Write two observations for the following chemical reaction:
$(a)$ Lead nitrate is strongly heated in a hard glass test tube.

Complete the following reaction:
$2 FeSO_{4} \xrightarrow{\text{Heat}} Fe_{2}O_{3} + \dots + \dots$

Give reasons for the following:
$(i)$ All decomposition reactions are endothermic reactions.
$(ii)$ Colour of copper sulphate solution changes when an iron nail is dipped in it.
$(iii)$ Respiration is an exothermic reaction.

Name the oxidising and reducing agent in the following reaction:
$CuO + H_{2} \longrightarrow Cu + H_{2}O$

Explain the type of reaction represented by the following equation:
$(i) \ CuSO_{4} + 2NaOH \longrightarrow Cu(OH)_{2} + Na_{2}SO_{4}$

$A$ reddish-brown coloured metal, used in electrical wires, when powdered and heated strongly in an open china dish, its colour turns black. When hydrogen gas is passed over this black substance, it regains its original colour. Based on the above information, answer the following questions:
$(i)$ Name the metal and the black coloured substance formed.
$(ii)$ Write balanced chemical equations for both the reactions.