Precipitate Reactions
L.O.
• I can state which are spectator ions in a
precipitation reaction.
• I can make a clean dry precipitate and write the
ionic equation for the formation of the precipitate
Precipitation reactions
An insoluble solid that forms during an aqueous reaction is
called a precipitate. A reaction which forms a precipitate is
called a precipitation reaction.
The limewater test for carbon dioxide is a precipitation reaction.
Limewater is actually a dilute
solution of calcium hydroxide.
The calcium hydroxide reacts with
carbon dioxide to form calcium
carbonate, which is insoluble in water:
calcium
carbon
+
hydroxide
dioxide
Ca(OH)2
+
CO2
calcium
+ water
carbonate
CaCO3
+ H2O
Uses of precipitation reactions
Most precipitation reactions are very
fast reactions that occur between ions.
This makes them very useful for
identifying specific ions based on the
type of precipitate formed.
Precipitation reactions have a
number of other uses:
 production of coloured pigments
for paints and dyes
 removal of toxic chemicals
from water
 separation of reaction products.
A lead iodide precipitate.
Isolating the precipitate
The precipitate from a precipitation
reaction can be separated from the
reaction mixture by filtration.
Buchner
funnel
vacuum
pump
A Buchner funnel and
flask can be used to
accelerate the process.
filter paper
This apparatus uses a
vacuum pump to draw the
mixture through the filter.
Buchner
flask
The filtrate is finally
washed and dried.
Spectator ions
In ionic precipitation reactions there are often ions that are not
involved in the reaction. These are known as spectator ions.
NaI (aq) + AgNO3 (aq)
AgI (s) + NaNO3 (aq)
The spectator ions are easily identified using the ionic equation.
Na+ (aq) + I– (aq) + Ag+ (aq) + NO3– (aq)
AgI (s) + Na+ (aq) + NO3– (aq)
This equation shows that the silver and the iodine ions have
reacted, joining together to make the precipitate.
The sodium (Na+) and nitrate (NO3–) ions are spectator ions.
This means the ionic equation can be simplified to:
I– (aq) + Ag+ (aq)
AgI (s)
Which ions are spectators?
Silver halides
The different silver halide precipitates can be distinguished
by their differing colours.
chloride
bromide
iodide
white AgCl
precipitate
cream AgBr
precipitate
yellow AgI
precipitate
Identifying negative ions: halides
Halide ions are formed from the Group VII elements, the
halogens. Halides are detected using silver nitrate solution.
The substance to be tested is first acidified with a small
amount of nitric acid before adding the silver nitrate solution.
If halides are present, a precipitate will form.
The precipitates formed are silver halides:
sodium
+
chloride
silver
nitrate
NaCl (aq) + AgNO3 (aq)
Cl– (aq)
+
Ag+ (aq)
silver
+
chloride
sodium
nitrate
AgCl (s) + NaNO3 (aq)
AgCl (s)
Identifying negative ions: sulfate
Sulfate ions (SO42–) are identified by adding a few drops of
barium chloride solution. The solution must be acidified first
with a few drops of hydrochloric acid.
A white precipitate of barium sulfate forms.
sodium
sulfate
Na2SO4 (aq)
+
barium
chloride
+ BaCl2 (aq)
barium
sulfate
BaSO4 (s) + 2NaCl (aq)
The ionic equation for this reaction is:
SO42– (aq) + Ba2+ (aq)
sodium
+ chloride
BaSO4 (s)
Method
Add the chemicals together and write the
symbol equations for each one, remember to
include state symbols
copper sulphate CuSO4 and sodium hydroxide NaOH
iron II chloride FeCl2 and sodium hydroxide NaOH
iron III chloride FeCl3 and sodium hydroxide NaOH
potassium chloride KCl and silver nitrate AgNO3
potassium bromide KBr and silver nitrate AgNO3
potassium iodide KI and silver nitrate AgNO3
potassium sulphate KSO4 and barium chloride BaCl2
lead nitrate Pb(N03)2 and potassium iodide KI
Finish symbol equation – balance – add state symbols
Equations
CuSO4(aq)
+
2NaOH (aq)
FeCl2 + 2NaOH
(aq)
(aq)
Cu(OH)2 + Na2SO4 (aq)
(s)
Fe(OH)2 (s)+ 2 NaCl (aq)
FeCl3(aq)+ 3NaOH(aq)
Fe(OH)3(s)+ 3 NaCl (aq)
KCl (aq)+ AgNO3
KNO3(aq) + AgCl (s)
(aq)
Finish symbol equation – balance – add state symbols
Equations
KBr
(aq)
+
AgNO3(aq)
KI (aq) + AgNO3(aq)
KNO3 +
AgBr(s)
KNO3(aq)+
AgI
(aq)
(s)
K2SO4 + BaCl2
2 KCl(aq)+ BaSO4 (s)
Pb(NO3)2 + 2 KI
2KNO3(aq) + PbI2 (s)
(aq)
(aq)
(aq)
(aq)
State symbols