Qualitative Analysis

Invariably involves detection or identification of cation(s) and or anion(s) present in given test substance in a series of reactions.

Theoretical basis

  • Mainly inorganic reactions.


Skills tested

  • Observational
  • Colour change detection
  • Scientific deductive skills nasedon chemistry concepts
  • Handling of apparatus


Preliminary Deductions

this is an informed guess about the possible identity of the test substance based on its physical appearance.


Guidelines to the identification of solids

Key Questions
  • is it coloured?
  • is it crystalline/ powder?


Nature of test substance Preliminary Deduction
White cystalline solid
  •  not likely to be a transition metal compound (usually coloured).
  • not likely to be an oxide/carnonates (powders).
  • likely to have water of crystallization.
White powder
  • not likely to be a transition metal compound
  • likely to be an oxide/carnonates/slphates of non-trans element.
  • manganese carbonate is almost white.
Coloured crystalline
  • likely to be a transition metal compound (nitrates /halides /some sulphates of trans metals)
Coloured powder
  • likely to be caronate of transition metal e.g CuCO3 is green powder.
  • it can be a sulphate/ another compound of a trans metal which has lost its ater of crystallisation.
  • red lead oxide, Pb3O4 (non-transition element).
Black powder
  • likely to be a metal sulphide/ oxide of a transition metal/ non
  • most metal sulphides are black powders eg PbS

Guidelines to the identification of species in solutions

Key questions

  • is it coloured or not?
  • is it clear coloured or clear colourless?
Nature of test substance Preliminary deduction
Clear colourless solution
  • no transition cation /oxoanion /complex present
  • substance is soluble
A strangely coloured solution
  • likely to be a mixture of trans metal cation(s).
  • if solution is a mixture of trans cations, 20 colours of two cations should be expected eg Fe3+(a) -redbrown and Cu2+(aq) -blue = grey green colour
A clear colured solution
  • a transition metal/ oxoanion/ complex is likely to be present.
  • maybe mixed with a soluble compound of a non transition metal


The colours of aqueous ions


Ion Colour
Cu2+ pale blue
Fe2+ light green
Fe3+ red brown
Cr3+ green
Mn2+ faint pink
MnO4 purple/violet
CrO42- yellow
Cr2O72- Orange


Tips and hints
  • Follow instructions
  • Do not put too much test reagent in the test tube
  • initially add one dropat a time with shaking and careful observations.


Golden rule
  • Before you add any reagent, always ask yourself the following question(s) first
  • What i this test for?
  • What does the reagent to be added detect?


Common Reagents and their uses

The commonly used reagents

dilute NaOH HCl(aq) dilute H2SO4 KI(aq) K2Cr2O7(aq) BaCl(aq) NaHCO3(solid & aq) Dilute NH3
dilute HNO3 dilute H2O2 CrO42- AgNO3 NaCO3(solid & aq) Starch solution Na2S2O3(aq)


Different Uses of reagents in Qualitative Analysis

Dilute H2SO4

  • to decompose carbonates to produce CO2(g)
  • expect and test for CO2(g) when suspected solid or liquid contain carbonates
  • the sulphates in dilute H2SO4 can detect Ba2+(aq) & Pb2+(aq) which form insoluble sulphates.
  • can be used to neutralise a basic solution.
  • to identify CrO42-(aq) and Cr2O72-(aq) whosecolors are characteristic in acid or alkaline solutions
  • to liberate SO2(g) form SO32-(aq) and NO2(g) from NO2(aq)
  • to provide an acidic medium for oxidizing agents eg MnO4(a), CrO42-(aq), H2O2 or Cr2O72-(aq)


Conc H2SO4

  • used to detect salts of waeker acids or less volatile acids from whose salts they are displaced
  • when conc H2SO4 is added to a sold test substance and the following observations and deductions are made;
Test Observation(s) Deduction(s)
add conc H2SO4 HCl(g) evolve Cl present
Br2 & HBr(g) produced Br
I2(g) I present
O2(g) form a dark brown solid. A white residue remains PbO2(s) present

Dilute HCl(aq)

  • the Cl (aq) can be used to detect Pb2+ by forming a white ppt of PbCl2(s) which dissolved when the sln is warmed but reappears after being cooled
  • same as dil H2SO4 in;
  • decomposing carbonates.
  • neutralizing base.
  • acidifying some reactions


Concentrated HCl

  • used to detect oxidizing agents eg MnO4
  • it is oxidised to Cl2
  • To detect presence of a basic oxide -the solid dissolves upon addition of con HCl


Dilute HNO3

  • can be used to decompose carbonates and neutralise bases
  • it is usually more preferable as a neutralizer since nitrate present does not form an insoluble ppt witth any cations that might be in the test substance
  • to dissolve an formed ppt ie the filtered residues before further tests on it.


Dilute sodium hydroxide NaOH(aq)

  • precipitates metal cations (except those of group [1]) in form of hydroxides which are insoluble/soluble in water.
  • M2+ + 2OH2- → M(OH)2(s)-ppt eg Cu2+ + OH-(aq) & Cu(OH)2(s)-skyblue ppt
  • some ppts dissolve upon adding excess NaOH eg Pb2+ and Zn2+
  • as the complex formed dissolves;
  • M2+ + 2OH2- → M(OH)2(s) + 2OH → M(OH)42-(aq) eg Al(OH)42-(aq), Zn(OH)42-(aq) & Pb(OH)42-(aq)
  • Colour of ppt depens on the cation present it is a transition metal.
  • Can be use to neutralizean acidic solution – no apparent observation
  • Used to dsplace H2(g) when metals eg Al or Zn are added NaOH(aq)
    eg 2Al(s) + 2NaOH(aq) + H2O(l) → 2NaAl(OH)3 + 3H2(g).
  • Also used to displace a weaker base eg ammonia in the test for nitrates.


Ammonia NH3(aq)

  • precipitates metal cations in form of metal hydrroxides justlike NaOH(aq)
  • Ammonia partially dissociates ie has is very low concentration of OH(aq)
  • NaOH(aq) fully dissociates ie has high concentration of OH(aq)
  • used for the confirmatory test for Cu2+(aq)- a pale ppt whichdissovless in excess NH3 to give a deep a blue sln
  • used to distinguish between silver halides AgCl(s) dissolves in dil NH3 while the AgBr(s) & AgI(s) do not.


Oxidizing Agents

  • Common used oxidizing agents include;
  • KMnO4
  • K2CrO7
  • H2O2
  • *usually requires acidic conditions eg adding sulphuric acid before carrying out the test except MnO4 requires alkaline conditions.


i)KMnO4(aq)- purple

  • used to detect Fe2+(aq) in acidic medium.
Test Observation Notes
add dil H2SO4 followed by KMnO4(aq) KMnO4 is decolourised Fe2+ is oxidised to Fe3+ and MnO4(aq) is reduced to Mn2+(aq)
ii)Oxidation of iodide ions
  • I ions are oxidised to yellowish brown iodine(aq)
  • MnO4(aq) are reduced to faint pink Mn2+(aq)


iii)Oxidation of hydrogen peroxide
  • MnO4(aq) oxidize H2O2 to waterr and oxygen
  • MnO4(aq) are reduced to faint pink/colourless Mn2+(aq)


iv)Oxidation of Ethanol
  • it is oxidized in alkaline medium to ethanoic acid
  • MnO4(aq) are decolourised



  • stabilized by an acidic environment.
  • acts as an oxidizing agent, while it is reduced to green Cr3+
  • like MnO4 ions, dichromate ions oxidize;
  • Fe(ii) to Fe(iii)- dirty green observed due to mixture of green & yellowish brown;
  • I ions to I2;
  • H2O2 to H2O & O2
  • Dichromate ions oxidize sulphite ions to sulphate and nitrate ions to nitrate.
  • Ethanol can be used to test for dichromate(vi) ions- the sln turns to green


Hydrogen peroxide(H2O2)- colourless

  • can work in both acidified and alkaline medium.
  • it is reduced to water and there is no O2(g) that is liberated
  • oxygen is liberated only when it is acting as a reducing agent
The table below shows the observations and deductions made when H2O2(aq) is added to a test substance containing some cations and anions;
Observations Deductions Notes
Solutions turns red-brown from pale green Fe(ii)(aq) present Fe(ii)(aq) is oxidized to Fe(iii)(aq)
Solutions turns dark brown & black-brown ppt from pale pink Mn2+(aq) prsent Oxidation of Mn2+ to MnO2(s)
Solution changes from colourless to orange-brown;
a black ppt settles at the bottom after a few minutes
I(aq) present I is oxidized to I2(s).
black ppt are crystals of iodine(s).
orange-brown sln consists of I3-(aq)

Reducing Agents

  • Commonly used reducing agents include;
  • KI-iodides;
  • Sodium thiosulphate -thiosulphate ions

Potassium Iodide, KI

  • I(aq) is oxidized to I2(aq)-brown or I2(s)-black ppt


Observations and deductions when KI is added to various test reagents
Observations Deductions Notes
Pale solution to brown ppt plus few white ppt Cu2+ present Cu2+ is reduced to Cu1+
white ppt is CuI(s)
a bright yellow ppt soluble in excess KI to give colourless sln Pb2+ present yellow PbI2 dissolves in excess KI bcz a complex PbI42- is formed
A dark brown sln from pale red brown sln; black ppt forms slowly Fe3+ Fe(iii) to Fe(ii)
purple sln to faint pink sln Mn2+ present Mn7+ to Mn2+

Hydrogen Peroxide

  • it can act as a reducing agent in some reactions
  • oxygen gas is liberated, you should prepare to test for oxygen.
  • examples include;
  • Acidified hydrogen peroxide reduces Mn(7+) to Mn(2+)
  • pH change may affect the behaviour of hydrogen peroxide in redox reactions eg;
  • in acidic medium: Mn(7+) is reduced to Mn(2+)
  • in alkaline medium: Mn(2+) is oxidized to Mn(4+) -dark brown ppt

Reactions Of Transition metal cations

Reaction of Cu2+

Cu2+(aq) (pale blue) 1ons can be obtained b
Dissolving a solub1e copper (II) salt in water, for example solid copper (II) sulphate (pale blue).
Dissolving black CuO in an acid such as sulphuric acid.

Test Observation Notes
(a) Add dil HCl and warm Solution turns form light blue to yellow-green warming drives out some water and thus increases the concentration of Cl ligands, resulting in the formation of yellow complex. Ggreenish-yellow is caused by mixing of blue of Cu2+ and yellow of [CuCl4]2
Add conc HCl Solution turns from light blue to yellowish-green Chlorine ligands replace water ligands from the aqua complex
Add NaOH(aq) Pale blue ppt formed, insoluble in exess NaOH Precipitation of Cu2+ by OH ions

Add KI(aq) followed by;


(i) Na2S2O3


(ii) Starch solution

Solution turns pale blue to brown. white ppt is formed.

Brown colour disappears, leaving a white solid


blue black colour appears

2Cu2+(aq) + 4I → 2CuI(s) + I2(aq). CuI(white ppt)

Brown iodine is reduced to colourless iodide by sodium thiosulphate


This is a test to confirm the presence of iodine

Add NH3(aq) A blue ppt is formed soluble in xs ammonia to give a deep blue solution Cu(OH)2(s) is first precipitated. The ppt dissolves in xs ammonia to form a deep blue complex


Reaction of Fe2+

Colour : pale green, almost colourless.

Test Observation Notes

Add H2O2


Then add dilute H2SO4(aq)


Followed by NaOH(aq)

Solution turns from light green to yellowish brown

Intensity of yellowish-brwon colour increases


A red-brown ppt is formed upon addition of a few drops of NaOH(aq)

Fe2+(aq) is oxidized by hydrogen peroxide to Fe3+(aq) which is yellowish-brown

The action of hydrogen peroxide as an oxidising agent is promoted by an acid


Fe3+(aq) from the oxidation of Fe2+(aq) by hydrogen peroxide is precipitated as Fe(OH)3(s), which is a red-brown solid.

Add NH3(aq)


Leave ppt in the air for a few minutes

A green ppt is formed


ppt changes from green to red-brown

Fe(OH)2(s) is produced.

Fe(OH)2(s) oxidized by oxygen to red-brown Fe(OH)3(s)

Add NaOH(aq) Pale green ppt is formed insoluble in xs. The ppt slowly turns brown in air Fe3+ precipitated as Fe(OH)3(s).
Add dilute H2SO4 followed by KMnO4(aq), drop by drop KMnO4 is decolourised. A yellowish-brown solution is formed Oxidation of Fe3+ by KMnO4 to form a yellowish-green solution of Fe3+. The yellowish-brown colour observed is due to Fe3+.


Reaction of Fe3+

Colour : yellow brown to orange when unmixed with other colored cations

Test Observation Notes
Add solid NaCO3 Effervescence occurs, gas which turns limewater milky is produced. A coffee brown ppt is formed Brown ppt is formed Fe(OH)3. Fe3+ is acidic, protonate water hence carbonate is dicomposed.

Add KI(sq) followed by either;


Strach solution



Solution turns deeper brown


a blue black colour appears


Brown colour disappears leaving a pale green solution

I oxidized to brown iodine. meanwhile Fe3+ is reduced to pale green Fe2+ whose color is masked by that of iodine.

The blue black colour is due to the formation of an iodine/starch complex


Brown iodine is reduced by thiosuphate ions to I leaving Fe3+


Reaction of Cr3+

Its colour is green. A solution of Cr3+ is commonly provided in form of CrCl3(aq). In that case the solution is blue-violet rather than green this is caused by the environment of Cl and hydrolysis

Test Observations Notes
Add NH3 an immediate green ppt soluble in xs is formed Cr(OH)3(s)
Add solid sodium carbonate Effervescence, gas which tunrs limewater milky is produced.Solution turns to grey-green. A pake green ppt is also formed Solution of Cr3+(aq) is acidic and so reacts witha carbonate to produce CO. Green ppt is Cr(OH)3(s)
Add NaOH(sq) an immediate grey-green ppt is formed whichis soluble in xs NaOH to form a dark green solution Grey-green ppt is Cr(OH)3(s). The ppt reacts with further xs OH ions in a ligand exchange reaction forming a dark green complex [Cr(OH)6]3+
Add NaOH(aq) followed by 20Vol Hydrogen peroxide. Solution truns yellow Cr3+ oxidized to CrO42-


Reaction of Mn2+

Color : pale pink, almost colourless

Test Observations Notes
Add NaOH(aq) An off-white ppt is formed insoluble in xs NaOH and rapidly turning brown on contact with air Off-white ppt is Mn(OH)2(s). In air it is rapidly oxidized to brown Mn(OH)3(s) The brown color of the ppt continues to darken due to dehydration to form Mn2O3(s)

Add NH3


Fliter the solid and leave it in the air

off-white ppt is formed insoluble in xs NH3 and rapidly turning brown on contact with air


Brown colour of ppt becomes deeper

Off-white ppt is Mn(OH)2(s). In air it is rapidly oxidized to brown Mn(OH)3(s) The brown color of the ppt continues to darken due to dehydration to form Mn2O3(s)

Leave a Reply

Your email address will not be published. Required fields are marked *