Explain the differences between pure air and polluted air.
Answer:
Pure air contains around 78% nitrogen, 21% oxygen and 0.03% carbon dioxide. Other gases such as argon, methane, ozone and water vapours are also present in small quantities. When this composition of air is altered by the addition of harmful substances or gases such as nitrogen dioxide, sulphur dioxide, carbon monoxide, and particulate matter, then the air is said to be polluted.