A heavy crystalline white solid on heating in a test tube crumbles with a crackling sound and gives a reddish brown residue which changes to yellow and partly fuses with the glass and stains it yellow. A brown gas is also evolved. A glowing wooden splinter relights when held near the products evolved.

(i) Name the brown gas evolved.

(ii) What is the residue left in the test tube?

(iii) Write the formula of the crystalline white solid.

Distinguish between the following pairs of substances :

(i) Sodium sulphide and sodium sulphite

(ii) HCl gas and chlorine gas

The electronic configuration of three elements A, B and C are as follows :

A — 2, 8, 8, 2;

B — 2, 8, 7

C — 2, 4

(i) Which of these can form cations ?

(ii) Which of these can form anions ?

(iii) Write a formula of a covalent compound formed using these elements

(i) What do you understand by the term 'isotopes'?

(ii) Why do isotopes of an element possess identical chemical properties ?

(iii) Name the isotope of hydrogen which do not have a neutron in it.