Sodium Hydroxide

Sodium hydroxide (NaOH) is the most-used alkali in 0620. It appears in neutralisation, salt preparation, the identification of metal cations, and as a product of brine electrolysis.

Key facts

Property	Value
Formula	NaOH
Mr	23 + 16 + 1 = 40
Bonding	Ionic — Na+ and OH- ions
Acid/base	Strong alkali — fully dissociated, pH 13-14
Key property	White solid, very soluble; dissolving is exothermic; corrosive
Main uses	Cation tests; making soap and paper; neutralising acids

Where sodium hydroxide appears in 0620

• Acids, bases and indicators: the reference strong alkali.
• Preparation of salts: titration to make soluble sodium salts.
• Tests for ions: identifying metal cations and the ammonium ion.
• Electrolysis of aqueous solutions: NaOH is the solution left after brine electrolysis.

As a strong alkali

NaOH(aq) -> Na+(aq) + OH-(aq)

It is fully dissociated, which is what “strong” means. Compare aqueous ammonia, a weak alkali that is only partially ionised.

Key reactions

Neutralising acids

NaOH(aq) + HCl(aq) -> NaCl(aq) + H2O(l)

2NaOH(aq) + H2SO4(aq) -> Na2SO4(aq) + 2H2O(l)

NaOH(aq) + HNO3(aq) -> NaNO3(aq) + H2O(l)

Because both reactants are soluble, these sodium salts are made by titration. Note the 2:1 ratio with diprotic sulfuric acid.

With ammonium salts (the ammonium-ion test)

NH4+(aq) + OH-(aq) -> NH3(g) + H2O(l)

Warm the salt with NaOH; ammonia is released, turning damp red litmus paper blue.

Identifying metal cations

Adding NaOH solution to a metal-ion solution gives a coloured hydroxide precipitate. Adding it in excess separates the amphoteric hydroxides, which redissolve:

Cation	Precipitate with NaOH	In excess NaOH
Cu2+	Light blue	Insoluble (no change)
Fe2+	Green	Insoluble (no change)
Fe3+	Red-brown	Insoluble (no change)
Ca2+	White	Insoluble (no change)
Al3+	White	Dissolves, colourless solution
Zn2+	White	Dissolves, colourless solution
Cr3+	Green	Dissolves, green solution

Example ionic equation: Fe3+(aq) + 3OH-(aq) -> Fe(OH)3(s)

Production: electrolysis of brine

Electrolysing concentrated NaCl solution discharges H+ at the cathode (2H+ + 2e- -> H2) rather than sodium, and Cl- at the anode (2Cl- -> Cl2 + 2e-). Na+ and OH- are left in solution as sodium hydroxide — one of three useful products alongside chlorine and hydrogen.

Aqueous sodium hydroxide is added to a colourless solution and a white precipitate forms. The solution could contain aluminium ions or calcium ions. Describe how you would identify which ion is present. (2 marks)

Mark scheme

• Add excess (more) sodium hydroxide solution [1]
• With aluminium ions the white precipitate dissolves to give a colourless solution; with calcium ions the precipitate stays / is insoluble in excess [1]

Examiner note: three cations give a white precipitate with NaOH — aluminium, calcium and zinc. Excess NaOH separates calcium (precipitate remains) from aluminium and zinc (precipitates dissolve). To tell aluminium from zinc you must use aqueous ammonia, in which only the zinc hydroxide redissolves.

Common exam mistakes

• Calling NaOH a weak alkali. It is fully dissociated, so it is a strong alkali; “alkali” simply means a soluble base.
• Saying the copper(II) or iron hydroxide precipitate dissolves in excess NaOH. Only the aluminium, zinc and chromium(III) hydroxides redissolve.
• Forgetting the 2:1 ratio with sulfuric acid: 2NaOH + H2SO4 -> Na2SO4 + 2H2O.
• Trying to distinguish aluminium from zinc with excess NaOH. Both dissolve, so this fails; use aqueous ammonia, in which only zinc redissolves.