36. (b) (i) When nitric acid is prepared by the action of concentrated sulphuric acid on potassium nitrate, what is the special feature of the apparatus used?   37. (ii) Write the equation for the laboratory preparation of nitric acid from potassium nitrate and concentrated sulphuric acid.   38. (iii) Potassium nitrate is prepared from potassium hydroxide and nitric acid. Name the type of this reaction.   39. (iv) Which gas is produced when potassium nitrate is heated? Write the equation for the reaction.