Which of the following are combination reactions?
$(i)$ $2 KClO_{3} \xrightarrow{\text{Heat}} 2 KCl + 3O_{2}$
$(ii)$ $MgO + H_{2}O \longrightarrow Mg(OH)_{2}$
$(iii)$ $4 Al + 3O_{2} \longrightarrow 2 Al_{2}O_{3}$
$(iv)$ $Zn + FeSO_{4} \longrightarrow ZnSO_{4} + Fe$

Give reason why:
$(i)$ White coloured silver chloride turns grey when kept in sunlight.

$A$ 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.

$(a)$ Why is the corrosion of iron a serious problem?
$(b)$ How can we prevent corrosion?

Write true or false for the following statement:
The insoluble substance formed during a chemical reaction is known as a precipitate.

Which among the following statement$(s)$ is(are) true? Exposure of silver chloride to sunlight for a long duration turns grey due to:
$(i)$ The formation of silver by decomposition of silver chloride
$(ii)$ Sublimation of silver chloride
$(iii)$ Decomposition of chlorine gas from silver chloride
$(iv)$ Oxidation of silver chloride