Q.When hydrogen peroxide is shaken with an acidified solution of potassium dichromate in presence of ether, the ethereal layer turns blue due to the formation of

In hydrogen peroxide, the oxygen atoms exist in an intermediate oxidation state of $-1$, which allows $H_2O_2$ to act as either an oxidising agent or a reducing agent depending on the substance it reacts with. As an oxidising agent (typically in acidic medium) it converts a lower oxidation state to a higher one while itself being reduced to water, for example $PbS(s) + 4H_2O_2(aq) \rightarrow PbSO_4(s) + 4H_2O(l)$, where sulphide sulfur is oxidised to sulfate. As a reducing agent, in the presence of a stronger oxidiser, it is itself oxidised to oxygen gas, for example $2MnO_4^-(aq) + 6H^+(aq) + 5H_2O_2(aq) \rightarrow 2Mn^{2+}(aq) + 8H_2O(l) + 5O_2(g)$, where manganese is reduced from $+7$ to $+2$. This dual redox character also underlies its use as a bleaching agent: on decomposition it releases nascent oxygen, $H_2O_2 \rightarrow H_2O + [O]$, and this nascent oxygen oxidises coloured o …