Precipitation Reactions

• 2 solutions react together to make an insoluble substance - precipitate
• the precipitate makes the solution turn cloudy
• most precipitate reactions involve ions

barium chloride + sodium sulfate -> barium sulfate + sodium chloride

     BaCl2 (aq)   +  Na2SO4 (aq)   ->    BaSO4 (s)   + 2NaCl (aq)

• sodium and chloride ions were dissolved in solution before the reaction = they're still dissolved afterwards
• ^ called spectator ions - as they don't change during the reaction
• An ionic equation ignores the spectator ions:

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

Test for Sulfate ions, SO4^-

• add dilute HCl
• then barium chloride, BaCl2
• white precipitate of barium sulfate means the origninal compound was a sulfate

Test for Chloride (Cl^-), Bromide (Br^-) or Iodide (I^-) ions

• add dilute nitric acid
• followed by lead nitrate
• Chorlide = white percipitate of lead chloride
• Bromide = cream precipitate of lead bromide
• Iodide = yellow precipitate of lead iodide