Carbon monoxide, sulphur and nitrogen oxides, and acid rain
Three groups of air pollutants matter here: carbon monoxide (CO), sulphur oxides (SO₂, SO₃) and nitrogen oxides (NO, NO₂). Carbon monoxide is not carbon dioxide: CO is toxic, CO₂ is not a poison; both come from burning. CO forms when fuel burns incompletely, with too little oxygen. It passes from the lungs into the blood and binds to haemoglobin 200 to 250 times more strongly than oxygen does, forming carboxyhaemoglobin, so less oxygen reaches the tissues (hypoxia). Cells switch to anaerobic respiration and lactic acid builds up, causing tiredness and headache; higher levels cause dizziness, unconsciousness and brain damage. Sulphur oxides come mainly from burning coal and oil that contain sulphur in power stations and factories, and from refining and smelting. Nitrogen oxides form in large amounts in very hot burning, as in engines and furnaces. Acid rain: in the air SO₂ is oxidised to SO₃ (2SO₂ + O₂ → 2SO₃), which dissolves in cloud droplets to give sulphuric acid (SO₃ + H₂O → H₂SO₄). Nitrogen dioxide with water gives nitrous and nitric acids (2NO₂ + H₂O → HNO₂ + HNO₃), and overall 4NO₂ + O₂ + 2H₂O → 4HNO₃. Normal rain is already slightly acidic (pH about 5.6, from dissolved CO₂); acid rain is lower, often below pH 5. Acid rain does not burn skin. Breathing these gases irritates the airways and can set off asthma.
| Gas | Main source | Effect |
|---|---|---|
| CO | incomplete burning | less oxygen to tissues |
| SO₂ | coal and oil with sulphur | acid rain, irritates airways |
| NO₂ | very hot engines and furnaces | acid rain, irritates airways |
Count atoms with beads, with a grown-up. Make the left and right sides of 2SO₂ + O₂ → 2SO₃ using two colours of beads (S and O). Count the S and O on each side. Then make a flow chart on paper that starts at a power station chimney and ends at acid rain, with each step in your own words. No experiment with burning, smoke or exhaust gas.