11.A small iodine crystal is added to each of the following aqueous solutions I (sodium sulphate), II (sodium thiosulphate), III (sodium tetrathionate and IV (sulphuric acid). In which solution the purple colour disappears.

12.Which of following substance contains magnesium as well as silicon?

13.In general, the boron trihalides act as

14.Carborundum is a very hard substance (only next to diamond in hardness) is mainly a

15.Which compound of boron is used as a mild antiseptic for washing of eyes under the name boric lotion?