CBSE Class 10 Science 2019 · Paper 31/1/1/C · Question 16

(a) Mercury and copper. Cinnabar, $\text{HgS}$, heated in air gives mercuric oxide, which on further heating gives mercury; copper glance, ${\text{Cu}}_2\text{S}$, heated in air gives copper. (b) Calcination is heating a carbonate ore strongly in limited air (or in the absence of air), which drives out carbon dioxide: ${\text{ZnCO}}_3(s)\stackrel{\text{heat}}{\to }\text{ZnO}(s)+{\text{CO}}_2(g)$. Roasting is heating a sulphide ore strongly in excess air, which gives off sulphur dioxide: $2\text{ZnS}(s)+3{\text{O}}_2(g)\stackrel{\text{heat}}{\to }2\text{ZnO}(s)+2{\text{SO}}_2(g)$. Both change the ore into zinc oxide. (c) The thermit reaction is the reaction of iron(III) oxide with aluminium, in which the more reactive aluminium displaces iron: ${\text{Fe}}_2{\text{O}}_3(s)+2\text{Al}(s)\to 2\text{Fe}(l)+{\text{Al}}_2{\text{O}}_3(s)+\text{heat}$. It is so highly exothermic that the iron is produced in the molten state. Significance: it is used to join railway tracks and cracked machine parts.