Write the chemical formula for the following.
a) Alumina b) Silica c) Rust d) Oleum e) Slag f) Sodium amalgam g) Bauxite h) Haematite.

20 Why are metals like sodium, calcium, magnesium, aluminium etc. obtained by electrolytic reduction? 21 State briefly how you will extract iron from its ore. Draw a neat and labelled diagram. Write all the chemical reactions involved in it. 23 In Frasch process of digging out sulphur from a deep underground, how has it made possible to life sulphur from the bed to surface of each? What is the basic principle on which extraction of sulphur is based? Draw a neat and labelled diagram. Besides elemental form, in what compound forms is sulphur found. Give two physical properties of sulphur. 24 The copper vessels used in homes are found to be coated with green colour. Answer the following.
a) What is the chemical composition of the green coating?
b) Why do copper vessels form such a green coating?
c) What is the name of the phenomenon, responsible for the green coating?
d) Copper vessels are best cleaned with tamarind or lemon juice, but not with soap. Why?