Similar to the hydration of sulfuric acid, the dissolution of solid sodium hydroxide in water is a highly exothermic reaction [15] in which a large amount of heat is released, posing a safety hazard through the possibility of splashing. The resulting solution is usually colorless and odorless. Like other alkaline solutions, it feels slippery on skin contact due to the saponification process that occurs between the NaOH and natural skin oils.

Concentrated aqueous sodium hydroxide solution (50%) has an intrinsic viscosity of 78 mPa s at room temperature, which is much greater than that of water (1.0 mPa s) and close to that of olive oil (85 mPa s). Like any liquid chemical, the viscosity of an aqueous NaOH solution is inversely proportional to the temperature at which it is used, i.e. its viscosity decreases with increasing temperature and vice versa. The viscosity of sodium hydroxide solution plays a direct role in its application and storage. [14]

Sodium hydroxide can form a variety of hydrates NaOH nH2O, resulting in a complex solubility diagram described in detail by Spencer Umfreville Pickering in 1893 [16]. The approximate ranges of temperature and concentration (mass percent of NaOH) of known hydrates and their saturated aqueous solutions are:[11]

Commercial "sodium hydroxide" is usually the monohydrate (density 1.829 g/cm3). Physical data in technical literature may refer to this form and not to the anhydrous compound.

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

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

Typically, such neutralization reactions are represented by a simple net ionic equation:

OH−(aq) + H+(aq) → H2O(l)