Sodium hydroxide
A highly corrosive base used in industry and chemistry education.
Sodium hydroxide (NaOH), commonly called caustic soda or lye, is a white, solid ionic compound made of sodium cations and hydroxide anions. It acts as a strong nucleophile, making it a frequent participant in SN2 reactions. As one of the simplest hydroxides, it is a standard classroom tool alongside water and hydrochloric acid for demonstrating the pH scale.
This substance is a highly corrosive base and alkali. At room temperature, it breaks down lipids and proteins, and in high concentrations it can cause severe chemical burns. It dissolves easily in water, pulling in moisture and carbon dioxide from the air. Sodium hydroxide forms several hydrates (NaOH·nH2O). The monohydrate (NaOH·H2O) crystallizes from water solutions between 12.3 and 61.8 °C, and this is the form commonly sold commercially; published data often refer to it rather than the anhydrous compound.
Industrially, sodium hydroxide is used in making wood pulp and paper, textiles, drinking water, soaps, detergents, and drain cleaners. Global production in 2022 reached about 83 million tons.
**Properties**
Pure sodium hydroxide is a colorless crystalline solid that melts at 318 °C (604 °F) without decomposing and boils at 1,388 °C (2,530 °F). It is highly soluble in water but less so in polar solvents like ethanol and methanol, and it does not dissolve in ether or other non-polar solvents. Dissolving solid NaOH in water releases a large amount of heat—similar to mixing sulfuric acid with water—posing a splash hazard. The resulting solution is colorless and odorless. Like other alkaline solutions, it feels slippery on skin because it saponifies natural skin oils.
A concentrated (50%) aqueous solution of sodium hydroxide has a viscosity of 78 mPa·s at room temperature, much thicker than water (1.0 mPa·s) and close to olive oil (85 mPa·s). As with most liquids, its viscosity drops as temperature rises and increases as temperature falls. This property affects both its use and storage.
Sodium hydroxide forms several hydrates, producing a complex solubility diagram first detailed by Spencer Umfreville Pickering in 1893. The known hydrates and their approximate temperature and concentration ranges (mass percent NaOH) in saturated solutions are: - Heptahydrate (NaOH·7H2O): –28 °C (18.8%) to –24 °C (22.2%) - Pentahydrate (NaOH·5H2O): –24 °C (22.2%) to –17.7 °C (24.8%) - Tetrahydrate, α form (NaOH·4H2O
- chemical_formula
- NaOH
- molar_mass
- 39.997 g/mol
- melting_point
- 318 °C (604 °F)
- boiling_point
- 1,388 °C (2,530 °F)
- density
- 1.829 g/cm³ (monohydrate)
- solubility_in_water
- Highly soluble
- worldwide_production_2022
- ~83 million tons
Lore & Background
Sodium hydroxide is a highly corrosive base and alkali that decomposes lipids and proteins at ambient temperatures, and may cause severe chemical burns at high concentrations. It is highly soluble in water, and readily absorbs moisture and carbon dioxide from the air. It forms a series of hydrates NaOH·nH2O, with the monohydrate NaOH·H2O crystallizing from water solutions between 12.3 and 61.8 °C. The commercially available 'sodium hydroxide' is often this monohydrate, and published data may refer to it instead of the anhydrous compound.
Sodium hydroxide is used in many industries: in the making of wood pulp and paper, textiles, drinking water, soaps and detergents, and as a drain cleaner. It reacts with protic acids to produce water and the corresponding salts, and with acidic oxides such as sulfur dioxide to scrub harmful gases. Glass reacts slowly with aqueous sodium hydroxide solutions at ambient temperatures to form soluble silicates, causing glass joints and stopcocks to 'freeze.' Sodium hydroxide does not attack iron at room temperature, but 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.
Reader's Guide
Sodium hydroxide is a foundational industrial chemical with a global production of approximately 83 million tons in 2022. Its significance spans multiple sectors, including the manufacture of wood pulp and paper, textiles, drinking water treatment, soaps and detergents, and drain cleaning. In education, it is frequently used alongside neutral water and acidic hydrochloric acid to demonstrate the pH scale to chemistry students. The compound's high corrosivity and ability to decompose lipids and proteins require careful handling, as it can cause severe chemical burns at high concentrations. Its dissolution in water is highly exothermic, posing a safety threat through splashing. The complex hydrate system, detailed by Spencer Umfreville Pickering in 1893, affects its storage and application, with the monohydrate being the common commercial form. Sodium hydroxide's role in neutralizing acids and scrubbing acidic gases like SO2 and H2S from coal combustion highlights its environmental importance. Its reactivity with glass limits its use in certain laboratory equipment, while its non-reactivity with iron at room temperature allows for safe storage in iron containers. The compound's legacy is as a versatile, essential alkali in both industrial processes and fundamental chemical education.
Did You Know?
- Sodium hydroxide is a strong nucleophile frequently used in SN2 reactions.
- The dissolution of solid sodium hydroxide in water is a highly exothermic reaction, similar to the hydration of sulfuric acid.
- Concentrated (50%) aqueous solutions of sodium hydroxide have a viscosity of 78 mPa·s, much greater than water (1.0 mPa·s) and near that of olive oil (85 mPa·s) at room temperature.
- In 1986, an aluminium road tanker in the UK was mistakenly used to transport 25% sodium hydroxide solution, causing pressurization due to hydrogen gas produced in the reaction between sodium hydroxide and aluminium.
More in Sodium Compounds 1-24
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