18. (i) Chlorine is passed into an aqueous solution of sulphur dioxide.   19. (ii) Aluminium powder is warmed with hot and concentrated caustic soda solution.   20. (iii) Concentrated nitric acid is added to copper turnings kept in a beaker.   21. (iv) Red lead (trilead tetroxide) is warmed with concentrated hydrochloric acid.   22. (v) Chlorine gas is passed through an aqueous solution of iron (II) sulphate acidified with dilute sulphuric acid.   23. (vi) Ethane is burnt in air.