CBSE Class 10 Science 2020 · Paper 31/C/1 · Question 25(b)

(a) X is ethene, ${\text{CH}}_2={\text{CH}}_2$ (${\text{C}}_2{\text{H}}_4$), and Y is ethane, ${\text{CH}}_3-{\text{CH}}_3$ (${\text{C}}_2{\text{H}}_6$, molecular mass $2(12)+6(1)=30{\text{g mol}}^{-1}$). Addition of hydrogen: ${\text{CH}}_2={\text{CH}}_2+{\text{H}}_2\stackrel{\text{Ni or Pd catalyst}}{\to }{\text{CH}}_3-{\text{CH}}_3$ Decolorising bromine water: ${\text{CH}}_2={\text{CH}}_2+{\text{Br}}_2⟶{\text{CH}}_2\text{Br}-{\text{CH}}_2\text{Br}$ Burning: ${\text{C}}_2{\text{H}}_4+3{\text{O}}_2⟶2{\text{CO}}_2+2{\text{H}}_2\text{O}+\text{heat and light}$; the flame is smoky because part of the carbon is left unburnt. (b) (i) Butanone, ${\text{CH}}_3-\text{CO}-{\text{CH}}_2-{\text{CH}}_3$: $$\begin{array}{ccccccccccc}& & \text{H}& & \text{O}& & \text{H}& & \text{H}& & \\ & & |& & \Vert & & |& & |& & \\ \text{H}& -& \text{C}& -& \text{C}& -& \text{C}& -& \text{C}& -& \text{H}\\ & & |& & & & |& & |& & \\ & & \text{H}& & & & \text{H}& & \text{H}& & \end{array}$$ (ii) Pentanoic acid, ${\text{CH}}_3-{\text{CH}}_2-{\text{CH}}_2-{\text{CH}}_2-\text{COOH}$: $$\begin{array}{ccccccccccccccc}& & \text{H}& & \text{H}& & \text{H}& & \text{H}& & \text{O}& & & & \\ & & |& & |& & |& & |& & \Vert & & & & \\ \text{H}& -& \text{C}& -& \text{C}& -& \text{C}& -& \text{C}& -& \text{C}& -& \text{O}& -& \text{H}\\ & & |& & |& & |& & |& & & & & & \\ & & \text{H}& & \text{H}& & \text{H}& & \text{H}& & & & & & \end{array}$$ (c) No. A detergent lathers well in both hard and soft water, because the charged ends of detergent molecules do not form insoluble precipitates (scum) with the calcium and magnesium ions of hard water; so hard and soft water behave alike with it. Soap, which forms scum and gives little lather in hard water, is used to check hardness.