What are the constituents of solder alloy? Which property of solder makes it suitable for welding electrical wires?

Explain the following:
$(a)$ $NaCl$ is not a conductor of electricity in solid state,whereas it does conduct electricity in aqueous solution as well as in molten state.
$(b)$ Iron articles are galvanised.
$(c)$ Metals like $Na, K, Ca$ and $Mg$ are never found in their free state in nature.

Give reasons for the following:
$(a)$ We can store copper sulphate solution in a silver vessel but not silver nitrate solution in a copper vessel.
$(b)$ The reaction of zinc with dilute nitric acid does not produce hydrogen gas.
$(c)$ Food cans are coated with tin rather than zinc.

An element $A$ burns with a golden yellow flame in air. It reacts with another element $B$ (atomic number $17$) to give a product $C$. An aqueous solution of product $C$ on electrolysis gives a compound $D$ and liberates hydrogen. Identify $A$,$B$,$C$,and $D$. Also,write down the equations for the reactions involved.

What is the typical carat value of gold used in jewelry?

Give reasons for the following:
$(i)$ Most metals conduct electricity well.
$(ii)$ The reaction of iron $(III)$ oxide,$Fe_{2}O_{3}$,with heated aluminium is used to join cracked machine parts.
$(iii)$ Sodium,potassium,and lithium are stored under oil.