Inside hydroxide
The O–H bond belongs to OH⁻.
Hydroxide is a covalently bonded diatomic ion carrying −1 charge. Sodium interacts ionically with the anion in the solid.
Ionic solid · strong base
Sodium hydroxide
NaOH
Sodium hydroxide is an ionic solid made from Na⁺ and OH⁻ in a 1:1 ratio. In water it separates into solvated ions, making hydroxide directly available for acid–base reactions. Its dissolution is strongly exothermic.
Product distinction: “lye” and caustic-soda products can be solid NaOH or aqueous formulations of specified concentration. The chemical identity NaOH does not by itself state the product composition.
Essentials
Solid NaOH is ionic: Na⁺ and OH⁻, not a little Na–O–H molecule.
The formula unit expresses electrical neutrality. Sodium is present as Na⁺ and hydroxide as the polyatomic anion OH⁻; drawing a covalent bond from Na to O would give the wrong first mental model.
Detail
In water, NaOH separates into solvated Na⁺ and OH⁻.
Sodium hydroxide is a strong electrolyte. The chemically important base species is hydroxide, while sodium acts mainly as a spectator ion in many simple aqueous acid–base reactions.
Thermal effect
Dissolution releases substantial heat.
Breaking the ionic lattice costs energy, but hydration of the ions releases more under ordinary conditions, so the overall dissolution is exothermic.
Understand
Neutralization is most clearly seen at the ionic level.
When a strong acid and sodium hydroxide react in water, spectator ions can be removed from the equation. The core proton-transfer chemistry is hydronium plus hydroxide forming water.
Strong base
Strong does not mean concentrated.
“Strong base” describes near-complete dissociation and acid–base behavior in water. A solution can still be dilute or concentrated independently of base strength.
Reference identifiers
Names and identifiers that pin down the chemical identity.
| Formula | NaOH |
|---|---|
| CAS Registry Number | 1310-73-2 |
| PubChem CID | 14798 |
| Standard InChI | InChI=1S/Na.H2O/h;1H2/q+1;/p-1 |
| Canonical SMILES | [OH-].[Na+] |
Advanced
Push past the first model without losing the simple picture.
Why is NaOH not drawn as Na–O–H?
That drawing suggests a discrete covalent molecule. The solid is ionic, and in water the relevant particles are solvated Na⁺ and OH⁻.
Why does dissolving NaOH heat water?
Ion hydration is strongly favorable. The net enthalpy change for dissolution is exothermic under ordinary conditions.
Does OH⁻ exist as a bare ion in water?
No. Aqueous hydroxide is strongly solvated and embedded in a fluctuating hydrogen-bond network. OH⁻(aq) is a compact chemical notation.
Constituent elements