Write two observations for the following chemical reaction:
$(a)$ Potassium iodide solution is added to lead nitrate solution.

The substances which get oxidised easily can be kept for longer duration by:

Which among the following is(are) double displacement reaction$(s)$?
$(i)$ $Pb + CuCl_{2} \rightarrow PbCl_{2} + Cu$
$(ii)$ $Na_{2}SO_{4} + BaCl_{2} \rightarrow BaSO_{4} + 2NaCl$
$(iii)$ $C + O_{2} \rightarrow CO_{2}$
$(iv)$ $CH_{4} + 2O_{2} \rightarrow CO_{2} + 2H_{2}O$

$A$ 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.

The reaction which is used in black and white photography is:

The coating formed on the metals such as iron, silver, and copper after corrosion will be: