When $Fe^{2+}$ changes to $Fe^{3+}$ in a reaction,

When copper turnings are added to silver nitrate solution,a blue coloured solution is formed after some time. It is because,copper

Which of the following correctly represents the process of reduction?

In the reaction $H_2S + NO_2 \to H_2O + NO + S$,$H_2S$ is

Identify the redox couples involved in the following reactions:
$(a) \ Cu + Zn^{+2} \to Cu^{+2} + Zn$
$(b) \ Mg + Fe^{+2} \to Mg^{+2} + Fe$
$(c) \ Br_{2} + 2Cl^{-} \to Cl_{2} + 2Br^{-}$
$(d) \ Fe + Cd^{+2} \to Cd + Fe^{+2}$

When $HCl$ reacts with finely powdered iron,it forms ferrous chloride and not ferric chloride. Why?