Test For Gas

Test For Gas: A gas can be known from its characteristics.The various experiments are performed by the scientist so as to use in practical application. The following are the experiments of the various gases in Laboratory with results.Test For Gas

Test for Hydrogen(H2)

  • The evolved gas is colourless, odourless and neutral to litmus.
  • Pure hydrogen burns with a pale blue flame when a burning splint is brought near it.
  • Hydrogen burns in air with a pop sound when a burning taper is brought near it.

Test: Oxygen(O2)

  • The gas is colorless, odorless and neutral to litmus.
  • It rekindles a glowing splinter.
  • The gas is absorbed in the colorless alkaline solution of pyrogallol and turns it dark brown.

Test for Carbon Dioxide (CO2)

  • The gas is colourless and odourless.
  • It turns moist blue litmus faint red.
  • When the gas is passed through limewater, it turns milky due to the formation of the white precipitate spirit of calcium carbonate.

Ca(OH)2 + CO  → CaCO3 ↓ +  H2O

The milkiness disappears on passing excess of CO2

CaCO3 + CO2 +  H2O → Ca(HCO)3

  • The gas has no effect on the filter paper dipped in acidified K2Cr2O7 or KMnO4

Test for Chlorine (Cl2)

  • The gas is greenish-yellow having a sharp pungent choking odour.
  • It turns, a moist blue litmus paper red and finally bleaches i.e., decolourises it.

CI2 + H2O → HCl + HClO

  • It turns moist starch iodide paper (Kl + Starch solution) blue-black.

CI2 + 2KI →  2KI  +  I2

Starch + l2 → Blue-Black colour

  • Pass the gas through silver nitrate solution, a white ppt. is formed.

Test: Hydrogen Chloride (HCl)

  • The gas is colourless with a pungent choking odour.
  • The gas turns moist blue litmus paper red.
  • If a rod dipped in ammonia solution is brought near the gas dense white fumes of ammonium chloride are formed.
  • NH3(aq) + HCl → NH4CI
  • HCl Gas, when passed through silver nitrate solution white precipitate, is formed.
  • AgNO3(aq) + HCl → AgCl ↓
  • The ppt dissolves in excess of NH4OH,
  • AgCl + 2NH4OH -→ Ag(NH3)2Cl + 2H2O

Test for Sulphur Dioxide (SO2)

  • The gas is colourless with a smell of burning sulphur i.e., suffocating odour.
  • It turns moist blue litmus red and finally bleaches it.
  • The gas turns lime water turbid (milky).
  • Ca(OH)2 + SO  → CaSO3 ↓ +  H2O
  • It decolourises pink potassium permanganate solution.
  • 2KMnO+ 2H2O + 5SO2 →K2SO+ 2MnSO4  + 2H2SO4.
  • It changes orange/yellow solution of acidified potassium dichromate green.
  • K2Cr2O7 + H2SO4 + 3SO Cr2(SO4)3 +K2SO4 +  H2O
  • It has no effect on lead acetate paper.

Test: Hydrogen Sulphide Gas( H2S)

  • It is colourless having a foul smell of rotten eggs.
  • It turns moist blue litmus to red.
  • It turns lead acetate paper black.
  • (CH3COO)2  + H2S  → PbS ↓ +  2CH3COOH
  • It also turns lead nitrate solution black.
  • Pb(NO3)2 + H2S s → PbS ↓ + 2HNO3

Test for Ammonia(NH3)

  • It is is colourless with a sharp рungent characteristic smell.
  • It turns moist red litmus to blue.
  • If a rod dipped in concentrated HCl is brought near the gas, dense fumes of ammonium chloride are formed.
  • It turns colourless Nessler’s reagent i.e. (K2HgI4) potassium mercuric iodide brown.

Test: Water Vapour (H2O)

  • Colourless, odourless forms a clear liquid on the cooler parts of the test-tribe.
  • The liquid is neutral litmus.
  • This liquid turns anhydrous copper sulphate from white to blue.
  • CuSO4 + 5H2O → CuSO4.5H2O
  •  It turns blue cobalt chloride paper pink.
  • CoCl + 6H2O → CoCl2.6H2O

Test for  Nitrogen Dioxide(NO2) Gas

  • It is brown in colour having an irritating (pungent) odour and non-combustible.
  • It turns moist blue litmus paper to red.
  • It turns starch Iodide paper from colourless to blue-black.
  • 2KI + 2NO2 → 2KNO2 + I2
  •  Turns potassium iodide paper brown.
  • It turns green acidified ferrous sulphate solution brown.

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