CBSE Class 10 Science 2019 · Paper 31/1/1/C · Question 17(a)

(a) In methane, ${\text{CH}}_4$, carbon (configuration $2,4$) shares one pair of electrons with each of four hydrogen atoms: C is in the centre with an H on each of its four sides, and each C–H link is a shared pair made of one electron from carbon and one from hydrogen. Carbon thus has an octet and each hydrogen a duplet, with four single covalent bonds and no lone pairs. (b) (i) ${\text{C}}_2{\text{H}}_6\text{O}$ is ethanol, ${\text{C}}_2{\text{H}}_5\text{OH}$: alcohol group, $-\text{OH}$. (ii) ${\text{C}}_2{\text{H}}_4\text{O}$ is ethanal, ${\text{CH}}_3\text{CHO}$: aldehyde group, $-\text{CHO}$. (c) Air contains only about $21\%$ oxygen, so ethyne burnt in air burns incompletely, with a sooty flame that is not hot enough for welding. In pure oxygen it burns completely and gives a very hot flame (about $3000^{\circ }$C) that melts metals: $2{\text{C}}_2{\text{H}}_2+5{\text{O}}_2\to 4{\text{CO}}_2+2{\text{H}}_2\text{O}+\text{heat}$.