Concentrated (50%) aqueous solutions of sodium hydroxide 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 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.^[16]

Sodium hydroxide can form several hydrates NaOH·nH₂O, which result in a complex solubility diagram that was described in detail by Spencer Umfreville Pickering in 1893.^[18] The known hydrates and the approximate ranges of temperature and concentration (mass percent of NaOH) of their saturated water solutions are:^[13]

• Heptahydrate, NaOH·7H₂O: from −28 °C (18.8%) to −24 °C (22.2%).^[18]
• Pentahydrate, NaOH·5H₂O: from −24 °C (22.2%) to −17.7 °C (24.8%).^[18]
• Tetrahydrate, NaOH·4H₂O, α form: from −17.7 °C (24.8%) to 5.4 °C (32.5%).^[18][19]
• Tetrahydrate, NaOH·4H₂O, β form: metastable.^[18][19]
• Trihemihydrate, NaOH·3.5H₂O: from 5.4 °C (32.5%) to 15.38 °C (38.8%) and then to 5.0 °C (45.7%).^[18][13]
• Trihydrate, NaOH·3H₂O: metastable.^[18]
• Dihydrate, NaOH·2H₂O: from 5.0 °C (45.7%) to 12.3 °C (51%).^[18][13]
• Monohydrate, NaOH·H₂O: from 12.3 °C (51%) to 65.10 °C (69%) then to 62.63 °C (73.1%).^[18][20]

Early reports refer to hydrates with n = 0.5 or n = 2/3, but later careful investigations failed to confirm their existence.^[20]

The only hydrates with stable melting points are NaOH·H₂O (65.10 °C) and NaOH·3.5H₂O (15.38 °C). The other hydrates, except the metastable ones NaOH·3H₂O and 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.^[13][20]

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 °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.^[13]

A hot water solution containing 73.1% (mass) of NaOH is a eutectic that solidifies at about 62.63 °C as an intimate mix of anhydrous and monohydrate crystals.^[21][20]

A second stable eutectic composition is 45.4% (mass) of NaOH, that solidifies at about 4.9 °C into a mixture of crystals of the dihydrate and of the 3.5-hydrate.^[13]

The third stable eutectic has 18.4% (mass) of NaOH. It solidifies at about −28.7 °C as a mixture of water ice and the heptahydrate NaOH·7H₂O.^[18][22]

When solutions with less than 18.4% NaOH are cooled, water ice crystallizes first, leaving the NaOH in solution.^[18]

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% NaOH 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 NaOH·3.5H₂O and a liquid solution.^[19]

The β form of the tetrahydrate is metastable, and often transforms spontaneously to the α form when cooled below −20 °C.^[19] 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.^[19]

The “sodium hydroxide” 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.

NaOH 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, 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 bonds between water molecules.^[23]

Sodium hydroxide reacts with protic acids to produce water and the corresponding salts. For example, when sodium hydroxide reacts with hydrochloric acid, sodium chloride is formed:

NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)

In general, such neutralization reactions are represented by one simple net ionic equation:

OH⁻(aq) + H⁺(aq) → H₂O(l)

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, sodium hydroxide is not used as a primary standard because it is hygroscopic and absorbs carbon dioxide from air.

Sodium hydroxide also reacts with acidic oxides, such as sulfur dioxide. Such reactions are often used to “scrub” harmful acidic gases (like SO₂ and H₂S) produced in the burning of coal and thus prevent their release into the atmosphere. For example,

2 NaOH + SO₂ → Na₂SO₃ + H₂O

Glass reacts slowly with aqueous sodium hydroxide solutions at ambient temperatures to form soluble silicates. Because of this, glass joints and stopcocks exposed to sodium hydroxide have a tendency to “freeze”. Flasks and glass-lined chemical reactors are damaged by long exposure to hot sodium hydroxide, which also frosts the glass. Sodium hydroxide does not attack iron at room temperature, 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 sodium hydroxide to form iron(III) oxide, sodium metal, and hydrogen gas.^[24] This is due to the lower enthal