CBSE Class 10 Science 2020 · Paper 31/2/1 · Question 18(b)

(a) (i) ${\text{O}}_2$; (ii) ${\text{O}}_3$; (iii) Oxygen is needed for respiration (breathing): aerobic organisms use it to release energy from food; (iv) Ozone absorbs the harmful ultraviolet (UV) radiation of the sun and shields living organisms from it. (b) At the higher levels of the atmosphere, high-energy UV radiation splits some oxygen molecules into free oxygen atoms, ${\text{O}}_2\stackrel{\text{UV}}{\to }\text{O}+\text{O}$. These atoms combine with oxygen molecules to form ozone, $\text{O}+{\text{O}}_2⟶{\text{O}}_3$.