Lesson 5 of 7

Use solubility rules to choose starting materials

An insoluble product can be separated as a precipitate.

O-Level 6092 (2026) / SEC G3 K324 (2027)

Pure only

I can apply salt-solubility rules.Syllabus
Syllabus K324, 5.2(b). Apply salt-solubility rules
I can select a salt-preparation route.Syllabus
Syllabus K324, 5.2(c). Select a salt-preparation route

Use solubility rules to choose starting materials

An insoluble product can be separated as a precipitate.

General salt-solubility rules
Family	Rule and important exceptions
Group 1 and ammonium salts	Soluble
Nitrates	Soluble
Chlorides	Generally soluble; silver chloride and lead(II) chloride are exceptions
Sulfates	Generally soluble; barium sulfate and lead(II) sulfate are insoluble; calcium sulfate is sparingly soluble
Carbonates	Generally insoluble except Group 1 and ammonium carbonates
Hydroxides	Generally insoluble except Group 1 hydroxides; calcium hydroxide is slightly soluble

Precipitation occurs when dissolved ions meet and form an insoluble solid. Mixing Ba(NO₃)₂(aq) and Na₂SO₄(aq) gives BaSO₄(s). The ionic equation is Ba²⁺(aq) + SO₄^2-(aq) -> BaSO₄(s). Choose soluble starting salts so both required ions are available to mix.

Check your understanding

Why mix barium nitrate solution with sodium sulfate solution, rather than mixing the two dry solids?

Water reacts with sodium sulfate to make the sulfate ions needed for the precipitate.
In solution the Ba²⁺ and SO₄^2- ions can move and meet to form barium sulfate.
The dry solids would react too violently, so water is added to slow the reaction.

Show the answer

In solution the Ba²⁺ and SO₄^2- ions can move and meet to form barium sulfate.

In the dry solids the ions are fixed in their lattices. Dissolving frees them, so Ba²⁺ and SO₄^2- collide and insoluble BaSO₄ precipitates.