Write chemical equations for the reactions taking place when:
$(i)$ iron is strongly heated in air.
$(ii)$ lead carbonate is calcined.
$(iii)$ chromium oxide $(Cr_{2}O_{3})$ is heated with aluminium powder.

(N/A) $(i)$ Iron does not burn in air even on strong heating. At high temperature,it combines with oxygen of the air to form $Fe_{3}O_{4}$.
$3Fe(s) + 2O_{2}(g) \xrightarrow{\text{Heat}} Fe_{3}O_{4}(s)$
$(ii)$ Calcination of lead carbonate involves heating the ore in the absence of air.
$PbCO_{3}(s) \xrightarrow{\text{Heat}} PbO(s) + CO_{2}(g)$
$(iii)$ This is a thermite reaction where aluminium acts as a reducing agent.
$Cr_{2}O_{3}(s) + 2Al(s) \rightarrow Al_{2}O_{3}(s) + 2Cr(l) + \text{Heat}$