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Detection of Some Gases

Test for hydrogen (H2):

Hydrogen gas is liberated when active metals such as Na, K, Mg react with dilute acids. M + H2SO4 →            MSO4 + H2M + HCl →           MCl + H2M=Ca, Mg, Zn, etc. Few characteristics of this gas are as follows. i. It is a colourless and odourless gas. ii. When this gas is allowed to pass through a moist red or blue litmus paper, the colour of the paper does not change. This shows that hydrogen gas is neutral to litmus. iii. A burning wooden splint, when brought near this gas, gets off and burns with a pale blue flame producing a pop sound. 2 H2 + O2 →         2 H2O  

Test for oxygen (O2):

Oxygen gas is liberated on heating metal nitrates, potassium chlorate, potassium dichromate, potassium permanganate, hydrogen peroxide, barium peroxide and oxides such as HgO, PbO2, Pb3O4. 2 PbO2 →     Δ      2 PbO + O22 HgO →      Δ       2 Hg + O2 Few characteristics of this gas are as follows.

i. It is a colourless and odourless gas.

ii. When this gas is allowed to pass through a moist red or blue litmus paper, the colour of the paper does not change. This shows that oxygen gas is neutral to litmus.

iii. A burning wooden splint when brought near this gas re-lights brightly which shows that it is a supporter of combustion.

 

Test for water vapour (H2O):

Water vapour is liberated on heating salts containing water of crystallisation, metallic hydroxides and metallic hydrogen carbonates. Na2CO3.10H2O →      Δ      Na2CO3 + 10 H2OCuSO4.5H2O →      Δ       CuSO4 + 5 H2O Few characteristics of this gas are as follows.

i. It is a colourless and odourless gas.

ii. When this gas is allowed to pass through a moist red or blue litmus paper, the colour of the paper does not change. This shows that water vapour is neutral to litmus.

iii. It turns anhydrous copper sulphate to blue. CuSO4 + 5 H2O →           CuSO4.5H2O

iv. It turns blue copper chloride to pink. CoCl2 + 2 H2O →            CoCl2.2H2O

 

Test for ammonia (NH3):

Ammonia is liberated by heating ammonium salts with alkalies and treating metallic nitrides with warm water. 2 NH4Cl + CaOH2 →          CaCl2 + 2 H2O + 2 NH3 Few characteristics of this gas are as follows.

i. It is a colourless gas with a strong biting odour that brings tears to eyes.

ii. When this gas is allowed to pass through a moist red litmus paper, the colour of the paper changes to blue. This shows that ammonia is basic in nature.

iii. Dense white fumes are formed when a rod dipped in HCl is brought near this gas. NH3 + HCl →         NH4Cl iv. It turns Nessler's reagent (K2HgI4) brown. v. It forms a pale blue precipitate when passed through copper sulphate solution. This precipitate is soluble in excess of the gas and the solution turns dark blue in colour.

 

Test for carbon dioxide (CO2):

Carbon dioxide is liberated by strong heating of metallic carbonates and hydrogen carbonates. It is also liberated when dilute mineral acids are treated with carbonates and hydrogen carbonates of metals. ZnCO3 →      Δ       ZnO + CO2CaCO3 + 2 HCl →                 CaCl2 + H2O + CO2NaHCO3 + HCl →                 NaCl + H2O + CO2 Few characteristics of this gas are as follows.

i. It is a colourless and odourless gas.

ii. It turns lime water milky. CaOH2 + CO2 →           CaCO3 + H2O

ii. When this gas is allowed to pass through a moist blue litmus paper, the colour of the paper changes to light red. This shows that carbon dioxide is acidic in nature.

iii. A burning wooden splint when brought near this gas goes off which shows that it is not a supporter of combustion. iv. It has no effect on filter paper dipped in acidified potassium dichromate (K2Cr2O7) or potassium permanganate (KMnO4) solution.

 

Test for sulphur dioxide (SO2):

Sulphur dioxide is liberated by strong heating of metallic sulphites and hydrogen sulphites. It is also liberated when dilute mineral acids are treated with sulphites and hydrogen sulphites of metals. CaSO3 →      Δ      CaO + SO2Na2SO3 + 2 HCl →              2 NaCl + H2O + SO22 NaHSO3 + H2SO4 →               Na2SO4 + 2 H2O + 2 SO2 Few characteristics of this gas are as follows.

i. It is a colourless gas having suffocating odour.

ii.  It turns lime water milky. CaOH2 + SO2 →           CaSO3 + H2O

iii. When this gas is allowed to pass through a moist blue litmus paper, the colour of the paper changes to red. This shows that sulphur dioxide is acidic in nature.

iv. It turns potassium permanganate solution colourless. 2 KMnO4 + 2 H2O + 5 SO2 →           K2SO4 + 2 MnSO4 + 2 H2SO4

v. It changes the colour of acidified potassium dichromate from orange to green. K2Cr2O7 + H2SO4 + 3 SO2 →             Cr2SO43 + K2SO4 + H2O

 

Test for hydrogen sulphide (H2S):

Hydrogen sulphide is liberated by the action of dil. HCl or dil. H2SO4 on metallic sulphides. FeS + H2SO4 →           FeSO4 + H2S Few characteristics of this gas are as follows.

i. It is a colourless gas having rotten egg like smell.

ii.  It turns lead acetate solution silvery black. CH3COO2Pb + H2S →              PbS + 2 CH3COOH

iii. It turns moist blue litmus paper red. This shows that it is acidic in nature. iv. It turns lead nitrate solution black i…

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