What is formed when sulfur trioxide gas is absorbed in concentrated sulfuric acid?

Give reasons for the following:
$(i)$ All decomposition reactions are endothermic reactions.
$(ii)$ Colour of copper sulphate solution changes when an iron nail is dipped in it.
$(iii)$ Respiration is an exothermic reaction.

Which of the following is not a physical change?

The reaction, $3 MnO_{2}(s) + 4 Al(s) \longrightarrow 3 Mn(l) + 2 Al_{2}O_{3}(s) + \text{Heat}$, is an example of:

Write true or false for the following statement:
The thermal decomposition reaction of calcium sulphate (gypsum) is used in black and white photography.

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