12. (i) Sodium hydroxide is added drop-wise till in excess to a solution of zinc sulphate.   13. (ii) Ammonium hydroxide is added first in a small quantity and then in excess to a solution of copper sulphate.   14. (iii) Excess of ammonium hydroxide is added to a substance obtained by adding hydrochloric acid in silver nitrate solution.   15. (iv) Moist starch iodide paper is put on the mouth of a test-tube containing chlorine gas.   16. (v) A paper dipped in potassium permanganate solution is put on the mouth of a test-tube containing sulphur dioxide gas.