Very pure hydrogen $(99.9\%)$ can be made by which of the following processes?

Formic acid is heated with conc. $H_2SO_4$ at $100^{\circ}C$ to form $A$ and $B$. When $Fe_2O_3$ is heated strongly with $B$,the products formed are $C$ and $D$. $C$ can also be obtained by reacting $CaCO_3$ with dil. $HCl$. What is $D$?

Describe the bulk preparation of dihydrogen by the electrolytic method. What is the role of an electrolyte in this process?

In all its properties,hydrogen resembles

The oxide which is not reduced by hydrogen is

The element used for welding metals with high melting points is: