CBSE Class 10 Science 2020 · Paper 31/3/2 · Question 19(b)

(a) At the higher levels of the atmosphere, high-energy UV radiation splits some molecular oxygen (${\text{O}}_2$) into free oxygen atoms ($\text{O}$); these free atoms then combine with molecular oxygen to form ozone (${\text{O}}_3$). (b) ${\text{O}}_2\stackrel{\text{UV}}{\to }\text{O}+\text{O}$ and $\text{O}+{\text{O}}_2⟶{\text{O}}_3$ (ozone). (c) CFCs, used as refrigerants and in fire extinguishers, deplete the ozone layer. Since the ozone layer shields the earth from the Sun’s UV radiation, more UV then reaches the earth, and it can cause skin cancer in human beings and harm other organisms.