ( sodium chloride), sweet (
lead diacetate Lead(II) acetate (Pb(CH3COO)2), also known as lead acetate, lead diacetate, plumbous acetate, sugar of lead, lead sugar, salt of Saturn, or Goulard's powder, is a white crystalline chemical compound with a slightly sweet taste. Like many other le ...
, which will cause lead poisoning if ingested), sour ( potassium bitartrate), bitter ( magnesium sulfate), and umami or
savory Savory or Savoury may refer to: Common usage * Herbs of the genus ''Satureja'', particularly: ** Summer savory (''Satureja hortensis''), an annual herb, used to flavor food ** Winter savory (''Satureja montana''), a perennial herb, also used to ...
( monosodium glutamate).


Odor

Salts of strong acids and strong bases (" strong salts") are non- volatile and often odorless, whereas salts of either weak acids or weak bases (" weak salts") may smell like the
conjugate acid A conjugate acid, within the Brønsted–Lowry acid–base theory, is a chemical compound formed when an acid donates a proton () to a base—in other words, it is a base with a hydrogen ion added to it, as in the reverse reaction it loses a ...
(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 the presence of water, since
hydrolysis Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution, elimination, and solvation reactions in which water is the nucleophile. Biological hydrolysi ...
is the other half of the
reversible reaction A reversible reaction is a reaction in which the conversion of reactants to products and the conversion of products to reactants occur simultaneously. : \mathit aA + \mathit bB \mathit cC + \mathit dD A and B can react to form C and D or, in the ...
equation of formation of weak salts.


Solubility

Many ionic compounds exhibit significant solubility in water or other polar solvents. Unlike molecular compounds, salts dissociate in solution into anionic and cationic components. The
lattice energy In chemistry, the lattice energy is the energy change upon formation of one mole of a crystalline ionic compound from its constituent ions, which are assumed to initially be in the gaseous state. It is a measure of the cohesive forces that bin ...
, the cohesive forces between these ions within a solid, determines the solubility. The solubility is dependent on how well each ion interacts with the solvent, so certain patterns become apparent. For example, salts of
sodium Sodium is a chemical element with the symbol Na (from Latin ''natrium'') and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 of the periodic table. Its only stable ...
,
potassium Potassium is the chemical element with the symbol K (from Neo-Latin '' kalium'') and atomic number19. Potassium is a silvery-white metal that is soft enough to be cut with a knife with little force. Potassium metal reacts rapidly with atmos ...
and ammonium are usually soluble in water. Notable exceptions include ammonium hexachloroplatinate and potassium cobaltinitrite. Most nitrates and many sulfates are water-soluble. Exceptions include barium sulfate, calcium sulfate (sparingly soluble), and lead(II) sulfate, where the 2+/2− pairing leads to high lattice energies. For similar reasons, most metal carbonates are not soluble in water. Some soluble carbonate salts are: sodium carbonate, potassium carbonate and ammonium carbonate.


Conductivity

Salts are characteristically insulators. Molten salts or solutions of salts conduct electricity. For this reason, liquified (molten) salts and solutions containing dissolved salts (e.g., sodium chloride in water) can be used as electrolytes.


Melting point

Salts characteristically have high melting points. For example, sodium chloride melts at 801 °C. Some salts with low lattice energies are liquid at or near room temperature. These include molten salts, which are usually mixtures of salts, and ionic liquids, which usually contain organic cations. These liquids exhibit unusual properties as solvents.


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 salt'' or ''acetate salt''). Common salt-forming cations include: * Ammonium *
Calcium Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar ...
*
Iron Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in ...
and * Magnesium *
Potassium Potassium is the chemical element with the symbol K (from Neo-Latin '' kalium'') and atomic number19. Potassium is a silvery-white metal that is soft enough to be cut with a knife with little force. Potassium metal reacts rapidly with atmos ...
* Pyridinium * Quaternary ammonium , R being an alkyl group or an
aryl In organic chemistry, an aryl is any functional group or substituent derived from an aromatic ring, usually an aromatic hydrocarbon, such as phenyl and naphthyl. "Aryl" is used for the sake of abbreviation or generalization, and "Ar" is used ...
group *
Sodium Sodium is a chemical element with the symbol Na (from Latin ''natrium'') and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 of the periodic table. Its only stable ...
*
Copper Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pink ...
Common salt-forming anions (parent acids in parentheses where available) include: * Acetate ( acetic acid) * Carbonate ( carbonic acid) * Chloride ( hydrochloric acid) * Citrate (
citric acid Citric acid is an organic compound with the chemical formula HOC(CO2H)(CH2CO2H)2. It is a colorless weak organic acid. It occurs naturally in citrus fruits. In biochemistry, it is an intermediate in the citric acid cycle, which occurs in ...
) *
Cyanide Cyanide is a naturally occurring, rapidly acting, toxic chemical that can exist in many different forms. In chemistry, a cyanide () is a chemical compound that contains a functional group. This group, known as the cyano group, consists of ...
( hydrocyanic acid) * Fluoride ( hydrofluoric acid) * Nitrate ( nitric acid) * Nitrite ( nitrous acid) * Oxide (
water Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as ...
) * Phosphate ( phosphoric acid) * Sulfate ( sulfuric acid) Salts with varying number of hydrogen atoms replaced by cations as compared to their parent acid can be referred to as ''monobasic'', ''dibasic'', or ''tribasic'', identifying that one, two, or three hydrogen atoms have been replaced; ''polybasic'' salts refer to those with more than one hydrogen atom replaced. Examples include: * Sodium phosphate monobasic (NaH2PO4) * Sodium phosphate dibasic (Na2HPO4) * Sodium phosphate tribasic (Na3PO4)


Formation

Salts are formed by a chemical reaction between: *A base and an acid, e.g., NH3 +
HCl HCL may refer to: Science and medicine * Hairy cell leukemia, an uncommon and slowly progressing B cell leukemia * Harvard Cyclotron Laboratory, from 1961 to 2002, a proton accelerator used for research and development * Hollow-cathode lamp, a s ...
NH4Cl *A
metal A metal (from ancient Greek, Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electrical resistivity and conductivity, e ...
and an acid, e.g., Mg + H2SO4MgSO4 + H2 *A metal and a non-metal, e.g., Ca + Cl2CaCl2 *A base and an acid anhydride, e.g., 2  NaOH + Cl2O → 2  NaClO + H2O *An acid and a
base anhydride A base anhydride is an oxide of a chemical element from group 1 or 2 (the alkali metals and alkaline earth metals, respectively). They are obtained by removing water from the corresponding hydroxide base. If water is added to a base anhydride, a ...
, e.g., 2  HNO3 + Na2O → 2  NaNO3 + H2O *In the salt metathesis reaction where two different salts are mixed in water, their ions recombine, and the new salt is insoluble and precipitates. For example: *: Pb(NO3)2 + Na2SO4 → PbSO4↓ + 2 NaNO3


Strong salt

Strong salts or strong electrolyte salts are chemical salts composed of strong electrolytes. These ionic compounds dissociate completely in
water Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as ...
. They are generally odorless and nonvolatile. Strong salts start with Na__, K__, NH4__, or they end with __NO3, __ClO4, or __CH3COO. Most group 1 and 2 metals form strong salts. Strong salts are especially useful when creating conductive compounds as their constituent ions allow for greater conductivity.


Weak salt

Weak salts or "weak electrolyte salts" are, as the name suggests, composed of weak electrolytes. They are generally more volatile than strong salts. They may be similar in odor to the acid or base they are derived from. For example, sodium acetate, CH3COONa, smells similar to acetic acid CH3COOH.


See also

*
Bresle method The Bresle method is used to determine concentration of soluble salts on metal surfaces prior to coating application, such as painting. These salts can cause serious adhesion problems after time. Importance Salt is in coastal areas. It can be t ...
(the method used to test for salt presence during coating applications) * Carboxylate * Fireworks/pyrotechnics (salts are what give color to fireworks) * Halide * Ionic bonds * Natron * Salinity


References

* Mark Kurlansky (2002). ''Salt: A World History''. Walker Publishing Company. . {{Authority control Chemical compounds Alchemical substances