Editing Sodium hydroxide (section)

===Physical properties===
Pure sodium hydroxide is a colorless crystalline solid that melts at {{convert|318|C|F}} without decomposition and boils at {{convert|1388|C|F}}. It is highly soluble in water, with a lower solubility in [[Chemical polarity|polar]] [[solvent]]s such as [[ethanol]] and [[methanol]].<ref name="Protank-2018">{{cite news|url=https://www.protank.com/sodium-hydroxide|title=Sodium Hydroxide Storage Tanks & Specifications|date=2018-09-08|work=Protank|access-date=2018-11-21|language=en-US}}</ref> Sodium hydroxide is insoluble in [[ether]] and other non-polar solvents.

Similar to the hydration of sulfuric acid, [[dissolution (chemistry)|dissolution]] of solid sodium hydroxide in water is a highly [[exothermic reaction]]<ref>{{cite web |last=<!-- not stated --> |date=August 29, 2014 |title=Exothermic vs. Endothermic: Chemistry's Give and Take |url=https://www.discoveryexpresskids.com/blog/exothermic-vs-endothermic-chemistrys-give-and-take |access-date=March 28, 2023 |website=Discovery Express Kids |archive-date=March 28, 2023 |archive-url=https://web.archive.org/web/20230328171940/https://www.discoveryexpresskids.com/blog/exothermic-vs-endothermic-chemistrys-give-and-take |url-status=dead }}</ref> where a large amount of heat is liberated, posing a threat to safety through the possibility of splashing. The resulting solution is usually colorless and odorless. As with other alkaline solutions, it feels slippery with skin contact due to the process of [[saponification]] that occurs between {{chem2|NaOH}} and natural skin oils.

==== Viscosity ====
Concentrated (50%) aqueous solutions of sodium hydroxide have a characteristic [[viscosity]], 78 m[[Pascal (unit)|Pa]]·s, that is much greater than that of water (1.0 mPa·s) and near that of olive oil (85 mPa·s) at room temperature. The viscosity of aqueous {{chem2|NaOH}}, as with any liquid chemical, is inversely related to its temperature, i.e., its viscosity decreases as temperature increases, and vice versa. The viscosity of sodium hydroxide solutions plays a direct role in its application as well as its storage.<ref name="Protank-2018" />

====Hydrates====
Sodium hydroxide can form several hydrates {{chem2|NaOH*''n''H2O}}, which result in a complex solubility diagram that was described in detail by [[Spencer Umfreville Pickering]] in 1893.<ref name="pick">{{cite journal | last1 = Umfreville Pickering | first1 = Spencer | year = 1893 | title = LXI.—The hydrates of sodium, potassium, and lithium hydroxides | url = https://zenodo.org/record/1860180| journal = Journal of the Chemical Society, Transactions | volume = 63 | issue = | pages = 890–909 | doi = 10.1039/CT8936300890 }}</ref> The known hydrates and the approximate ranges of temperature and concentration (mass percent of NaOH) of their [[Saturated solution|saturated]] water solutions are:<ref name="siem" />
* Heptahydrate, {{chem2|NaOH*7H2O}}: from −28&nbsp;°C (18.8%) to −24&nbsp;°C (22.2%).<ref name="pick" />
* Pentahydrate, {{chem2|NaOH*5H2O}}: from −24&nbsp;°C (22.2%) to −17.7&nbsp;°C (24.8%).<ref name="pick" />
* Tetrahydrate, {{chem2|NaOH*4H2O}}, α form: from −17.7&nbsp;°C (24.8%) to 5.4&nbsp;°C (32.5%).<ref name="pick" /><ref name="mrawIII">{{cite journal | last1 = Mraw | first1 = S. C. | last2 = Giauque | first2 = W. F. | year = 1974 | title = Entropies of the hydrates of sodium hydroxide. III. Low-temperature heat capacities and heats of fusion of the α and β crystalline forms of sodium hydroxide tetrahydrate | url = | journal = Journal of Physical Chemistry | volume = 78 | issue = 17| pages = 1701–1709 | doi = 10.1021/j100610a005 }}</ref>
* Tetrahydrate, {{chem2|NaOH*4H2O}}, β form: metastable.<ref name="pick" /><ref name="mrawIII" />
* Trihemihydrate, {{chem2|NaOH*3.5H2O}}: from 5.4&nbsp;°C (32.5%) to 15.38&nbsp;°C (38.8%) and then to 5.0&nbsp;°C (45.7%).<ref name="pick" /><ref name="siem" />
* Trihydrate, {{chem2|NaOH*3H2O}}: metastable.<ref name="pick" />
* Dihydrate, {{chem2|NaOH*2H2O}}: from 5.0&nbsp;°C (45.7%) to 12.3&nbsp;°C (51%).<ref name="pick" /><ref name="siem">{{cite journal | last1 = Siemens | first1 = P. R. | last2 = Giauque | first2 = William F. | year = 1969 | title = Entropies of the hydrates of sodium hydroxide. II. Low-temperature heat capacities and heats of fusion of NaOH·2H2O and NaOH·3.5H2O | url = | journal = Journal of Physical Chemistry | volume = 73 | issue = 1| pages = 149–157 | doi = 10.1021/j100721a024 }}</ref>
* Monohydrate, {{chem2|NaOH*H2O}}: from 12.3&nbsp;°C (51%) to 65.10&nbsp;°C (69%) then to 62.63&nbsp;°C (73.1%).<ref name="pick" /><ref name="murch">{{cite journal | last1 = Murch | first1 = L. E. | last2 = Giauque | first2 = W. F. | year = 1962 | title = The thermodynamic properties of sodium hydroxide and its monohydrate. Heat capacities to low temperatures. Heats of solution | url = | journal = Journal of Physical Chemistry | volume = 66 | issue = 10| pages = 2052–2059 | doi = 10.1021/j100816a052 }}</ref>

Early reports refer to hydrates with ''n'' = 0.5 or ''n'' = 2/3, but later careful investigations failed to confirm their existence.<ref name="murch" />

The only hydrates with stable melting points are {{chem2|NaOH*H2O}} (65.10&nbsp;°C) and {{chem2|NaOH*3.5H2O}} (15.38&nbsp;°C). The other hydrates, except the metastable ones {{chem2|NaOH*3H2O}} and {{chem2|NaOH*4H2O}} (β) can be crystallized from solutions of the proper composition, as listed above. However, solutions of NaOH can be easily supercooled by many degrees, which allows the formation of hydrates (including the metastable ones) from solutions with different concentrations.<ref name="siem" /><ref name="murch" />

For example, when a solution of NaOH and water with 1:2 mole ratio (52.6% NaOH by mass) is cooled, the monohydrate normally starts to crystallize (at about 22&nbsp;°C) before the dihydrate. However, the solution can easily be supercooled down to −15&nbsp;°C, at which point it may quickly crystallize as the dihydrate. When heated, the solid dihydrate might melt directly into a solution at 13.35&nbsp;°C; however, once the temperature exceeds 12.58&nbsp;°C it often decomposes into solid monohydrate and a liquid solution. Even the ''n'' = 3.5 hydrate is difficult to crystallize, because the solution supercools so much that other hydrates become more stable.<ref name="siem" />

A hot water solution containing 73.1% (mass) of NaOH is a [[eutectic]] that solidifies at about 62.63&nbsp;°C as an intimate mix of anhydrous and monohydrate crystals.<ref name="brodale">{{cite journal | last1 = Brodale | first1 = G. E. | last2 = Giauque | first2 = W. F. | year = 1962 | title = The freezing point-solubility curve of aqueous sodium hydroxide in the region near the anhydrous-monohydrate eutectic | url = | journal = Journal of Physical Chemistry | volume = 66 | issue = 10| pages = 2051 | doi = 10.1021/j100816a051}}</ref><ref name="murch" />

A second stable eutectic composition is 45.4% (mass) of NaOH, that solidifies at about 4.9&nbsp;°C into a mixture of crystals of the dihydrate and of the 3.5-hydrate.<ref name="siem" />

The third stable eutectic has 18.4% (mass) of NaOH. It solidifies at about −28.7&nbsp;°C as a mixture of water ice and the heptahydrate {{chem2|NaOH*7H2O}}.<ref name="pick" /><ref name="conde">M. Conde Engineering: "[http://www.aldacs.com/DocBase/AqNaOHSLEVLE.pdf Solid-Liquid Equilibrium (SLE) and Vapour-Liquid Equilibrium (VLE) of Aqueous NaOH] {{Webarchive|url=https://web.archive.org/web/20201007122113/http://www.aldacs.com/DocBase/AqNaOHSLEVLE.pdf |date=2020-10-07 }}". Online report, accessed on 2017-04-29.</ref>

When solutions with less than 18.4% NaOH are cooled, water [[ice]] crystallizes first, leaving the NaOH in solution.<ref name="pick" />

The α form of the tetrahydrate has density 1.33 g/cm<sup>3</sup>. It melts congruously at 7.55&nbsp;°C into a liquid with 35.7% NaOH and density 1.392 g/cm<sup>3</sup>, and therefore floats on it like ice on water. However, at about 4.9&nbsp;°C it may instead melt incongruously into a mixture of solid {{chem2|NaOH*3.5H2O}} and a liquid solution.<ref name="mrawIII" />

The β form of the tetrahydrate is metastable, and often transforms spontaneously to the α form when cooled below −20&nbsp;°C.<ref name="mrawIII" /> Once initiated, the exothermic transformation is complete in a few minutes, with a 6.5% increase in volume of the solid. The β form can be crystallized from supercooled solutions at −26&nbsp;°C, and melts partially at −1.83&nbsp;°C.<ref name="mrawIII" />

The "sodium hydroxide" of commerce is often the monohydrate (density 1.829 g/cm<sup>3</sup>). Physical data in technical literature may refer to this form, rather than the anhydrous compound.

====Crystal structure====
NaOH and its monohydrate form orthorhombic crystals with the space groups Cmcm ([[Pearson symbol|oS8]]) and Pbca (oP24), respectively. The monohydrate cell dimensions are a = 1.1825, b = 0.6213, c = 0.6069 [[nanometer|nm]]. The atoms are arranged in a [[hydrargillite]]-like layer structure, with each sodium atom surrounded by six oxygen atoms, three each from hydroxide ions and three from water molecules. The hydrogen atoms of the hydroxyls form strong bonds with oxygen atoms within each O layer. Adjacent O layers are held together by [[hydrogen bond]]s between water molecules.<ref name="jacobs">{{cite journal|author=Jacobs, H. and Metzner, U. |year=1991|title=Ungewöhnliche H-Brückenbindungen in Natriumhydroxidmonohydrat: Röntgen- und Neutronenbeugung an NaOH·H<sub">2</sub">O bzw. NaOD·D<sub">2</sub">O|journal=Zeitschrift für anorganische und allgemeine Chemie|volume =597| issue =1| pages =97–106|doi=10.1002/zaac.19915970113}}</ref>