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
  • ┬ánot 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

Procedure

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

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