How to Prepare Insoluble Salts Using the Precipitation Method

In this video, we will focus on making insoluble salts.

When preparing salts, the first step is to determine whether the salt is soluble or insoluble in water. If the salt we want to prepare is insoluble, we can use the precipitation method.

In the precipitation method, we mix two soluble salts together. One salt provides the cation, and the other provides the anion. When mixed, the insoluble salt forms as a precipitate (a solid that settles out of the solution).

Example 1: Preparing Lead(II) Sulfate (PbSO₄)

To prepare the insoluble salt lead(II) sulfate, we need:

A soluble salt containing Pb²⁺ ions
A soluble salt containing SO₄²⁻ ions

Step 1: Choose suitable soluble salts

All nitrate salts are soluble, so we can use lead(II) nitrate, Pb(NO₃)₂.
All Group I salts are soluble, so we can use sodium sulfate, Na₂SO₄.

Step 2: Mix the two solutions When lead(II) nitrate solution is mixed with sodium sulfate solution, lead(II) ions combine with sulfate ions to form lead(II) sulfate, which is insoluble.

Ionic equation:

$\text{Pb}^{2+}(aq) + \text{SO}_4^{2-}(aq) \rightarrow \text{PbSO}_4(s)$

PbSO₄ forms as a white precipitate (solid).
Sodium ions (Na⁺) and nitrate ions (NO₃⁻) remain in solution.

Example 2: Preparing Silver Chloride (AgCl)

To prepare the insoluble salt silver chloride, we need:

A soluble salt containing Ag⁺ ions
A soluble salt containing Cl⁻ ions

Step 1: Choose suitable soluble salts

All nitrate salts are soluble, so we can use silver nitrate, AgNO₃.
All Group I salts are soluble, so we can use potassium chloride, KCl.

Step 2: Mix the two solutions When silver nitrate solution is mixed with potassium chloride solution, silver ions combine with chloride ions to form silver chloride, which is insoluble.

Ionic equation:

$\text{Ag}^{+}(aq) + \text{Cl}^{-}(aq) \rightarrow \text{AgCl}(s)$

AgCl forms as a white precipitate.
Potassium ions (K⁺) and nitrate ions (NO₃⁻) remain in solution.

Summary

Insoluble Salt	Soluble Salt 1	Soluble Salt 2	Precipitate Formed
Lead(II) sulfate	Lead(II) nitrate	Sodium sulfate	PbSO₄ (s)
Silver chloride	Silver nitrate	Potassium chloride	AgCl (s)

Key Point: The precipitation method works only when both starting salts are soluble, but the salt we want to prepare is insoluble.

Take some time to decide what are the possible starting reagents to make the insoluble salts.

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