Sodium hydroxide

Na 1
O 1
H 1

3 atoms in one formula unit of an ionic lattice.

The drawing shows one formula unit of an ionic lattice, not a molecule; sodium hydroxide has no molecular shape.

IUPAC name: sodium hydroxide
Molar mass: 39.9970(22) g/mol
Atoms: 3
SMILES: [OH-].[Na+]
InChIKey: HEMHJVSKTPXQMS-UHFFFAOYSA-M
PubChem CID: 14798

Made of

Many commercial drain and oven cleaners contain the same compound, sold on its own as lye or caustic soda: sodium hydroxide. It is a manufactured substance, according to ATSDR (the US Agency for Toxic Substances and Disease Registry), and an alkali metal hydroxide in the ChEBI database. It combines sodium cations with hydroxide anions, each an oxygen atom bonded to a hydrogen atom, and its molar mass is 39.997 g/mol.

The solid is white and deliquescent, drawing in water from the air until it dissolves, according to HSDB, the Hazardous Substances Data Bank quoted on PubChem. It dissolves in water very freely: a gram needs less than a millilitre. Ethanol also takes it up readily. The alkali-metal hydroxides NaOH and KOH are, on Chem1’s account, the only truly strong bases that everyday chemistry is likely to meet, their solutions being in effect dissolved hydroxide ions.

Structure and bonding

The drawing shows one formula unit: a sodium ion with a charge of +1 beside a hydroxide ion with a charge of −1, and no line between them. The one bond drawn is the single O–H bond inside the hydroxide ion, a polyatomic anion. The real solid forms orthogonal crystals, whose axes meet at right angles, and changes from an alpha form to a beta form at 299.6 °C, according to HSDB.

A crystal holds far more than one pair. In the solid, the hydroxides of the Group 1 metals form interpenetrating lattices, one of metal cations and one of hydroxide anions, as Chem1 describes them. In the idealised picture of an ionic solid, each ion is kept in position by a balance of coulombic forces, the electrical attractions and repulsions between charges.

The O–H bond survives because of which bond gives way. In a metal–oxygen–hydrogen unit, a metal–oxygen link that breaks more easily leaves the OH group whole, carrying the negative charge as hydroxide, Chem1 explains. Water breaks the lattice completely into Na⁺(aq) and OH⁻(aq), and ion–dipole attractions, between an ion and the partially charged ends of water molecules, then surround each freed ion.

Where it turns up

Beyond drain and oven cleaners, sodium hydroxide reaches the bathroom shelf. The Cosmetic Ingredient Review judges it safe in hair straighteners and hair removers under recommended conditions of use, with skin contact kept low. The US Food and Drug Administration (FDA) affirms it as generally recognised as safe as a direct food additive. JECFA, the international expert committee on food additives, classes it as an acidity regulator.

Workshops and factories use it as well, according to Haz-Map. Photographic developing baths contain it, and chip makers use it to develop photoresist, a light-sensitive coating, and to etch away unwanted material. Hides are prepared with it for leather, and hobbyists meet it when colouring metal surfaces and in textile arts. In fracking fluid, used to crack rock, it sets the pH so that other components keep working.

Industry buys the solid as flakes, beads or cast blocks, though a solution of 50 per cent by weight is the commonest form, per HSDB.

Why it matters

Most sodium hydroxide comes from passing electricity through brine, a solution of sodium chloride. Sodium hydroxide solution, chlorine and hydrogen leave the cells in mass ratios of 1 to 0.88 to 0.025, on HSDB’s figures. Because caustic soda and chlorine emerge together, output of the base has been held back by how much chlorine the market wants. The US volume for 2023 stood at 20 to 25 billion pounds in the US Environmental Protection Agency’s (EPA) Chemical Data Reporting.

The base is bought, on Haz-Map’s list from the Merck Index, for neutralising acids, making sodium salts, hydrolysing fats into soap and precipitating metals. Large amounts also go to petroleum refining, rayon and rubber reclaiming. Pulp and paper take 22 per cent of consumption in HSDB’s breakdown, organic chemicals 20 per cent and inorganic chemicals 11 per cent.

The compound also led to its own metal. Davy first isolated sodium in 1807 by electrolysis of caustic soda, Los Alamos records, a route since replaced by cheaper electrolysis of molten salt.

Properties

Properties of sodium hydroxide Molar mass summed from IUPAC/CIAAW 2024 standard atomic weights; formula, atom and bond counts and PubChem's molecular weight from PubChem CID 14798.
Property	Value	Unit	Source
Formula (Hill order)	HNaO
Molar mass	39.9970(22) g/mol	g/mol	IUPAC/CIAAW 2024
Sodium atoms	1
Oxygen atoms	1
Hydrogen atoms	1
PubChem molecular weight	39.997 g/mol	g/mol	PubChem

Safety

Solid sodium hydroxide and its solutions are corrosive to the eyes, skin and airways, a hazard for users of drain cleaners and for workers. It carries the statement “Causes severe skin burns and eye damage” in over 99.9 per cent of GHS (Globally Harmonized System) reports on PubChem. Brief contact can cause second- and third-degree burns, HSDB warns, and dissolving or neutralising it can release enough heat to ignite combustible materials. The PubChem Laboratory Chemical Safety Summary and the NIOSH (US National Institute for Occupational Safety and Health) Pocket Guide card, which lists workplace exposure limits, are the authoritative sheets.

Questions

Is sodium hydroxide a molecule?

No. Solid sodium hydroxide is a crystal of ions: sodium cations and hydroxide anions arranged as two interpenetrating lattices, the form Chem1 gives for the solid hydroxides of Group 1 metals. The drawing is one formula unit, Na⁺ beside OH⁻, with no line joining the pair. The single bond it does draw belongs to the hydroxide ion, which keeps its oxygen and hydrogen together as one charged unit.

Are lye and caustic soda the same as sodium hydroxide?

Yes. Caustic soda and lye are other common names for sodium hydroxide, according to ATSDR, the US Agency for Toxic Substances and Disease Registry, and NIOSH (the US National Institute for Occupational Safety and Health) also lists it as soda lye and sodium hydrate. All of them mean the same compound, NaOH, whether sold as flakes, pellets or a solution. ATSDR notes that commercial drain and oven cleaners commonly contain it.

Why is sodium hydroxide a strong base?

Its solid is already made of ions, so when it dissolves the crystal simply falls apart into free Na⁺ and OH⁻, and HSDB describes the solution as completely dissociated. The hydroxide ion is the base itself. Dissolved NaOH and KOH are in effect solutions of hydroxide, on Chem1's account, which puts these alkali-metal hydroxides among the few really strong bases in ordinary chemistry.

Why does an open container of lye turn crusty?

Sodium hydroxide takes up carbon dioxide from the air and turns into sodium carbonate, which CAMEO Chemicals, the US hazardous-chemicals database, says can appear as crusts or precipitates. The solid also pulls water out of the air until it dissolves in it, a behaviour called deliquescence, so an open container takes in both water and carbon dioxide.

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An acid gives a proton and a base takes it: Arrhenius, Brønsted–Lowry and Lewis Hydrogen chloride gives its proton to water effectively completely; acetic acid gives only a small fraction.