Which of the following gases can be used for the storage of a fresh sample of an oil for a long time?

$A$ dilute ferrous sulphate solution was gradually added to the beaker containing acidified permanganate solution. The light purple colour of the solution fades and finally disappears. Which of the following is the correct explanation for the observation?

The reaction, $3 MnO_{2}(s) + 4 Al(s) \longrightarrow 3 Mn(l) + 2 Al_{2}O_{3}(s) + \text{Heat}$, is an example of:

Why do fireflies glow at night?

$A$ metal $X$ acquires a green colour coating on its surface on exposure to air.
$(i)$ Identify the metal $X$ and name the process responsible for this change.
$(ii)$ Name and write the chemical formula of the green coating formed on the metal.
$(iii)$ List two important methods to prevent this process.

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