Which gas has the potential to become a major energy source in the future?

Why do ionic compounds have high melting points? State the reason.

Give two examples each of metals that are good conductors and poor conductors of heat,respectively.

What is ammonia $(NH_3)$?

$(a)$ When calcium metal is added to water,the gas evolved does not catch fire,but the same gas evolved on adding potassium metal to water catches fire. Explain why?
$(b)$ Name a metal for each case:
$(i)$ It displaces hydrogen gas from nitric acid.
$(ii)$ It does not react with any physical state of water.
$(iii)$ It does not react with cold as well as hot water but reacts with steam.

State whether the following statement is true or false:
Copper reacts vigorously with dilute $HCl$.