Salt (chemistry)
In chemistry, a salt or ionic compound is a chemical compound consisting of an assembly of positively charged ions (cations) and negatively charged ions (anions),[1] which results in a compound with no net electric charge (electrically neutral). The constituent ions are held together by electrostatic forces termed ionic bonds.
"Ionic compound" redirects here. Not to be confused with Salt or Sodium chloride.
The component ions in a salt can be either inorganic, such as chloride (Cl−), or organic, such as acetate (CH
3COO−
). Each ion can be either monatomic (termed simple ion), such as fluoride (F−), and sodium (Na+) and chloride (Cl−) in sodium chloride, or polyatomic, such as sulfate (SO2−
4), and ammonium (NH+
4) and carbonate (CO2−
3) ions in ammonium carbonate. Salt containing basic ions hydroxide (OH−) or oxide (O2−) are classified as bases, for example sodium hydroxide.
Individual ions within a salt usually have multiple near neighbours, so they are not considered to be part of molecules, but instead part of a continuous three-dimensional network. Salts usually form crystalline structures when solid.
Salts composed of small ions typically have high melting and boiling points, and are hard and brittle. As solids they are almost always electrically insulating, but when melted or dissolved they become highly conductive, because the ions become mobile. Some salts have large cations, large anions, or both. In terms of their properties, such species often are more similar to organic compounds.
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Uses[edit]
Salts have long had a wide variety of uses and applications. Many minerals are ionic.[82] Humans have processed common salt (sodium chloride) for over 8000 years, using it first as a food seasoning and preservative, and now also in manufacturing, agriculture, water conditioning, for de-icing roads, and many other uses.[83] Many salts are so widely used in society that they go by common names unrelated to their chemical identity. Examples of this include borax, calomel, milk of magnesia, muriatic acid, oil of vitriol, saltpeter, and slaked lime.[84]
Soluble salts can easily be dissolved to provide electrolyte solutions. This is a simple way to control the concentration and ionic strength. The concentration of solutes affects many colligative properties, including increasing the osmotic pressure, and causing freezing-point depression and boiling-point elevation.[85] Because the solutes are charged ions they also increase the electrical conductivity of the solution.[86] The increased ionic strength reduces the thickness of the electrical double layer around colloidal particles, and therefore the stability of emulsions and suspensions.[87]
The chemical identity of the ions added is also important in many uses. For example, fluoride containing compounds are dissolved to supply fluoride ions for water fluoridation.[88]
Solid salts have long been used as paint pigments, and are resistant to organic solvents, but are sensitive to acidity or basicity.[89] Since 1801 pyrotechnicians have described and widely used metal-containing salts as sources of colour in fireworks.[90] Under intense heat, the electrons in the metal ions or small molecules can be excited.[91] These electrons later return to lower energy states, and release light with a colour spectrum characteristic of the species present.[92][93]
In chemical synthesis, salts are often used as precursors for high-temperature solid-state synthesis.[94]
Many metals are geologically most abundant as salts within ores.[95] To obtain the elemental materials, these ores are processed by smelting or electrolysis, in which redox reactions occur (often with a reducing agent such as carbon) such that the metal ions gain electrons to become neutral atoms.[96][97]
According to the nomenclature recommended by IUPAC, salts are named according to their composition, not their structure.[98] In the most simple case of a binary salt with no possible ambiguity about the charges and thus the stoichiometry, the common name is written using two words.[99] The name of the cation (the unmodified element name for monatomic cations) comes first, followed by the name of the anion.[100][101] For example, MgCl2 is named magnesium chloride, and Na2SO4 is named sodium sulfate (SO2−
4, sulfate, is an example of a polyatomic ion). To obtain the empirical formula from these names, the stoichiometry can be deduced from the charges on the ions, and the requirement of overall charge neutrality.[102]
If there are multiple different cations and/or anions, multiplicative prefixes (di-, tri-, tetra-, ...) are often required to indicate the relative compositions,[103] and cations then anions are listed in alphabetical order.[104] For example, KMgCl3 is named magnesium potassium trichloride to distinguish it from K2MgCl4, magnesium dipotassium tetrachloride[105] (note that in both the empirical formula and the written name, the cations appear in alphabetical order, but the order varies between them because the symbol for potassium is K).[106] When one of the ions already has a multiplicative prefix within its name, the alternate multiplicative prefixes (bis-, tris-, tetrakis-, ...) are used.[107] For example, Ba(BrF4)2 is named barium bis(tetrafluoridobromate).[108]
Compounds containing one or more elements which can exist in a variety of charge/oxidation states will have a stoichiometry that depends on which oxidation states are present, to ensure overall neutrality. This can be indicated in the name by specifying either the oxidation state of the elements present, or the charge on the ions.[108] Because of the risk of ambiguity in allocating oxidation states, IUPAC prefers direct indication of the ionic charge numbers.[108] These are written as an arabic integer followed by the sign (... , 2−, 1−, 1+, 2+, ...) in parentheses directly after the name of the cation (without a space separating them).[108] For example, FeSO4 is named iron(2+) sulfate (with the 2+ charge on the Fe2+ ions balancing the 2− charge on the sulfate ion), whereas Fe2(SO4)3 is named iron(3+) sulfate (because the two iron ions in each formula unit each have a charge of 3+, to balance the 2− on each of the three sulfate ions).[108] Stock nomenclature, still in common use, writes the oxidation number in Roman numerals (... , −II, −I, 0, I, II, ...). So the examples given above would be named iron(II) sulfate and iron(III) sulfate respectively.[109] For simple ions the ionic charge and the oxidation number are identical, but for polyatomic ions they often differ. For example, the uranyl(2+) ion, UO2+
2, has uranium in an oxidation state of +6, so would be called a dioxouranium(VI) ion in Stock nomenclature.[110] An even older naming system for metal cations, also still widely used, appended the suffixes -ous and -ic to the Latin root of the name, to give special names for the low and high oxidation states.[111] For example, this scheme uses "ferrous" and "ferric", for iron(II) and iron(III) respectively,[111] so the examples given above were classically named ferrous sulfate and ferric sulfate.
Common salt-forming cations include:
Common salt-forming anions (parent acids in parentheses where available) include:
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:
Many metals such as the alkali metals react directly with the electronegative halogens gases to salts.[7][8]
Salts form upon evaporation of their solutions.[9] Once the solution is supersaturated and the solid compound nucleates.[9] This process occurs widely in nature and is the means of formation of the evaporite minerals.[10]
Insoluble salts can be precipitated by mixing two solutions, one with the cation and one with the anion in it. Because all solutions are electrically neutral, the two solutions mixed must also contain counterions of the opposite charges. To ensure that these do not contaminate the precipitated salt, it is important to ensure they do not also precipitate.[11] If the two solutions have hydrogen ions and hydroxide ions as the counterions, they will react with one another in what is called an acid–base reaction or a neutralization reaction to form water.[12] Alternately the counterions can be chosen to ensure that even when combined into a single solution they will remain soluble as spectator ions.[11]
If the solvent is water in either the evaporation or precipitation method of formation, in many cases the ionic crystal formed also includes water of crystallization, so the product is known as a hydrate, and can have very different chemical properties compared to the anhydrous material.[13]
Molten salts will solidify on cooling to below their freezing point.[14] This is sometimes used for the solid-state synthesis of complex salts from solid reactants, which are first melted together.[15] In other cases, the solid reactants do not need to be melted, but instead can react through a solid-state reaction route. In this method, the reactants are repeatedly finely ground into a paste and then heated to a temperature where the ions in neighboring reactants can diffuse together during the time the reactant mixture remains in the oven.[8] Other synthetic routes use a solid precursor with the correct stoichiometric ratio of non-volatile ions, which is heated to drive off other species.[8]
In some reactions between highly reactive metals (usually from Group 1 or Group 2) and highly electronegative halogen gases, or water, the atoms can be ionized by electron transfer,[16] a process thermodynamically understood using the Born–Haber cycle.[17]
Salts are formed by salt-forming reactions
Zwitterion[edit]
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.[112]
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Strength[edit]
Strong salts or strong electrolyte salts are chemical salts composed of strong electrolytes. These salts dissociate completely or almost completely in water. 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 salts or weak electrolyte salts are composed of weak electrolytes. These salts do not dissociate well in water. 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.
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