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
+ 4
On warming, a colourless, pungent gas evolved which turned moist red litmus paper blue. The gas is ammonia.
No visible reaction.
-
No visible reaction. No visible reaction.
No visible reaction. No visible reaction.
-
Cu2+
NH
Na+ K+
Lead(II) hydroxide Pb(OH)2
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 PbI2.
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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 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.
NO
SO
3
Barium sulphate BaSO4
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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).
Limewater turns milky.
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.
Colourless gas with a pungent smell
Place a drop of acidified potassium dichromate(VI) solution on a piece of filter paper, and then place it in the gas.
Acidified orange potassium dichromate(VI) solution turns green.
Sulphur dioxide
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+
SUMMARY (ANIONS) 1.
Test for carbonate (CO3 2-) dilute acid carbonate
carbon dioxide gas 2.
Test for nitrate (NO3 -) sodium hydroxide and aluminium powder (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
iodide
yellow precipitate 5.
Test for sulphate ( SO42- ) acidified barium nitrate / barium chloride
sulphate
white precipitate 5
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
White
Zinc hydroxide
-
-
Zn2+ 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 sulphate
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-
m-
X Cu2+
Cu2+
OH-
2+
Cu Xm-
Xm-
Na+
OHCu2+
Na+
Test solution
OH-
OH-
OHNa+
Reagent
XmNa+
Xm-
Na+ Na+
Xm-
spectator ions
Xm-
OH- Cu2+
OH-
Cu2+
OH-
Cu2+ OH-
Cu2+
OH-
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) sulphate, 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.
+ Cl-
n+
Y
Yn+
ClCl-
Cl
Ag+
-
Yn+ Yn+
NO3-
Ag+ Cl-
Ag+
NO3-
Ag+
Ag+ NO3-
Reagent
Test solution
Yn+
Yn+
NO3-
3
NO
Y
NO3-
n+
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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