Which is the oxidising agent in the following reaction?
$CuO(s) + H_2(g) \longrightarrow Cu(s) + H_2O(l)$

$A$ substance $X$, which is an oxide of a group $2$ element, is used intensively in the cement industry. This element is present in bones also. On treatment with water, it forms a solution which turns red litmus blue. Identify $X$ and write the chemical reactions involved.

Write a balanced chemical equation for the reaction in which phosphorus burns in chlorine to form phosphorus pentachloride.

Complete and balance the following chemical equations:
$(i)$ $CaCO_{3} + HCl \longrightarrow$
$(ii)$ $Al + HCl \longrightarrow$
$(iii)$ $MnO_{2} + HCl \longrightarrow$

Balance the following chemical equations and identify the type of chemical reaction.
$(a)$ $Mg(s) + Cl_2(g) \to MgCl_2(s)$
$(b)$ $HgO(s) \xrightarrow{\text{Heat}} Hg(l) + O_2(g)$
$(c)$ $Na(s) + S(s) \xrightarrow{\text{Fuse}} Na_2S(s)$

$A$ dilute ferrous sulphate solution was gradually added to the beaker containing acidified permanganate solution. The light purple colour of the solution fades and finally disappears. Which of the following is the correct explanation for the observation?