1.5 Gas Test


Example of Preparation



Decomposition of hydrogen peroxide

H2O2 -----> H2O + ½O2(g)

Thermal decomposition of some metallic oxides

2PbO -----> 2Pb + O2(g)

Ignites a glowing splint

since oxygen supports combustion


acid + metal -----> salt + hydrogen gas

2HCl + Zn -----> ZnCl2 + H2(g)

active metal + water -----> hydroxide + hydrogen gas

Ca(s)+ 2H2O--->Ca(OH)2+ H2(g)

"Pop Test" : a lit taper will produce an audible "pop" when placed in hydrogen gas

Carbon dioxide

acid + carbonate -----> salt + water + carbon dioxide


Thermal decomposition of some carbonates

CuCO3 -----> CuO + CO2(g)

When CO2(g) is bubbled through colourless limewater (Ca(OH)2(aq))the limewater turns milky due to the formation of a calcium carbonate (CaCO3(s)) precipitate.

When CO2(g) is bubbled through Ba(OH)2(aq) a white precipitate of BaCO3 forms.

Nitrogen dioxide

Thermal decomposition of some nitrates

2Ni(NO3)2--->2NiO + O2+ 4NO2(g)

2Pb(NO3)2--->2PbO + O2+4NO2(g)

Less active metals + nitric acid -----> nitrate + water + nitrogen dioxide

Cu + 4HNO3
---> Cu(NO3)2 + 2H2O + 2NO2(g)

Nitrogen dioxide is a red-brown gas which is soluble in water and has a pungent odour.

Sulfur dioxide

Combustion of sulfur

S(s) + O2 -----> SO2(g)

Sulfites + acid -----> water + sulfur dioxide

SO32- + 2H+ -----> H2 + SO2(g)

Hot concentrated sulfuric acid + copper

3H2SO4 + Cu(s)
--->Cu2++2HSO4-+2H2O +SO2(g)

SO2(g) is bubbled through hydrogen peroxide solution (H2O2) to produce sulfuric acid (H2SO4). The addition of barium chloride solution (BaCl2(aq)) to this solution results in a precipitate of barium sulfate (BaSO4(s)).

SO2(g) passed through a concentrated, acidified solution of potassium dichromate (K2Cr2O7) causes the dichromate solution to change colour from orange to green due to the formation of chromium (III) ions (Cr3+).

Hydrogen sulfide

sulfide + acid -----> salt + hydrogen sulfide

FeS(s) + 2HCl ---> FeCl2 + H2S(g)

H2S is often referred to as "rotten egg gas" as it smells just like rotten eggs!

H2S(g) is passed through a solution of zinc chloride (ZnCl2) and a white precipitate of zinc sulfide forms (ZnS(s)).

Pass H2S(g) over a damp filter paper impregnated with lead (II) nitrate (Pb(NO3)2), the paper turns black as lead (II) sulfide (PbS(s)) forms.


MnO2(s) + 2Cl- + 4H+
--->Mn2+ + 2H2O + Cl2(g)

Electrolysis of molten sodium chloride (NaCl)

2NaCl -----> 2Na(Hg) + Cl2(g)
(Na(Hg) is a mercury amalgam of sodium metal since sodium is slightly soluble in mercury but doesn't react with mercury as it does with other solvents like water.)

Cl2(g) is a pale yellow-green gas which bleaches coloured materials and has a characteristic choking odour.


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