In which of the following reactions is there no change in the valency (oxidation state) of the elements?

Which of the following can act as both a reducing and an oxidising agent?

$50 \ mL$ of $0.1 \ M$ solution of a salt reacted with $25 \ mL$ of $0.1 \ M$ solution of sodium sulphite. The half-reaction for the oxidation of sulphite ion is:
$SO_{3}^{2-} (aq) + H_{2}O (l) \to SO_{4}^{2-} (aq) + 2H^{+} (aq) + 2e^{-}$
If the oxidation number of the metal in the salt was $+3$,what would be the new oxidation number of the metal?

Assign $A, B, C, D$ from the given type of reaction:
$2FeCl_3 + 3KI \longrightarrow 2FeCl_2 + I_2 + 2KCl$
$I_2 + KI \longrightarrow KI_3$
Identify the observation for the product $KI_3$ in the reaction $FeCl_3 + KI \longrightarrow Fe^{2+}(aq.) + KI_3$.

$A$ white crystalline salt $[A]$ reacts with dilute $HCl$ to produce a suffocating gas $[B]$ and also forms a yellow precipitate. When gas $[B]$ is passed through acidified potassium dichromate,it turns the solution green,forming $[C]$. Identify $A, B$,and $C$ respectively.

How do you account for the following observations?
$(a)$ Though alkaline potassium permanganate and acidic potassium permanganate both are used as oxidants,yet in the manufacture of benzoic acid from toluene,we use alcoholic potassium permanganate as an oxidant. Why? Write a balanced redox equation for the reaction.
$(b)$ When concentrated sulphuric acid is added to an inorganic mixture containing chloride,we get a colourless pungent-smelling gas $HCl$,but if the mixture contains bromide,then we get red vapour of bromine. Why?