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Practice Make Perfect 3!

Practice Make Perfect 2!

1.5 Gas Test Video

Dear students, this video help you to understand the gast test.

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.

Announcement 1!

1.4 Qualitative Analysis

Next we are going to learn about qualitative analysis. What is qualitative analysis? Qualitative analysis is a Techniques for identify species in chemistry. It involves several techniques such as
  • Physical properties (colour and solubility)
  • Gas test
  • Action of heat
  • Cation Test
  • Anion Test
  • Confirmatory Test