Write the essential function performed by ozone at the higher levels of the atmosphere. How is it formed in the upper atmosphere? Write the name of the group of chemicals mainly responsible for the depletion of ozone layer.

This is a direct recall-and-explain question grounded in Section 13.2.1 'Ozone Layer and How it is Getting Depleted'. It has three distinct parts each carrying roughly 1 mark:

1. Function of ozone — the chapter explicitly states ozone 'shields the surface of the earth from ultraviolet (UV) radiation from the Sun,' and that this radiation 'is highly damaging to organisms, for example, it is known to cause skin cancer in human beings.' Note the contrast the NCERT draws: ozone ($O_3$) at ground level would be a 'deadly poison,' unlike the $O_2$ essential for aerobic respiration — but at high altitude it performs this protective role.

1. Formation mechanism — ozone is a product of UV radiation acting on molecular oxygen. The two-step reaction given in the text is: high-energy UV radiation splits $O_2$ into free oxygen atoms, and each free oxygen atom then combines with another $O_2$ molecule to give $O_3$.

1. Chemicals causing depletion — the NCERT names chlorofluorocarbons (CFCs), synthetic chemicals used as refrigerants and in fire extinguishers, as the group linked to the sharp drop in atmospheric ozone since the 1980s. It also notes that in 1987 the UNEP forged an agreement to freeze CFC production at 1986 levels, and it is now mandatory for manufacturers to produce CFC-free refrigerators.