First we added a few drops of the indicator, bromothymol blue, to 10 mL of dem. water in a test tube until the water became slightly blue. Then we placed a match in a clean beaker and lighted it. We covered the beaker with a lid. When the match had burned out we added bromothymol blue solution from the test tube to the beaker. Afterwards we shook the beaker to make sure it mixed.
In the cupboard we heated a spatula of lead nitrate in a test tube. We observed how the lead nitrate decomposed to lead(II)oxide, PbO, and nitrogen dioxide, NO2. When heavy fumes of nitrogen dioxide occurred we immediately poured them into a clean beaker. Again we added bromothymol blue solution from the test tube to the beaker and shook it vigorously.
Observations
When we added bromothymol blue solution to the beakers, they turned yellowish. In the second experiment the fumes were brown.
Explanation
The chemical reactions taken place when lighting the match and heating the lead nitrate, were:
S(s) + O2(g) → SO2(g)
2Pb(NO3)2(s) + O2(g) → 2PbO(g) + 4NO2(g) + 2O2(g)
Since the colour changed from blue to yellow when we added bromothymol blue, we know that an acid is present in both experiments.
What caused these changes to occur was the oxides of sulphur and nitrogen. The reason why we can see these changes is because we have added demineralised water and the indicator bromothymol blue.
Nitrogen oxides occur naturally from electrical storms and bacterial action.
Nitrogen monoxide is produced in the internal combustion engine and in jet engines:
N2(g) + O2(g) → 2NO(g)
Oxidation to nitrogen dioxide occurs in the air:
2NO(g) + O2(g) → 2NO2(g)
The nitrogen dioxide then reacts with the water to form nitric acid and nitrous acid:
2NO2(g) + H2O(l) → HNO3(aq) + HNO2(aq)
or is oxidized directly to nitric acid by oxygen in the presence of water:
4NO2(g) + O2(g) + 2H2O(l) → 4HNO3(aq)
Sulphur dioxide occurs naturally from volcanoes. It is produced industrially from the combustion of sulphur containing fossil fuels and the smelting of sulphur ores:
S(s) + O2(g) → SO2(g)
In the presence of sunlight sulphur dioxide is oxidized to sulphur trioxide:
SO2(g) + ½O2(g) → SO3(g)
The oxides can react with water in the air to sulphuric acid:
SO2(g) + H2O(l) → H2SO3(aq)
and
SO3(g) + H2O(l) → H2SO4(aq)
Conclusion and evaluation
We have made a model of how gases can dissolve in water to make water more acidic. We have also found out how acid rain is formed.
The experiment was successful.
To improve the experiment knowing more exact amounts of each reactant would be preferable, e.g. the amount of sulphur in the match, in order to do some calculations and achieve some precise numbers which could be compared with official limits etc.
