Editing Salt (chemistry) (section)

==Formation==
[[File:Halite-57430.jpg|alt=White crystals form a mineral sample of halite, shown against a black background.|thumb|[[Halite]], the mineral form of [[sodium chloride]], forms when salty water evaporates leaving the ions behind.]]
[[Image:Lead(II) sulfate.jpg|thumb|Solid lead(II) sulfate (PbSO<sub>4</sub>)]]

Many metals such as the [[alkali metal]]s react directly with the [[Electronegativity|electronegative]] [[halogen]]s gases to form salts.{{sfn|Zumdahl|1989|p = 312}}{{sfn|Wold|Dwight|1993|page=71}}

Salts form upon evaporation of their [[Solution (chemistry)|solution]]s.{{sfn|Wold|Dwight|1993|page=82}} Once the solution is [[supersaturation|supersaturated]] and the solid compound nucleates.{{sfn|Wold|Dwight|1993|page=82}} This process occurs widely in nature and is the means of formation of the [[evaporite]] minerals.<ref>{{cite book|last1=Wenk|first1=Hans-Rudolf|last2=Bulakh|first2=Andrei|title=Minerals: their constitution and origin|date=2003|publisher=Cambridge University Press|location=New York|isbn=978-0-521-52958-7|page=351|edition=Reprinted with corrections.|url=https://books.google.com/books?id=Z5r5M5ebK7YC&pg=PA351|url-status=live|archive-url=https://web.archive.org/web/20171203204320/https://books.google.com/books?id=Z5r5M5ebK7YC&lpg=PA358&pg=PA351|archive-date=2017-12-03}}</ref> 

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 [[counterion]]s of the opposite charges. To ensure that these do not contaminate the precipitated salt, it is important to ensure they do not also precipitate.{{sfn|Zumdahl|1989|p=133–140}} 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#Arrhenius theory|acid–base reaction]] or a [[neutralization reaction]] to form water.{{sfn|Zumdahl|1989|p=144–145}} Alternately the counterions can be chosen to ensure that even when combined into a single solution they will remain soluble as [[spectator ions]].{{sfn|Zumdahl|1989|p=133–140}}

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.{{sfn|Brown|2009|page=417}}

Molten salts will solidify on cooling to below their [[freezing point]].{{sfn|Wold|Dwight|1993|page=79}} This is sometimes used for the [[Solid-state chemistry|solid-state synthesis]] of complex salts from solid reactants, which are first melted together.{{sfn|Wold|Dwight|1993|pages=79–81}} 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.{{sfn|Wold|Dwight|1993|page=71}} Other synthetic routes use a solid precursor with the correct [[Stoichiometry|stoichiometric]] ratio of non-volatile ions, which is heated to drive off other species.{{sfn|Wold|Dwight|1993|page=71}}

In some reactions between highly reactive metals (usually from [[Alkali metal|Group 1]] or [[Alkaline earth metal|Group 2]]) and highly electronegative halogen gases, or water, the atoms can be ionized by [[electron transfer]],{{sfn|Zumdahl|1989|p=312–313}} a process thermodynamically understood using the [[Born–Haber cycle]].{{sfn|Barrow|1988|p=161–162}}

Salts are formed by [[salt-forming reaction]]s

* A [[base (chemistry)|base]] and an [[acid]], e.g., [[ammonia|NH<sub>3</sub>]] + [[hydrochloric acid|HCl]] → [[ammonium chloride|NH<sub>4</sub>Cl]]
* A [[metal]] and an [[acid]], e.g., [[magnesium|Mg]] + [[sulfuric acid|H<sub>2</sub>SO<sub>4</sub>]] → [[magnesium sulfate|MgSO<sub>4</sub>]] + [[hydrogen|H<sub>2</sub>]]
* A metal and a non-metal, e.g., [[calcium|Ca]] + [[chlorine|Cl<sub>2</sub>]] → [[calcium chloride|CaCl<sub>2</sub>]]
* A [[base (chemistry)|base]] and an [[acid anhydride]], e.g., 2&nbsp;[[Sodium hydroxide|NaOH]] + [[Dichlorine monoxide|Cl<sub>2</sub>O]] → 2&nbsp;[[Sodium hypochlorite|NaClO]] + [[Water|H<sub>2</sub>O]]
* An [[acid]] and a [[base anhydride]], e.g., 2&nbsp;[[nitric acid|HNO<sub>3</sub>]] + [[Sodium oxide|Na<sub>2</sub>O]] → 2&nbsp;[[Sodium nitrate|NaNO<sub>3</sub>]] + [[Water|H<sub>2</sub>O]]
* 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(NO<sub>3</sub>)<sub>2</sub> + Na<sub>2</sub>SO<sub>4</sub> → PbSO<sub>4</sub>↓ + 2&nbsp;NaNO<sub>3</sub>