| Gas |
Example of Preparation |
Test |
|---|
Oxygen
(O2) |
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 |
|---|
Hydrogen
(H2) |
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
(CO2) |
acid + carbonate -----> salt + water + carbon dioxide
2HCl+CaCO3-->CaCl2+H2O+CO2(g)
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
(NO2) |
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
(SO2) |
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
(H2S) |
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. |
|---|
Chlorine
(Cl2) |
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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