CBSE Class 10 Science 2020 · Paper 31/C/2 · Question 25(b)

(a) Two reasons: catenation, the ability of carbon atoms to link with one another into long chains, branched chains and rings; and tetravalency, a valency of four, so each carbon atom can bond with four atoms of carbon or of other elements such as hydrogen, oxygen, nitrogen, sulphur and chlorine. (i) Carbon has four valence electrons (configuration $2,4$); gaining four to form ${\text{C}}^{4-}$ or losing four to form ${\text{C}}^{4+}$ is very difficult, so it shares its electrons and forms covalent bonds. (ii) Carbon compounds are made of molecules held together by weak forces of attraction between the molecules, so little energy is needed to separate them, and their melting and boiling points are low. (b) (i) Sodium reacts with ethanol, giving bubbles of hydrogen gas and sodium ethoxide: $2\text{Na}+2{\text{CH}}_3{\text{CH}}_2\text{OH}⟶2{\text{CH}}_3{\text{CH}}_2\text{ONa}+{\text{H}}_2$. (ii) Ethanol burns with a clean blue flame, forming carbon dioxide and water and releasing heat and light: ${\text{CH}}_3{\text{CH}}_2\text{OH}+3{\text{O}}_2⟶2{\text{CO}}_2+3{\text{H}}_2\text{O}+\text{heat and light}$.