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)-sky-blue 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 neutralize an acidic solution – no apparent observation
  • Used to displace 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 hydroxides just like 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 which dissovless in excess NH3 to give a deep a blue solution
  • used to distinguish between silver halides AgCl(s) dissolves in dil NH3 while the AgBr(s) & AgI(s) do not.

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