Why do ionic compounds have high melting points?

Which of the following pairs will give displacement reactions?

Food cans are coated with tin and not with zinc because

Pratyush took sulphur powder on a spatula and heated it. He collected the gas evolved by inverting a test tube over it,as shown in the figure below.
$(a)$ What will be the action of the gas on
$(i)$ dry litmus paper?
$(ii)$ moist litmus paper?
$(b)$ Write a balanced chemical equation for the reaction taking place.

Samples of four metals $A$,$B$,$C$ and $D$ were taken and added to the following solutions one by one. The results obtained have been tabulated as follows.

Metal	Iron $(II)$ sulphate	Copper $(II)$ sulphate	Zinc sulphate	Silver nitrate
$A$	No reaction	Displacement	No reaction	Displacement
$B$	Displacement	Displacement	No reaction	Displacement
$C$	No reaction	No reaction	No reaction	Displacement
$D$	No reaction	No reaction	No reaction	No reaction

Use the table above to answer the following questions about metals $A$,$B$,$C$ and $D$:
$(i)$ Which is the most reactive metal?
$(ii)$ What would you observe if $B$ is added to a solution of Copper $(II)$ sulphate?
$(iii)$ Arrange the metals $A$,$B$,$C$ and $D$ in the order of decreasing reactivity.

Which gas is produced when dilute hydrochloric acid is added to a reactive metal? Write the chemical reaction when iron reacts with dilute $H_2SO_4$.