Physical properties Pure<unk>  is a colorless crystalline solid that melts at without decomposition, and with a boiling point of . It is highly soluble in water, with a lower solubility in polar solvents such as ethanol and methanol.<unk>  is insoluble in ether and other non-polar solvents. Similar to the hydration of sulfuric acid, dissolution of solid<unk>  in water is a highly exothermic reaction 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<unk>  and natural skin oils. Viscosity Concentrated (50%) aqueous solutions of<unk>  have a characteristic viscosity, 78 mPa·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<unk> , as with any liquid chemical, is inversely related to its service temperature, i.e., its viscosity decreases as temperature increases, and vice versa. The viscosity of<unk>  solutions plays a direct role in its application as well as its storage. Hydrates<unk>  can form several hydrates , which result in a complex solubility diagram that was described in detail by S. U. Pickering in 1893. The known hydrates and the approximate ranges of temperature and concentration (mass percent of<unk>  of their saturated water solutions are: *Heptahydrate, : from −28 °C (18.8%) to −24 °C (22.2%). *Pentahydrate, : from −24 °C (22.2%) to −17.7 (24.8%). *Tetrahydrate, , α form: from −17.7 (24.8%) to +5.4 °C (32.5%). *Tetrahydrate, , β form: metastable. *Trihemihydrate, : from +5.4 °C (32.5%) to +15.38 °C (38.8%) and then to +5.0 °C (45.7%). *Trihydrate, : metastable. *Dihydrate, : from +5.0 °C (45.7%) to +12.3 °C (51%). *Monohydrate, : from +12.3 °C (51%) to 65.10 °C (69%) then to 62.63 °C (73.1%). Early reports refer to hydrates with n = 0.5 or n = 2/3, but later careful investigations failed to confirm their existence. The only hydrates with stable melting points are (65.10 °C) and (15.38 °C). The other hydrates, except the metastable ones and (β) can be crystallized from solutions of the proper composition, as listed above. However, solutions of<unk>  can be easily supercooled by many degrees, which allows the formation of hydrates (including the metastable ones) from solutions with different concentrations. For example, when a solution of<unk>  and water with 1:2 mole ratio (52.6%<unk>  by mass) is cooled, the monohydrate normally starts to crystallize (at about 22 °C) before the dihydrate. However, the solution can easily be supercooled down to −15 °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 °C; however, once the temperature exceeds 12.58 °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. A hot water solution containing 73.1% (mass) of<unk>  is an eutectic that solidifies at about 62.63 °C as an intimate mix of anhydrous and monohydrate crystals. A second stable eutectic composition is 45.4% (mass) of<unk> , that solidifies at about 4.9 °C into a mixture of crystals of the dihydrate and of the 3.5-hydrate. The third stable eutectic has 18.4% (mass) of<unk> . It solidifies at about −28.7 °C as a mixture of water ice and the heptahydrate . When solutions with less than 18.4%<unk>  are cooled, water ice crystallizes first, leaving the<unk>  in solution. The α form of the tetrahydrate has density 1.33 g/cm3. It melts congruously at 7.55 °C into a liquid with 35.7%<unk>  and density 1.392 g/cm3, and therefore floats on it like ice on water. However, at about 4.9 °C it may instead melt incongruously into a mixture of solid and a liquid solution. The β form of the tetrahydrate is metastable, and often transforms spontaneously to the α form when cooled below −20 °C. 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 °C, and melts partially at −1.83 °C. The<unk> " of commerce is often the monohydrate (density 1.829 g/cm3). Physical data in technical literature may refer to this form, rather than the anhydrous compound. Crystal structure<unk>  and its monohydrate form orthorhombic crystals with the space groups Cmcm (oS8) and Pbca (oP24), respectively. The monohydrate cell dimensions are a = 1.1825, b = 0.6213, c = 0.6069 nm. The atoms are arranged in a hydrargillite-like layer structure /O Na O O Na O/... Each sodium atom is surrounded by six oxygen atoms, three each from hydroxyl anions 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 bonds between water molecules. Chemical properties Reaction with acids<unk>  reacts with protic acids to produce water and the corresponding salts. For example, when<unk>  reacts with hydrochloric acid, sodium chloride is formed: : In general, such neutralization reactions are represented by one simple net ionic equation: : This type of reaction with a strong acid releases heat, and hence is exothermic. Such acid–base reactions can also be used for titrations. However,<unk>  is not used as a primary standard because it is hygroscopic and absorbs carbon dioxide from air. Reaction with acidic oxides<unk>  also reacts with acidic oxides, such as sulfur dioxide. Such reactions are often used to "scrub" harmful acidic gases (like and ) produced in the burning of coal and thus prevent their release into the atmosphere. For example, : Reaction with metals and oxides Glass reacts slowly with aqueous<unk>  solutions at ambient temperatures to form soluble silicates. Because of this, glass joints and stopcocks exposed to<unk>  have a tendency to "freeze". Flasks and glass-lined chemical reactors are damaged by long exposure to hot<unk> , which also frosts the glass.<unk>  does not attack iron at room temperatures, since iron does not have amphoteric properties (i.e., it only dissolves in acid, not base). Nevertheless, at high temperatures (e.g. above 500 °C), iron can react endothermically with<unk>  to form iron(III) oxide, sodium metal, and hydrogen gas. This is due to the lower enthalpy of formation of iron(III) oxide (−824.2 kJ/mol) compared to<unk>  (-500 kJ/mol) and positive entropy change of reaction, which imply spontaneity at high temperatures (ΔST>ΔH, ΔG0). Consider the following reaction between molten<unk>  and finely divided iron filings: : A few transition metals, however, may react vigorously with<unk>  under milder conditions. In 1986, an aluminium road tanker in the UK was mistakenly used to transport 25%<unk>  solution, causing pressurization of the contents and damage to the tanker. The pressurization was due to the hydrogen gas which is produced in the reaction between<unk>  and aluminium: : Precipitant Unlike<unk> , which is soluble, the<unk> s of most transition metals are insoluble, and therefore<unk>  can be used to precipitate transition metal<unk> s. The following colours are observed: *Copper - blue *Iron(II) - green *Iron(III) - yellow / brown Zinc and lead salts dissolve in excess<unk>  to give a clear solution of or . Aluminium<unk>  is used as a gelatinous flocculant to filter out particulate matter in water treatment. Aluminium<unk>  is prepared at the treatment plant from aluminium sulfate by reacting it with<unk>  or bicarbonate. : : Saponification<unk>  can be used for the base-driven hydrolysis of esters (as in saponification), amides and alkyl halides. However, the limited solubility of<unk>  in organic solvents means that the more soluble potassium<unk>  (KOH) is often preferred. Touching a<unk>  solution with bare hands, while not recommended, produces a slippery feeling. This happens because oils on the skin such as sebum are converted to soap. Despite solubility in propylene glycol it is unlikely to replace water in saponification due to propylene glycol's primary reaction with fat before reaction between<unk>  and fat. Mass fraction of<unk>  (wt%) 4 10 20 30 40 50 Molar concentration of<unk>  (M) 1.04 2.77 6.09 9.95 14.30 19.05 Mass concentration of<unk>  (g/l) 41.7 110.9 243.8 398.3 572.0 762.2 Density of solution (g/ml) 1.043 1.109 1.219 1.328 1.430 1.524
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