Q.(i) What is heavy water?

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 …