How will you test whether the gas evolved in a reaction is hydrogen?

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

State the reason for the following:
$(a)$ Potato chips manufacturers usually flush bags of chips with nitrogen gas.
$(b)$ Iron articles lose their shine gradually.
$(c)$ Foods should be kept in airtight containers.

The changes which take place when fats and oils are oxidised:

Observe the following diagram and answer the questions that follow:
$(i)$ Identify the gases evolved at anode and cathode.
$(ii)$ Why are the amounts of gases collected in the two test tubes not of the same volume?
$(iii)$ What type of reaction is this?
$(iv)$ Why should we use salt water?

Give differences between the displacement and double displacement reactions.