Salts - preparing insoluble salts by precipitation

Tips: Before preparing a salt, find out if the salt is soluble or insoluble. A soluble salt is made from an acid. An insoluble salt is prepared by precipitation.

Table 1 Soluble Salts and Insoluble Salts

Soluble Salts	Insoluble Salts
All (group I) sodium, potassium, and ammonium
All nitrates
Most chlorides	Silver chloride and lead chloride
Most sulfates (sulphates)	Calcium sulfate, barium sulfate and lead sulfate
Most bromide and iodides	Silver bromide, silver iodide, lead bromide and lead iodide
Sodium, potassium(group I), and ammonium carbonates	All other carbonates

Preparing Insoluble Salts by Precipitation

Example: 

preparing insoluble barium sulfate salt.

• Barium sulfate salt is made of Ba²⁺ ions and SO₄^2- ions.
• So, we have to prepare two soluble salts solutions of which one contains Ba²⁺, its positive metal ions and the other contains the SO₄^2-, its negative non-metal ions. 
• These two ions will react and form a precipitation as barium sulfate salt. 

          Ba²⁺ (aq) + SO₄^2-  (aq) ---> BaSO₄ (s)

The precipitate of barium sulfate salt(residue) is gathered using a filter funnel, rinsed with distilled water to remove any impurities and dried. The filtrate contains the spectator ions which remained unchanged in the reaction.

Another way to separate the precipitate from the solution is using a centrifuge. This technique is used when the solid particles are very small and dispersed in the solution. The fast spinning of the tube in a centrifuge (see picture below) causing the solid gathers at the bottom of the tube due to higher density and gravity force. The liquid can be then decanted leaving the solid in the tube.

A centrifuge

More examples of preparing insoluble salts:

Table 2. Preparing Insoluble Salts 

Preparing insoluble salts	‘+’ ions	‘- ' ions	Solution used	Solution used	Salt precipitation	Spectator ions (no changes in reaction)
Barium sulfate BaSO4	Ba2+	SO42-	Barium nitrate with Ba2+	Sodium sulfate with SO42-	Barium  sulfate BaSO4	NaNO3 sodium nitrate soluble and remained unchanged as Na+ ions and NO3- ions
Reaction: Ba(NO3)2 (aq) + Na2SO4(aq) ---> BaSO4 (s) + 2 NaNO3 (aq)
Barium sulfate BaSO4	Ba2+	SO42-	Barium chloride  with Ba2+	Sodium sulfate with SO42-	Barium  sulfate BaSO4	NaCl  sodium chloride soluble and remained unchanged as Na+ ions and Cl- ions
Reaction: BaCl2 (aq) + Na2SO4(aq) ---> BaSO4 (s) + 2 NaCl2 (aq)
Barium sulfate BaSO4	Ba2+	SO42-	Barium chloride  with Ba2+	Magnesium sulfate with SO42-	Barium  sulfate BaSO4	MgCl2  magnesium chloride soluble and remained unchanged as Na+ ions and Cl- ions
Reaction: BaCl2 (aq) + MgSO4(aq) ---> BaSO4 (s) + 2 MgCl2 (aq)
Silver chloride AgCl	Ag+	Cl-	Silver nitrate with  Ag+	Sodium chloride with Cl-	Silver chloride AgCl	NaNO3  sodium nitrate soluble and remained unchanged as Na+ ions and NO3- ions
Reaction: AgNO3 (aq) + NaCl(aq) ---> AgCl (s) + NaNO3 (aq)
Lead iodide PbI2	Pb2+	I-	Lead nitrate with  Pb2+	Sodium iodide with  I-	Lead iodide PbI2	NaNO3 sodium nitrate soluble and remained unchanged as Na+ ions and NO3- ions
Reaction: Pb(NO3)2 (aq) + NaI (aq) ---> PbI2 (s) + NaNO3 (aq)

Pictures of insoluble salts:

White precipitate
silver chloride, AgCl 

Creamy precipitate
silver bromide, AgBr

Yellow precipitate
silver Iodide, AgI

White precipitate
barium sulfate, BaSO₄

Yellow precipitate
lead iodide, PbI₂