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QUALITATIVE ANALYSIS NOTES
Tests for Cations (Positive Ions):
Cations are identified based on their reactions with 2 reagents: aqueous sodium hydroxide and
aqueous ammonia. The colour of the precipitates (ppt) formed and their solubility in excess of
the reagent serves as a useful guide for the identification of cations in aqueous solutions.
Cation
Observations:
Reaction with aqueous sodium
Reaction with aqueous ammonia
hydroxide
Al3+
White precipitate formed, soluble in
excess aqueous sodium hydroxide to
give a colourless solution.
Ca2+
White precipitate formed, insoluble
in excess aqueous sodium hydroxide.
White precipitate formed, insoluble
in excess aqueous ammonia.
Identity of
precipitate
Aluminium hydroxide
Al(OH)3
No visible reaction.
Calcium hydroxide
Ca(OH)2
Blue precipitate formed, insoluble in
excess aqueous sodium hydroxide.
Blue precipitate formed, soluble in
excess aqueous ammonia to give a
dark blue solution.
Copper(II) hydroxide
Cu(OH)2
Fe2+
Dirty-green precipitate formed,
insoluble in excess aqueous sodium
hydroxide. Dirty-green precipitate
turns reddish-brown on standing in
air.
Dirty-green precipitate formed,
insoluble in excess aqueous
ammonia. Dirty-green precipitate
turns reddish-brown on standing in
air.
Iron(II) hydroxide
Fe(OH)2
Fe3+
Reddish-brown precipitate formed,
insoluble in excess aqueous sodium
hydroxide.
Reddish-brown precipitate formed,
insoluble in excess aqueous
ammonia.
Iron(III) hydroxide
Fe(OH)3
Pb2+
White precipitate formed, soluble in
excess aqueous sodium hydroxide to
give a colourless solution.
White precipitate formed, insoluble
in excess aqueous ammonia.
Zn2+
White precipitate formed, soluble in
excess aqueous sodium hydroxide to
give a colourless solution.
White precipitate formed, soluble
in excess aqueous ammonia to give
a colourless solution.
Zinc hydroxide
Zn(OH)2
No visible reaction.
-
Cu2+
NH4+
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Lead(II) hydroxide
Pb(OH)2
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On warming, a colourless, pungent
gas evolved which turned moist red
litmus paper blue. The gas is
ammonia.
Na+
K+
No visible reaction.
No visible reaction.
No visible reaction.
No visible reaction.
Note: To distinguish between Al3+ and Pb2+, add aqueous potassium iodide (KI).
If Pb2+ is present, a bright yellow ppt is formed. This ppt is PbI 2.
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-
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Tests for Anions (Negative Ions):
Anions are identified based on their reactions with specific reagents.
Anion
Test
Observations
Identity of precipitate
Add aqueous sodium hydroxide.
Then add a little aluminium
powder/ foil or Devarda’s alloy.
Warm.
A colourless, pungent gas evolved
which turned moist red litmus paper
blue. The gas is ammonia.
-
CO32-
Add any dilute acid.
Brisk effervescence. A colourless,
odourless gas evolved which gives a
white precipitate with limewater
(calcium hydroxide). The gas is
carbon dioxide.
A white precipitate is
obtained because of the
calcium carbonate
(CaCO3) formed.
Cl-
Add dilute nitric acid.
Add aqueous silver nitrate.
A white precipitate is formed.
Silver chloride
AgCl
I-
Add dilute nitric acid.
Add aqueous lead(II) nitrate.
A yellow precipitate is formed.
Lead(II) iodide
PbI2
Add dilute nitric acid.
Add aqueous barium nitrate.
A white precipitate is formed.
Add dilute hydrochloric acid.
Add aqueous barium chloride.
A white precipitate is formed.
NO3
SO4
-
Barium sulfate
BaSO4
2-
Note:
The purpose of adding dilute nitric acid before adding the specific reagents is to prevent the
precipitation of CO32- ions (i.e. to eliminate the possible presence of CO32- ions) and at the
same time, not interfere with the test (because nitrates ions do not form precipitates as nitrate
compounds are soluble in water).
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Tests for Gases:
Gas
Colour & Odour
Test
Observations
Ammonia
Colourless gas with
a pungent smell
Place a piece of moist red litmus
paper in the gas.
Red litmus paper turns blue.
Carbon
dioxide
Colourless and
odourless gas
Bubble the gas through aqueous
calcium hydroxide (limewater).
White precipitate is formed.
Chlorine
Greenish-yellow gas
with a pungent smell
Place a piece of moist blue
litmus paper in the gas.
Blue litmus paper turns red, and is
then bleached.
Hydrogen
Colourless and
odourless gas
Place a lighted wooden splint
near the gas.
Lighted splint is extinguished with a
`pop’ sound.
Oxygen
Colourless and
odourless gas
Place a glowing wooden splint
near the gas.
Glowing splint is relighted.
Sulphur
dioxide
Colourless gas with
a pungent smell.
Smells like burning
rubber.
Place a drop of acidified
potassium
manganate(VII)
solution on a piece of filter
paper, and then place it in the
gas.
Purple acidified purple potassium
manganate(VII) solution is
decolourised.
Tests for Water or Water Vapour:
There are 2 chemical tests to detect the presence of water or water vapour:
1. Use anhydrous copper(II) sulphate
Water will change the colour of anhydrous copper(II) sulphate from white to blue.
2. Use dry cobalt(II) chloride paper
Water will change the colour of dry cobalt(II) chloride paper from blue to pink.
Note that these 2 tests only show the presence of water. They cannot be used to test for the
purity of water.
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SUMMARY (CATIONS)
Concept Map 1:
Reaction with aqueous sodium hydroxide
Salt solution
aqueous sodium
hydroxide
White ppt:
Al3+, Ca2+, Pb2+, Zn2+
Blue ppt:
Cu2+
Dirty-green ppt:
Fe2+
Reddish-brown ppt:
Fe3+
Excess aqueous
sodium hydroxide
No ppt:
NH4+, Na+, K+
Warm
Ammonia produced:
NH4+
White ppt soluble:
Al3+, Pb2+, Zn2+
Concept Map 2:
White ppt insoluble:
Ca2+
Reaction with aqueous ammonia
Salt solution
aqueous ammonia
White ppt:
Al3+, Pb2+, Zn2+
Blue ppt:
Cu2+
Dirty-green ppt:
Fe2+
Reddish-brown ppt:
Fe3+
Excess aqueous
ammonia
White ppt soluble:
Zn2+
White ppt insoluble:
Al3+, Pb2+
Blue ppt soluble:
Cu2+
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No ppt:
Ca2+, NH4+, Na+, K+
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SUMMARY (ANIONS)
1.
Test for carbonate (CO3 2-)
dilute acid
carbonate
carbon dioxide gas
2.
Test for nitrate (NO3 -)
sodium hydroxide and aluminium powder/ foil (Devarda’s alloy)
nitrate
warm
ammonia gas
3.
Test for chloride (Cl - )
acidified silver nitrate
chloride
white precipitate
4.
Test for iodide ( I- )
acidified lead (II) nitrate / silver nitrate
iodide
yellow precipitate
5.
Test for sulfate ( SO42- )
acidified barium nitrate / barium chloride
sulfate
white precipitate
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IONIC EQUATIONS (CATIONS)
Cation
Add aqueous sodium
hydroxide/aqueous ammonia:
Colour of
Identity of precipitate
precipitate
Al3+
White
Aluminium hydroxide
Ca2+
White
Calcium hydroxide*
Cu2+
Blue
Copper(II) hydroxide
Fe2+
Dirty-green
Iron(II) hydroxide
Fe3+
Reddish-brown
Iron(III) hydroxide
Pb2+
White
Lead(II) hydroxide
Zn2+
White
Zinc hydroxide
NH4+
-
-
Ionic Equation:
Al3+ (aq) + 3OH- (aq)
NH4+ (aq) + OH- (aq)
Al(OH)3 (s)
NH3 (g) + H2O (l)
* For the calcium ion (Ca2+), very little or no precipitate is formed with aqueous ammonia.
Note:
For the ammonium ion (NH4+), ammonia gas (NH3) is evolved when warmed with aqueous
sodium hydroxide.
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IONIC EQUATIONS (ANIONS)
Test:
Identity of
precipitate
Ionic Equation:
Aqueous sodium
hydroxide +
Aluminium powder
(with warming)
-
-
CO32-
Dilute hydrochloric
acid
-
Cl-
Dilute nitric acid +
Silver nitrate
Silver
chloride
I-
Dilute nitric acid +
Lead(II) nitrate
Lead(II)
iodide
Anion
NO3
-
CO32- (aq) + 2H+ (aq)
CO2 (g) + H2O (l)
Dilute nitric acid +
Barium nitrate
SO42Dilute hydrochloric
acid + Barium
chloride
Barium
sulfate
Note:
For the nitrate ion (NO3-), ammonia gas (NH3) is evolved when warmed with aqueous sodium
hydroxide and aluminium powder.
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PRECIPITATION ILLUSTRATION (CATIONS)
To illustrate how a precipitation reaction takes place for cations, consider the following model
for the test for Cu2+ ions.
Cu2+
Xm-
+
XmCu2+
Cu2+
OH-
Cu2+
Xm-
Xm-
Na+
OHCu2+
Na+
Test solution
OH-
OH-
OHNa+
Reagent
XmNa
+
Xm-
Na+
spectator ions
Na+
m-
X
Xm-
OH- Cu2+ OH-
Cu2+
OH-
Cu2+ OH-
OH-
Cu2+
Cu2+
precipitate (ppt)
1. Suppose the above test solution is copper(II) nitrate, can you work out the ionic equation
for the precipitation reaction?
2. Suppose the above test solution is copper(II) sulfate, can you work out the ionic equation
for the precipitation reaction?
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PRECIPITATION ILLUSTRATION (ANIONS)
To illustrate how a precipitation reaction takes place for anions, consider the following model
for the test for Cl- ions.
ClYn+
ClCl-
+
Yn+
Ag+
ClYn+
Yn+
Ag+
Cl-
NO3-
Ag+
NO3Ag+
Ag+
NO3-
Reagent
Test solution
Yn+
Yn+
NO3Yn+
NO3-
NO3-
spectator ions
Yn+
Cl-
Ag+
Cl-
Ag+
Cl-
Ag+
Cl-
Ag+
Cl-
Ag+
precipitate (ppt)
1. Suppose the above test solution is potassium chloride, can you work out the ionic equation
for the precipitation reaction?
2. Suppose the above test solution is hydrochloric acid, can you work out the ionic equation
for the precipitation reaction?
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