Saturday 15 September 2012

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 SO42- ions.
  • So, we have to prepare two soluble salts solutions of which one contains Ba2+, its positive metal ions and the other contains the SO42-, its negative non-metal ions. 
  • These two ions will react and form a precipitation as barium sulfate salt. 
          Ba2+ (aq) + SO42-  (aq) ---> BaSO(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 NO3ions  
                                     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 Clions  
                                        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 Clions 
                                        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 NO3ions  
                                        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 NO3ions  
                                        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