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Practical Work

Detection of Some Gases

Identification of Ions

Salts are composed of cations and anions. They are dissolved in the following solvents to make solutions:
  • Water 
  • Nitric acid (if the salt does not dissolve in water)
Identification of cations:
  • by using of sodium hydroxide solution
  • by using of potassium hydroxide solution
  • by using of ammonia solution
 
Metal Sodium Hydroxide Solution Ammonium Solution
Colour of ppt   With Excess Sodium Hydroxide Solution Colour of ppt   With Excess Ammonium Hydroxide Solution
Ca White curdy Insoluble No ppt. No change
Pb White chalky Soluble White chalky Insoluble
Zn White gelatinous Soluble White gelatinous Soluble
Cu Pale blue Insoluble Pale blue Soluble
Fe (II) Pale green turning brown Insoluble Pale green turning brown Insoluble
Fe(III) Reddish brown Insoluble Reddish brown Insoluble


Identification of anions:
  • by using dilute sulphuric acid
  • by using concentrated sulphuric acid
  • by using nitric acid and barium chloride
 
  • CO32-, S2-, NO2- and SO32- react with dil. H2SO4 to give out CO2, H2S, NO2 and SO2 gases respectively.
  • Cl-, Br-, I-, NO3-, C2O42- and CH3COO- react with conc. H2SO4 but not with dil. H2SO4 to produce characteristic gases.
  • SO42- and PO43- react neither with dil H2SO4 nor with conc. H2SO4. These are, therefore, identified by individual tests.

Dil. H2SO4 Tests
  Experiment Observation Inference
1 Salt + dil. H2SO4
  • Brisk effervescence
  • Colourless gas
  • Odourless gas
  • Does not support combustion
  • Turns moist litmus red
  • Turns lime water milky
  • The gas evolved is carbon dioxide
  • Salt contains carbonate (CO32-)
2 Salt + dil. H2SO4
  • Rotten egg smelling gas
  • Turns moist blue litmus paper red
  • Turns moist lead acetate paper black
  • The gas evolved is hydrogen sulphide
  • Salt contains sulphide  (S2-)
3. Salt + dil. H2SO4
  • Suffocating gas
  • Turns golden yellow or orange coloured filter paper moist with acidified potassium dichromate green
  • The gas evolved is sulphur dioxide
  • Salt contains sulphite  (SO32-)


Conc. H2SO4 Tests
  Experiment Observation Inference
1 Salt + conc. H2SO4 + Heat
  • Colourless pungent gas (HCl)
  • Produces dense white fumes when a glass rod dipped in ammonia solution is brought near it
  • Greenish yellow gas evolves that turns moist starch iodine paper blue-black when pinch of manganese is added to solution
  • Chloride ion confirmed (Cl-)
2 Salt + conc. H2SO4 + Heat
  • Reddish brown fumes that turn thick on adding copper turnings
  • A brown ring form at the junction of two liquids when iron (II) sulphate solution followed by conc. sulphuric acid is added to salt (Brown ring test)
  • The gas evolved is nitrogen dioxide
  • Salt contains nitrate   (NO3-)

Test for Sulphate Ion
  Experiment Observation Inference
1 Salt + Nitric acid + Barium chloride solution
  • White ppt insoluble in mineral acid
  • When solution is treated with acetic acid and lead acetate solution, white ppt is obtained which is soluble in exc…

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