In the manufacture of sulphuric acid by the contact process , $SO_3$ is not added to water directly to form $H_2SO_4$ because

Question

$\begin{array}{1 1} \text{The reaction does not go to completion } \\ \text{The reaction is exothermic} \\ \text{the reaction is exothermic } \\ \text{colloidal sulphur} \end{array}$